Are needle jets and needles matched in VM carbs? In other words, if I change the size of the needle should I change the needle jets as well?
'78 GS750E with VM 26 carbs.
'78 GS750E with VM 26 carbs.
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Needle Jets need to match Needes?
- Thread starter Sam 78 GS750
- Start date
Fair enough. I actually have no baseline as it is, I'm just making a best guess on where I should be.
Long story, but the short version is I lost my stock pipes and airbox in the middle of a complete tear-down/rebuild, couldn't find stock replacements and had to go with a 4-1 and pods.
I planned on using stock needles, but going up a few sizes on the main jets and 1 size on the pilots to start with. I had a bent needle though, and ordered 4 stock needles from what was listed on the fiche. Ended up being a lot narrower than what was in the carbs...
I do know that the PO had replaced the carbs and did make some jetting changes and he said he still never got the jetting quite right. When I dug into it, I found that the valves were really tight. If he would have adjusted them, he wouldn't have wasted time and money on carbs, and probably would have kept the bike. I got quite the deal.
That leaves me with a lot of unknowns on where I should start with jetting though. I'm just trying to get to a decent starting point so the bike runs well enough to break in/seat new piston rings. After that, I can fine tune the jetting.
Long story, but the short version is I lost my stock pipes and airbox in the middle of a complete tear-down/rebuild, couldn't find stock replacements and had to go with a 4-1 and pods.
I planned on using stock needles, but going up a few sizes on the main jets and 1 size on the pilots to start with. I had a bent needle though, and ordered 4 stock needles from what was listed on the fiche. Ended up being a lot narrower than what was in the carbs...
I do know that the PO had replaced the carbs and did make some jetting changes and he said he still never got the jetting quite right. When I dug into it, I found that the valves were really tight. If he would have adjusted them, he wouldn't have wasted time and money on carbs, and probably would have kept the bike. I got quite the deal.
That leaves me with a lot of unknowns on where I should start with jetting though. I'm just trying to get to a decent starting point so the bike runs well enough to break in/seat new piston rings. After that, I can fine tune the jetting.
As others said, you only need to adjust or change the jet needles. The needle jets are fine if they're factory.
As for the new jet needles you bought, if they're obviously more narrow than the stockers, then you could have problems jetting the bike. You say you ordered 4 stock needles from the fiche. If they're stock, they would be identical to the ones in your carbs, assuming the ones in the carbs were stock. Who did you order them from? Are they genuine Mikuni or a cheaper version? Just sounds wrong to me. I'd look up the factory jet needle info and buy the correct needles. I'm sure someone here has some needles you could buy.
For a QUALITY 4-1 pipe and pods, your stock #15 pilot jets are fine. For the pilot circuit, I'd turn the pilot fuel screws (underneath) 1 to 1 1/4 turns out from lightly seated and go from there. Then set the side air screws to 1 3/4 turns out. With the bike fully warmed up, set the base idle at 1,000 rpm's by turning the throttle adjuster knob. Turn the first air screw slowly to achieve the highest rpm, usually around 1 1/2 to 2 turns out. Then return the idle back to 1,000 by using the throttle adjuster knob. Repeat to the other 3 carbs. Test the pilot circuit at throttle position of closed to approx' 1/5 throttle. Ride around at 1/5 or less for several miles and check the sparkplugs. Do what the plugs say. If lean, you will NOT need to go richer on the pilot jets, only a little richer on the pilot fuel screws. If a little rich, then turn the pilot fuel screws in 1/4 turn and re-test. The side air screws are ALWAYS adjusted using the highest rpm method I describe.
For the main jets, if your stock mains are #100 (?), I'd try 127.5's. From what I remember with owners at this site, size 125 to 130 mains worked best. With a quality pipe and pods, you'd have to go up at least 4 full sizes (120) but I'd go richer and work back. Test at full throttle, note performance/check plugs.
For the jet needles, the most important jetting circuit, since you spend the most the most time riding on the jet needles, I'd try a minimum 2 positions richer on the KNOWN stock needles. So if your stock needles have the e-clip in the second position from the top, them lower the e-clips to the fourth position from the top and test at approx' 1/2 throttle. Remember, you re-jet by throttle position, not rpm's. The main jets take over at 3/4 throttle. There is some overlap between the jet needles and mains as the needles rise but the mains regulate fuel at 3/4 throttle to full throttle. The needle jets/jet needles regulate fuel from approx' 1/5 throttle to 3/4 throttle. The pilot jets and mixture screws regulate fuel from idle to approx' 1/5 throttle.
If you ride on freeways or are subject to crosswinds/windy conditions, I suggest removing the two carb bowl vent lines and leave the vent ports open to breath easier. This is recommended by Dynojet with their stage three jetting kits (for 4-1 pipes and pods) and I can personally tell you that removing the lines helps eliminate fuel starvation under some conditions.
