1977 Gs400 rejetting

[West]

Gentlefolk,

Please bear wiht me, I know rejetting is a perennial issue, but I beg your patience for one more iteration. I have a GS400, it's running pretty nicely. The air filter element was rotted out when I started rebuilding the bike, and right now I have about 5 layers of air-conditioner filter foam stuck in the filter holder.

Now, a new element seems to be abvout $20, so I figgered that if I had to spend $20, I would spend $25 & get pod filters that are cleanable, and cheaper & easier in the long run. (This is before I read about GS's and pod filters here.)

The other day I took the cover off the air filter box and was aghast at the change in the engine's performance! The whole nature of the beastie changes, from a flat midrange to heavy missing at mid-high throttle & RPM's.

Now, on to the gist.

IF I mount these filters (The Suzuki oval (NOT round) chrome pod filters. supposedly with an internal velocity stack), I will have to rejet. I know this. All I am looking for is a starting place, so I do not have to buy an enormous pile of jets to experiment with. I just want to narrow down the range I am targeting to something reasonable.

The major change in the bike's personality indicates to me that I am dealing with a slightly more dramatic change than just 1 or 2 jet sizes. Can anybody give me an idea of where to start?

The stock jets are 112.5's, increments are 2.5.

I was thinking of buying some 115's and 120' to start. Is this a good idea, or should I go with 120's and 130's? Or 130 and 140?

Stock pipes, everything else stock. Carbs completely rebuilt, operating perfectly. Air screws set with Colortune (although I was pretty close btyear), Plugs running clean, lightbrown/grey at full throttle in stock config.

Since observing the major change in the bike without the cover , I am approaching this with a lot more trepidation, I want to have my ducks in a row before i start mucking with things. I am also going to keep all the stock parts, unmodified, so I can always go back to a known good configuration.

Any help would be appreciated & thanks for your forbearance. FYI, I have scanned the forums for info on this & have come up blank on the specific info I am looking for. Also, I am only interested in the MAIN jetting right now, I'll zero in on needle jet. etc, after I am happy with the mains.

[ptm]

I think you'd be better off if you stay with the stock air filter. You'll spend as much on the pods and all the jets for trial and error tuning - plus a lot of time...

Rather than buying replacement foam from Suzuki, go to any auto or motorcycle parts store and ask for the bulk green air filter foam from Uni (Uni part number BF-1). A sheet costs about $15 and is big enough to cut out 2 or 3 air filters for the GS400. Each one will last for many years. You can also get the UNI filter oil and cleaner, but regular motor oil will do fine if you're not in a desert or other extremely dusty place. If you are, just clean it more often.

If you're really set on the pods, I think there was one other thread here recently about rejetting a 400 for pods and an aftermarket exhaust. Do a search. It's not done very often, so it might be hard to get much advice about it.

Good luck.

[West]

Thanks for the response, Paul.

The pod filters are on the way, so not much I can do about that. $15 will buy me about 5 jets - that gives me two tries, and I have some 145's (probably waay too big) from the used carbs I bought to obtain a replacement diaphragm.

I'm interested in giving it a try, some time spent mucking around on rainy days won't cost me anything. My current plan is to try the 145's first - see just how overrich they are (should be! That's a 29% increase!). Then I will buy a NEW set of 112.5's and drill out the original 112.5's to increasing diameters between the 112.5's and the 145's - I have a set of fine jobber drill bits, and there are 6 drill sizes between the 112.5's and the 145's. Then, I can purchase a range of jets in the area that my drilling tells me is in the ball bark.

I figure this will take me 4 main jet sets, about $24.

All that extra air has me intrigued, now. 'cause you can mix fuel with all that air and get more power, my guess at this point, a fair amount of power.

I also have some Y-2 needle jets - that's 2 sizes bigger than the Y-0's that are stock on the 400 - I think they may be just what I need.

I have the original needles, 4F23's and the ones that came with the Y-2 needle jets, which are 4H11's.

That gives me a fair amount to play with, and I will always be able to go back to stock. I am fair up on carb theory, got my originals running fine, so now it's time to break things and learn even more.