I also suggest to be SURE the carbs are clean and the float levels set correctly. Also, be sure the intake manifold O-rings are good. Any intake leak will make jetting attempts pointless. Be sure the carbs are AT LEAST carefully bench synched after adjusting the needles. Any further jet needle changes requires a new synch. A vacuum tool synch is BEST, especially when re-jetting.
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I had originally planned on using the needles I had, and just moving the clips. What started this was that the slide fell out of one of the carbs when I was pulling it off the shelf and landed right on the tip of the needle...the needle got bent pretty badly and needed to be replaced. I was confused because with the '78 GS750, there seems to be 2 stock carb setups with 2 separate fiches depending on the VIN #. I got the needle that wasn't showing as unavailable (and got 4 just in case they were different).
I got the needles from Partzilla, and they're genuine Mikuni. One fiche is labeled "emission style" for the assembly. That has the needle as 5DL36-2, which is the one with a thinner taper, the new ones I just got. It shows it matched to an O-4 needle jet with a 102.5 main. ---->https://www.partzilla.com/catalog/s...0c-f-no-gs750-41493-gs750ec-f-no-gs750e-19996
The other fiche has the needle as a 5F21-3, the thicker ones that were in my carbs. It's matched with an O-6 needle jet and a 100 main (with 92.5 through 107.5 mains as optional). --->https://www.partzilla.com/catalog/s...0c-f-no-gs750-41492-gs750ec-f-no-gs750e-19995
Both needles are the same diameter at the top, one just has more of a taper so it's thinner at the bottom.
I'm assuming it was because of US emissions regulations halfway through the model year, but I'm a bit confused why the "emission style" carb setup would have a larger main, and more of a taper on the needle...but then a smaller needle jet.
I'm installing the 5dl36 needles, since I have 4 good ones and the 5F21 is unavailable (even Jets-R-Us doesn't list it)... Do you think it will give me tuning issues, or should it be ok, since it's a needle that Suzuki put in these carbs for this bike at one point (even if it's not the one my particular bike had in the carbs when I got it)? I'm thinking since they're more of a taper, a bit richer than what I pulled out may not be a bad thing...???
I got the Delkevic 4-1 and 2 of the K&N 'lunchbox' filters (RC-2222), so no issues with the quality. I ordered #17 pilots, and had planned on trying those first. On your advice here, I'll stick with the #15 and adjust the screw. From what I've been reading, if you need to go more than 3 turns out on the fuel screws you need bigger pilot jets. Does that sound about right? If I have to go way out on the screws, at least I have the 17s on hand...
Like I said, the stock was either 100 or 102.5 depending on the VIN. I ordered 115 to 122.5, and was going to start at 117 and either work up or down depending on plug readings. Again, I'll take your advice and start with the richest I have on hand which is the 122.5.
Again, I hope I'll be OK with the "other" stock needles... The thicker ones had the clips in the middle, so I went one down from there to the 4th from the top on the more tapered ones. Hopefully these needles work out, since I can't seem to get the 5F21 anymore. If I have too, I guess I can try to straighten the bent one out.
I'll give this a shot. Do you have any screen or anything over them to keep random tiny critters or dust/dirt out?
Carbs have been meticulously cleaned...Dipped/sprayed/scrubbed until you can eat off of them, and all passages blown out with compressed air. I have them bench synced. I can't seem to locate my Carbtune, so may have to order another one. The last time I saw it was 2010 which was the last time this bike was on the road, so...
I cannot be sure, but my memory is the 5DL36-2 jet needles are the most common for the 750's. I helped jet a couple of 750's many years ago and I believe those were the needles and the mains were 102.5. The 1000, same VM26 carbs, uses the same needles. The "-2" in 5DL36-2 means the e-clip is in position 2 from the top per factory. I don't know enough about the "emission style" needles and how they effected main jet and needle jet size, so I have no comment on that. So I think your needles will work.
The stock #15 pilot jets will work for your basic 4-1 pod set up. A 4-1 and pods doesn't effect the pilot circuit as much as the jet needle and main circuits. So many owners have increased the pilot jet size because it seemed like the right thing to do. Your mod's don't justify changing the pilot jet size. It will only run rich with larger pilot jets. Dynojet makes quality jetting kits and their stage 3 kits (for 4-1/pods) are designed to be used with the stock pilot jets. The pilot fuel screws adjustments are all that are needed to richen the pilot circuit. As a rule of thumb, once you have turned out the pilot fuel screws 3 additional turns and you're still lean, then you need to go with larger pilot jets. I've never known a 750 or 1000 with pipe/pod mod's to run well with larger pilot jets.
Your needles have 5 positions for the e-clip. Placing the e-clip in the 4th position from the top represents 2 full positions richer. I've found that's the minimum change needed. It works many times. I've tried position 5 on a few 1000's and the bikes ran rich, including my own. Position "4 1/2" often works too, but you'll need the jetting spacers that are part of stage 3 kits. I'm a perfectionist about my bike and I wasn't happy about using the stock needles. I'd heard about Dynojet and their needles. Motorcyclist magazine also backed the Dynojet needles. I installed the Dynojet kit and got the needles right in 2 tries. The DJ needles are just better needles.
So I think position 4 on the 5DL36 needles has a good chance of working. I really doubt they'll be rich. They may still be lean but only testing will tell. If a little on the lean side, then position 4 1/2 would be my next try, depending on plug color. Assuming the bike is in good tune otherwise, be sure the plugs are clean/fresh and gapped correctly, ignition timing spot on, fresh gas, test the bike in a safe place at 1/2 throttle. If you don't feel any surging (lean) during level and various uphill riding and 1/4 to 3/4 throttle performance is good, then check the plugs. When I used to test the jetting, I always "chopped off" the throttle to get the most accurate reads, as simply slowing down with the motor running until stopping can compromise the plug color. "Chopping off" means to test the bike at a certain throttle position, then close the throttle, pull in the clutch, turn the ignition off and roll to a safe place to check the plugs. I carried a plug wrench, rag and a piece of hose that fit tightly over the top of the hot plugs. The hose allowed me to grip the plug when it was almost out of the threads. A couple turns and the plug came out stuck in the hose. Just be careful when re-installing so you don't cross-thread them. I'll add that today's fuels run cleaner and plugs are harder to read than years ago. Trying to get that perfect tan/brownish color is difficult. Anyway, do what the plugs and performance say. Also, keep in mind that reasonably uniform vacuum in all 4 carbs helps make jetting easier. If you only bench synch, you may have a difference in vacuum that exceeds factory requirements and it will effect the performance and make plug reading more difficult.
As for the mains, I think 4 full sizes larger (122.5) is the absolute minimum for a your set up. Personally, I'd go 5 full sizes (127.5). I think 4 sizes will be on the lean side and just my opinion. Many 750 owners here have shared their experience. I remember some said 125 to 130 worked well. I've also read that some said smaller mains worked but I have my doubts the bikes truly ran well. Test at full throttle. I used to have a safe place to jet. Once fully warmed up, I ran the bike at a particular throttle position for about 1 1/2 miles, then turned around and repeated that and then chopped off. Worked for me and several of my friends did similar.
I've always kept the float bowl vent line ports open, no screen or whatever. I suppose a bug could enter a port and gunk it up but I've never had it happen or heard of it.
Hope this helps. I'll be gone in a few days to go on a trip with my sons. I'll be gone about a week. If I can still help, I'll tune in later. Good luck.
The stock #15 pilot jets will work for your basic 4-1 pod set up. A 4-1 and pods doesn't effect the pilot circuit as much as the jet needle and main circuits. So many owners have increased the pilot jet size because it seemed like the right thing to do. Your mod's don't justify changing the pilot jet size. It will only run rich with larger pilot jets. Dynojet makes quality jetting kits and their stage 3 kits (for 4-1/pods) are designed to be used with the stock pilot jets. The pilot fuel screws adjustments are all that are needed to richen the pilot circuit. As a rule of thumb, once you have turned out the pilot fuel screws 3 additional turns and you're still lean, then you need to go with larger pilot jets. I've never known a 750 or 1000 with pipe/pod mod's to run well with larger pilot jets.
Your needles have 5 positions for the e-clip. Placing the e-clip in the 4th position from the top represents 2 full positions richer. I've found that's the minimum change needed. It works many times. I've tried position 5 on a few 1000's and the bikes ran rich, including my own. Position "4 1/2" often works too, but you'll need the jetting spacers that are part of stage 3 kits. I'm a perfectionist about my bike and I wasn't happy about using the stock needles. I'd heard about Dynojet and their needles. Motorcyclist magazine also backed the Dynojet needles. I installed the Dynojet kit and got the needles right in 2 tries. The DJ needles are just better needles.
So I think position 4 on the 5DL36 needles has a good chance of working. I really doubt they'll be rich. They may still be lean but only testing will tell. If a little on the lean side, then position 4 1/2 would be my next try, depending on plug color. Assuming the bike is in good tune otherwise, be sure the plugs are clean/fresh and gapped correctly, ignition timing spot on, fresh gas, test the bike in a safe place at 1/2 throttle. If you don't feel any surging (lean) during level and various uphill riding and 1/4 to 3/4 throttle performance is good, then check the plugs. When I used to test the jetting, I always "chopped off" the throttle to get the most accurate reads, as simply slowing down with the motor running until stopping can compromise the plug color. "Chopping off" means to test the bike at a certain throttle position, then close the throttle, pull in the clutch, turn the ignition off and roll to a safe place to check the plugs. I carried a plug wrench, rag and a piece of hose that fit tightly over the top of the hot plugs. The hose allowed me to grip the plug when it was almost out of the threads. A couple turns and the plug came out stuck in the hose. Just be careful when re-installing so you don't cross-thread them. I'll add that today's fuels run cleaner and plugs are harder to read than years ago. Trying to get that perfect tan/brownish color is difficult. Anyway, do what the plugs and performance say. Also, keep in mind that reasonably uniform vacuum in all 4 carbs helps make jetting easier. If you only bench synch, you may have a difference in vacuum that exceeds factory requirements and it will effect the performance and make plug reading more difficult.
As for the mains, I think 4 full sizes larger (122.5) is the absolute minimum for a your set up. Personally, I'd go 5 full sizes (127.5). I think 4 sizes will be on the lean side and just my opinion. Many 750 owners here have shared their experience. I remember some said 125 to 130 worked well. I've also read that some said smaller mains worked but I have my doubts the bikes truly ran well. Test at full throttle. I used to have a safe place to jet. Once fully warmed up, I ran the bike at a particular throttle position for about 1 1/2 miles, then turned around and repeated that and then chopped off. Worked for me and several of my friends did similar.
I've always kept the float bowl vent line ports open, no screen or whatever. I suppose a bug could enter a port and gunk it up but I've never had it happen or heard of it.
Hope this helps. I'll be gone in a few days to go on a trip with my sons. I'll be gone about a week. If I can still help, I'll tune in later. Good luck.
If you need some 5F21 needles, I would trade you for the 5DL36, or sell them to you.
For the quarter throttle range, the diameter of the needle and the needle jet are very critical, a very very tiny difference such as 1/2 of one thousandth of an inch (.0005"), makes a big difference. The 5DL36 needles are slightly thicker at the base area, but thinner as the taper is more aggressive for more performance at larger throttle openings. But a slightly cleaner idle to satisfy the stricter fuel emissions standards.
Some specs list the 5 F 21 needles as being used with a P4 needle jet, but I have never seen this combination in real life, they are always with o-6 needle jets in the carbs, as are all of the 5 DL 36 needles that I have seen matched with o-6 needle jets.
Jetting really depends a lot on the engine condition and compression, cam timing, at which even different "identical" bikes rolling off the assembly line came with slightly different camshaft timing due to manufacturing tolerances. I have found on 2 or more of my GS 750 combinations with pods and pipe, that raising the needle one position and running around 112.5 through 115 Main Jets was near perfect on the wide open throttle plug chops and the wideband O2 sensor air fuel ratio gauge readings. Altitude also has a lot to do with that. I am at around 900 feet above sea level, and on road trips never really get above 4000 feet averaging, with a 6000 foot maximum. The bike always has lots of power and never get so rich as to foul plugs at high altitude.
I do read of a lot of people running main Jets around 120 on gs750's, where that has never worked for me in any combination. In fact with 850 pistons or larger, I need even smaller main jets, because the engine sucks more air, and when more air velocity goes through the carbs, it sucks more fuel at the same time. This is where I decided that o-4 needle jets would be perfect for a big bore 750, as the quarter throttle area always became a slight bit on the rich side when going with big bore pistons while retaining the same carburetors.
By the way I was shooting for air fuel ratios of around 12.9 to 13.2, with high 13's being acceptable for around 1/4 throttle, shooting for around 13 at WOT, and mid 12's at idle. I was also doing WOT 5500-9500rpm hard acceleration plug chops after a 10 second solo drag race up the biggest hills I could find (scary fast once you get close on jetting!!!
For the quarter throttle range, the diameter of the needle and the needle jet are very critical, a very very tiny difference such as 1/2 of one thousandth of an inch (.0005"), makes a big difference. The 5DL36 needles are slightly thicker at the base area, but thinner as the taper is more aggressive for more performance at larger throttle openings. But a slightly cleaner idle to satisfy the stricter fuel emissions standards.
Some specs list the 5 F 21 needles as being used with a P4 needle jet, but I have never seen this combination in real life, they are always with o-6 needle jets in the carbs, as are all of the 5 DL 36 needles that I have seen matched with o-6 needle jets.
Jetting really depends a lot on the engine condition and compression, cam timing, at which even different "identical" bikes rolling off the assembly line came with slightly different camshaft timing due to manufacturing tolerances. I have found on 2 or more of my GS 750 combinations with pods and pipe, that raising the needle one position and running around 112.5 through 115 Main Jets was near perfect on the wide open throttle plug chops and the wideband O2 sensor air fuel ratio gauge readings. Altitude also has a lot to do with that. I am at around 900 feet above sea level, and on road trips never really get above 4000 feet averaging, with a 6000 foot maximum. The bike always has lots of power and never get so rich as to foul plugs at high altitude.
I do read of a lot of people running main Jets around 120 on gs750's, where that has never worked for me in any combination. In fact with 850 pistons or larger, I need even smaller main jets, because the engine sucks more air, and when more air velocity goes through the carbs, it sucks more fuel at the same time. This is where I decided that o-4 needle jets would be perfect for a big bore 750, as the quarter throttle area always became a slight bit on the rich side when going with big bore pistons while retaining the same carburetors.
By the way I was shooting for air fuel ratios of around 12.9 to 13.2, with high 13's being acceptable for around 1/4 throttle, shooting for around 13 at WOT, and mid 12's at idle. I was also doing WOT 5500-9500rpm hard acceleration plug chops after a 10 second solo drag race up the biggest hills I could find (scary fast once you get close on jetting!!!
Thanks for taking the time to write out all that detail, Keith. Definitely helpful.
Chuck - Thanks for the offer on the needles. I'll see how I make out with the 5DL36 needles, but if I need to adjust I'll definitely take you up on it. Makes sense to me now about the difference in the "emission style" having a more aggressive taper. Thicker just at the top for idle. Smart...
You guys are pretty far apart on the main jet recommendations... I'm pretty much at sea level, but do get into the mountains on occasion. Jersey mountains would be called "hills" in some parts of the country though. With the airbox, the bike was equally rideable on the Pacific Coast Highway, and going through Aspen... I'm not expecting the same from pods, but I'd be happy if I could get it running where I can go up to 6000' with no issues...
Chuck - Thanks for the offer on the needles. I'll see how I make out with the 5DL36 needles, but if I need to adjust I'll definitely take you up on it. Makes sense to me now about the difference in the "emission style" having a more aggressive taper. Thicker just at the top for idle. Smart...
You guys are pretty far apart on the main jet recommendations... I'm pretty much at sea level, but do get into the mountains on occasion. Jersey mountains would be called "hills" in some parts of the country though. With the airbox, the bike was equally rideable on the Pacific Coast Highway, and going through Aspen... I'm not expecting the same from pods, but I'd be happy if I could get it running where I can go up to 6000' with no issues...
Sea level would require slightly larger jets, maybe one size, then where I am at and where I ride.
I tried 125 mains in my 750 when I first started trying to jet it for a 4 into 1 exhaust with large performance baffle, and and K&N RC-2222 dual oval lunch box type air filters, and it was basically unrideable at wide open throttle, very poor performance, grossly rich. The smaller I went, the more power it would make from 5,500 up to 10,000 RPM. Down to around 112.5 or so I believe is where I found it was the best, nice light brown/tan color ring at the base of the spark plug insulator. The only area where a larger jet would really help is if you think you can just take a mechanical linkage slide type carb and whack the throttle open very far from a low RPM. The extra size in the main jet helps this respond better, but these carbs are not really intended to do that. If you think about how a CV carb works on air pressure differential, it only allows the slide to open as much as the air velocity through the venturi requires. So if you whack it wide open on a CV carb, the slide doesn't necessarily go anywhere immediately until the revs pick up substantially.
I tried 125 mains in my 750 when I first started trying to jet it for a 4 into 1 exhaust with large performance baffle, and and K&N RC-2222 dual oval lunch box type air filters, and it was basically unrideable at wide open throttle, very poor performance, grossly rich. The smaller I went, the more power it would make from 5,500 up to 10,000 RPM. Down to around 112.5 or so I believe is where I found it was the best, nice light brown/tan color ring at the base of the spark plug insulator. The only area where a larger jet would really help is if you think you can just take a mechanical linkage slide type carb and whack the throttle open very far from a low RPM. The extra size in the main jet helps this respond better, but these carbs are not really intended to do that. If you think about how a CV carb works on air pressure differential, it only allows the slide to open as much as the air velocity through the venturi requires. So if you whack it wide open on a CV carb, the slide doesn't necessarily go anywhere immediately until the revs pick up substantially.
Tonight I will be testing out another 77 gs750 that I just refurbished, and I have 115 main jets in it. I will let you know how that does. 15 Pilots, 5 DL 36 in the fourth position, fuel screws were at just under 1.5 turns out, I put them about 1.35 turns out this time to try it out. Too much on the fuel screws will give you a poor rough idle, given everything else is adjusted such as the valves, good sealing on the intake boots, good synchronization. Too little on the fuel screws and you will always have a stumble off idle.
I'm not going to "argue" about jetting suggestions.
If Chuck says 112.5 mains work for him and 1 position richer on the needles is all you need to compensate for a quality pipe and pods, then that's what he's found.
I don't believe 2 full sizes richer is enough for your mod's. At least I've never heard of it until now. Nor have I ever heard that 1 position richer is enough for your mod's. Every bike I ever jetted or helped jet required jetting twice that more richer. Every article I've ever read about jetting says the same. I have much more experience jetting 1000's than 750's, but the basic rules of thumb still apply to both bikes. Why does Dynojet supply 2 sizes of main jets in their stage 3 kits, sizes 138 and 142? DJ's 138 equals a Mikuni 130. Some bikes require the larger 142 because they may have mod's beyond pipe/pods that need the extra fuel. Anyway, my point is, the most commonly used DJ 138 is the same as a Mikuni 130 and that represents 7 full sizes richer than the stock 95. That surprised me years ago, but it works. On the 1000's, 2 full sizes richer would be severely lean. One position richer on the needles would burn the motor up on anything other than very short trips. Motorcyclist magazine built a custom 1000 back in the early '80's and they first tried to get the stock needles to work but even in the richest position they still had lean issues, so they went with the DJ kit. The 750 and 1000 are not THAT different. They even share the same carbs, just small changes in factory jetting.
I've also read here many times that owners said mains in the 122.5 -130 range worked best. That's what they said. I suggest you read up on jetting. There are lots of sites and you can pick out bits of information that all the sites agree on, or come close. I respect Chuck's input and experience, it just differs from mine. Maybe his suggestions would work for you and mine would be terrible. Jetting isn't that difficult. Do some reading. You'll learn a lot in no time.
If Chuck says 112.5 mains work for him and 1 position richer on the needles is all you need to compensate for a quality pipe and pods, then that's what he's found. I don't believe 2 full sizes richer is enough for your mod's. At least I've never heard of it until now. Nor have I ever heard that 1 position richer is enough for your mod's. Every bike I ever jetted or helped jet required jetting twice that more richer. Every article I've ever read about jetting says the same. I have much more experience jetting 1000's than 750's, but the basic rules of thumb still apply to both bikes. Why does Dynojet supply 2 sizes of main jets in their stage 3 kits, sizes 138 and 142? DJ's 138 equals a Mikuni 130. Some bikes require the larger 142 because they may have mod's beyond pipe/pods that need the extra fuel. Anyway, my point is, the most commonly used DJ 138 is the same as a Mikuni 130 and that represents 7 full sizes richer than the stock 95. That surprised me years ago, but it works. On the 1000's, 2 full sizes richer would be severely lean. One position richer on the needles would burn the motor up on anything other than very short trips. Motorcyclist magazine built a custom 1000 back in the early '80's and they first tried to get the stock needles to work but even in the richest position they still had lean issues, so they went with the DJ kit. The 750 and 1000 are not THAT different. They even share the same carbs, just small changes in factory jetting.
I've also read here many times that owners said mains in the 122.5 -130 range worked best. That's what they said. I suggest you read up on jetting. There are lots of sites and you can pick out bits of information that all the sites agree on, or come close. I respect Chuck's input and experience, it just differs from mine. Maybe his suggestions would work for you and mine would be terrible. Jetting isn't that difficult. Do some reading. You'll learn a lot in no time.
Keith, I am four sizes larger than stock, 102.5 was roughly the stock size but it varied depending on the model year. I am at 112.5. I'm not quite sure how one would go up a half size unless they drilled the jets themselves. Perhaps if the jets were listed as 100, 100.5, 101, 101.5, and might be a little more understandable about how one could refer to them as a half size up. Perhaps you could shed some light on how you describe a numerical jump in 2.5 increments as a half size, and 5 increments as one full size?
102.5 -> 105 -> 107.5 -> 110 -> 112.5, those are the jet sizes between stock and what I am running.
Generally it seemed to me that people always refer to jet sizes as one jet larger, so we may have some terminology confusion on the board here, extending far beyond this thread, as I have never read mention of half sizes being discussed. Aside from shimming a needle 1/2 position. That half position is not available without adding shims from an outside source.
I have tested this jetting many times with a wideband O2 sensor and air fuel ratio gauge, and in the 120's would give extremely rich air fuel ratios on my setups on multiple bikes, wirh 3/4 - full throttle at high rpm's reading in the 10's & low 11's.
Yesterday I did discover that this 78 GS550 did need about 1.5 positions richer on the jet needle, however. And the GS550's I have worked on support your comparisons with other models in that they need to jump from an 80 main up to 102.5, 105, and with a Wiseco 610cc kit, up to 110's... 15 pilots.
So that was 9, 10, and 12 sizes larger than stock. Or possibly the nomenclature or terminology referenced by Kieth, 4.5 sizes, 5 sizes, and 6 sizes larger than stock.
The other peculiar discovery from yesterday was that this really cool old Jardine 2 into 1 exhaust on a 77 GS750 with K&N big oval pods really was borderline on needing 17.5 pilots, as it took 3.5 turns out on the fuel screws to get out of lean misfire territory on the pilots at idle...
One last thing that I should always state here as that at one position richer on the needles, I am generally always referring to the 5DL36 needles on a GS750, as I usually swap out the 5F21 's for 5DL35/5DL36's with pods and pipe. On a Mac baffle, not their performance upgrade baffle but the chambered baffle that comes with the pipes new, I would run the 5F21 's.
I have plug chop photos somewhere on my hard drive or online host, I will dig this up at some point here.
Thank you for your insights on this debated issue.
Chuck
102.5 -> 105 -> 107.5 -> 110 -> 112.5, those are the jet sizes between stock and what I am running.
Generally it seemed to me that people always refer to jet sizes as one jet larger, so we may have some terminology confusion on the board here, extending far beyond this thread, as I have never read mention of half sizes being discussed. Aside from shimming a needle 1/2 position. That half position is not available without adding shims from an outside source.
I have tested this jetting many times with a wideband O2 sensor and air fuel ratio gauge, and in the 120's would give extremely rich air fuel ratios on my setups on multiple bikes, wirh 3/4 - full throttle at high rpm's reading in the 10's & low 11's.
Yesterday I did discover that this 78 GS550 did need about 1.5 positions richer on the jet needle, however. And the GS550's I have worked on support your comparisons with other models in that they need to jump from an 80 main up to 102.5, 105, and with a Wiseco 610cc kit, up to 110's... 15 pilots.
So that was 9, 10, and 12 sizes larger than stock. Or possibly the nomenclature or terminology referenced by Kieth, 4.5 sizes, 5 sizes, and 6 sizes larger than stock.
The other peculiar discovery from yesterday was that this really cool old Jardine 2 into 1 exhaust on a 77 GS750 with K&N big oval pods really was borderline on needing 17.5 pilots, as it took 3.5 turns out on the fuel screws to get out of lean misfire territory on the pilots at idle...
One last thing that I should always state here as that at one position richer on the needles, I am generally always referring to the 5DL36 needles on a GS750, as I usually swap out the 5F21 's for 5DL35/5DL36's with pods and pipe. On a Mac baffle, not their performance upgrade baffle but the chambered baffle that comes with the pipes new, I would run the 5F21 's.
I have plug chop photos somewhere on my hard drive or online host, I will dig this up at some point here.
Thank you for your insights on this debated issue.
Chuck
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Yup, definitely not looking to start an argument about exact final jetting...because the answer to that question seems to be "what works for your bike, with your mods, in your geographic location." There do seem to be some general guidelines, and accepted wisdom around starting points though.
I have read a lot, but in what I read needle jets weren't really talked about since most folks seem to stick with the stock needle jets (even if they use a Dynojet kit and swap to the DJ needles). That's what prompted me to start this thread. Since there are multiple "stock" jetting configurations for the '78 750 and I'm switching the needles from 1 stock set to a different stock set (since I have 4 good needles of that variety and only 3 good needles of the other variety), I was wondering if I should change the needle jets to match. Chuck, the fact that you usually switch from the 5F21's to the 5DL36's tells me I'm on the right track in using the 5DL36 needles...still not sure about the needle jets, but I guess plug chops will let me know about that? Or should I go with O-6 from the start?
You both definitely know what you're talking about when it comes to jetting for mods, and I'm grateful for the advice. Since you're both here, I'd appreciate it if you could both stick around to help out even after answering my initial question. I have no real experience in this area...
According to that I should go up 6 sizes on the mains, and +2 on the pilots. However in most threads I came across on the GSR forum, folks seem to either stick with stock or maybe go up 1 size on the pilots... A lot of the threads had people ending up between 115 and 120 on the mains, so I figured I'd start at 117.5 (which is 6 sizes above stock 102.5). Keith, you've seen folks ending up at 122.5-130 in threads. Makes sense to start richer and work back from there, even if I end up at 117.5 or lower... So I'm starting at 122.5, the largest jets I have on hand. If it's too rich or lean, I'll adjust.
Chuck, in my searching I came across one of your really old threads where you were jetting a heavily modified 750. Reading through that was really educational. The actual jet sizes you ended up with wouldn't necessarily apply to me, since I think you ported the head, and bored out the cylinders...changed out/degreed the cams, etc. I've just added pipes and filters, and added hotter coils. The process you went through definitely applies though. That thread is actually where I learned about the RC-2222 filters, and is why I ended up buying them over individual pods...
I have read a lot, but in what I read needle jets weren't really talked about since most folks seem to stick with the stock needle jets (even if they use a Dynojet kit and swap to the DJ needles). That's what prompted me to start this thread. Since there are multiple "stock" jetting configurations for the '78 750 and I'm switching the needles from 1 stock set to a different stock set (since I have 4 good needles of that variety and only 3 good needles of the other variety), I was wondering if I should change the needle jets to match. Chuck, the fact that you usually switch from the 5F21's to the 5DL36's tells me I'm on the right track in using the 5DL36 needles...still not sure about the needle jets, but I guess plug chops will let me know about that? Or should I go with O-6 from the start?
You both definitely know what you're talking about when it comes to jetting for mods, and I'm grateful for the advice. Since you're both here, I'd appreciate it if you could both stick around to help out even after answering my initial question. I have no real experience in this area...
This is essentially what I've done. I've seen this same rough guideline in a lot of places --->http://motorcycles.wikia.com/wiki/Rejetting
According to that I should go up 6 sizes on the mains, and +2 on the pilots. However in most threads I came across on the GSR forum, folks seem to either stick with stock or maybe go up 1 size on the pilots... A lot of the threads had people ending up between 115 and 120 on the mains, so I figured I'd start at 117.5 (which is 6 sizes above stock 102.5). Keith, you've seen folks ending up at 122.5-130 in threads. Makes sense to start richer and work back from there, even if I end up at 117.5 or lower... So I'm starting at 122.5, the largest jets I have on hand. If it's too rich or lean, I'll adjust.
Chuck, in my searching I came across one of your really old threads where you were jetting a heavily modified 750. Reading through that was really educational. The actual jet sizes you ended up with wouldn't necessarily apply to me, since I think you ported the head, and bored out the cylinders...changed out/degreed the cams, etc. I've just added pipes and filters, and added hotter coils. The process you went through definitely applies though. That thread is actually where I learned about the RC-2222 filters, and is why I ended up buying them over individual pods...
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Chuck, I visited Mikuni Corp in California in 1980 to ask some questions. During my conversation the Mikuni rep' said all of their jets (back then) were in 2.5 increments, with 2.5 and 7.5 representing a "step" and whole numbers as a "full" size. I often heard the term "half size" used instead of a step when referring to Mikuni jets. I've always used the same terminology since. The terminology was common back then.
I've never given it much thought since. Now you bring it up so I did some looking thinking it would be easy to find info on how Mikuni sizes their jets. I found only a couple of sites that mentioned the word "step" and nothing as concrete as my conversation that day.
Sam, all I can say is go with Chuck's advice on the mains since he's worked on 750's much more than me. I have no idea why a very similar motor 3/4 the size of a 1000 with nearly identical carbs would require much leaner main jetting to run correctly, while the other jetting circuits appear to require similar changes. I also have no idea why a 750 comes with larger mains than a 1000 from the factory. I can tell you a typical 1000 needs to go from 95 to 130 mains with identical mods. Why a 750 would require a change of only half that is strange to me.
I've never given it much thought since. Now you bring it up so I did some looking thinking it would be easy to find info on how Mikuni sizes their jets. I found only a couple of sites that mentioned the word "step" and nothing as concrete as my conversation that day.
Sam, all I can say is go with Chuck's advice on the mains since he's worked on 750's much more than me. I have no idea why a very similar motor 3/4 the size of a 1000 with nearly identical carbs would require much leaner main jetting to run correctly, while the other jetting circuits appear to require similar changes. I also have no idea why a 750 comes with larger mains than a 1000 from the factory. I can tell you a typical 1000 needs to go from 95 to 130 mains with identical mods. Why a 750 would require a change of only half that is strange to me.
Sam, I'm off on a vacation for the next week or so, just so you know. Sounds like Chuck can help you out if you need it. Good luck with it!
Interesting read, thanks for clearing that up. I was curious as I've never heard it referenced that way before, but typically only on the number of jets larger, not counting whole numbers vs half steps. Not trying to argue with you, but was just curious where the origins of that reference came from.
Every bike is different... even side by side GS750's of same model year may have noticeably different camshaft timing and engine traits due to widely varying machining tolerances. And old used bikes add a whole lot more variables to this...
The difference in 750 vs 1000 is due a lot to significantly different air jet sizes in carburetors which externally appear identical, camshafts with more valve lift and less opening duration, and different bore and stroke...
I'll keep this in mind for my two 1000 engines! I have two sets of Keihin CR31's, however, which are very likely to end up on those two bikes... And I have several GS 1100 GK Deport top ends. I suppose the jetting on that is a little less commonly known, but advice on a good starting point there would be very appreciated!