P.S. I found the other thread earlier - no help. No one will commit to a recommendation - who the hell hops up a GS400? And there are no jet kits for it! However, if I get the bike running right with the pods, I will post the info here.

[ptm]

One thing that might help you get the right setup sooner is that the 400 is basically the middle two cylinders of the 750. You may be able to learn something (at least a starting point) from the recommendations for rejetting a 750 with stock pipe and pods.

The problem is that the 77-79 750 used the VM slide carbs, and the 400 used the BS constant velocity diaphram carbs from the start. The 750 didn't use them until 1980 when it went to the 16 valve setup, so they may be too different - just a thought...

[West]

Thanks again. The problem with the GS750 is that they DO make carb kits for them, and I have not yet found anyone who has done it using stock Mikuni jets. The jet kit manufacturers jealously guard any info on what sizes they provide with their kits, too (not that I blame them). The only hint I have found so far (it was here) is a GS750, pods and exhaust mods, using a main 137.5. If that's the case, maybe my 145's are not that far off after all!

The style of carb certainly would matter for the fine points, but what I am looking for is a percentage change for ballpark, so I think that any info along those lines would help. I'll keep my eyes out for more.

[West]

Here is what I gleaned from the forums on GS750's:

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GS750 2V -> Increase the main jet to a 115. Pilot Jet should be increased to a 17.5 and the needle may need to be raised a notch (lower clip).
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Stock jet on the 2-valve is 100 or 102.5 depending on the country, with pods you go to a 115, mine had 116's. Yours has about that jet to begin with!
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I have a 77 and it is not bored out, but run velocity stacks and open header. I went from stock (97 ?) to 122's. I'm working on my boss's 1977 GS750. It has an after market free flowing 4 into 2 slash cut

exhaust. I don't know who the manufacturer is. His air box is in rough shape, so he has decided to go to a set of pod filters. I need to know the details on doing a rejet on this particular model with these upgrades.
According to the parts fiche, the stock main jet is a 100.

reply - if you are determined go up at least 4 sizes & raise the needles 2-3 grouves if they are adjustable

reply - I read some jetting mods a guy did to his 750. said he out 120 mains and put the needle to the lowest position. im gonna try it when i get home next week.

reply - Keith - 4 full sizes up on the mains..120 Mikuni.
I would raise the jet needles to their highest position (e-clip in the 5th/bottom groove).
I would try to make the stock 15 pilot jet work, along with richer pilot fuel screw (underneath) adjustment. An additional 1/2 turn out from where they were (if it was running well before) is a good starting point. Probably will be about 1 1/4 turns out?
Then adjust the side air screws using the highest rpm method. Usually about 1 3/4 turns out. Remove the two floatbowl vent lines and leave their ports open to breath.

*reply from original poster - I went with Keith's recomendations. 120 mains, stock needle in the highest position. I used the stock pilot jets, but had to tweak the mixture screws individually a few times before it ran right.
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I have learned to listen to Keith's recommendations. Since I am starting with 112.5's, I'm guessing that 125's or 130's are the new target, and the needle will go to the lowest notch.

Going rich first is safer, so I think 130's will be a good starting place. I am still going to try the 'drill out' method to confirm this range.

[West]

For anyone who's interested, here is an update with some rough calculations I made.

The original 112.5 jet has a bore diameter of .045"
The 145 jet has a bore diameter of .060"

The difference is 32.5 jet numbers, 15 thousandths, so 2.17 jets/thousandth

By drill bit collection yields the following (rough) equivalencies.

bit .046, 1*2.17=2.17 jet sizes, brings it to 114.67
bit .052, 7*2.17=15.19 jet sizes, brings it to 127.69 jet size
bit .055 10*2.17=21.7 jet sizes, brings it to 134.2 jet size
bit .058 13*2.17=28.21 jet sizes brings it to 140.71
bit .059 14*2.17=30.38 jet sizes brings it to 142.88

I don't have the .055 bit yet, on order.

I have fabbed a 'jet removal tool', so I can get the main jets out without dismounting the carbs. Drop out the bowl plugs, jets out, jets in, replace plug and test.

I have a large half-mile hill near my house, if I set the bike at 55MPH in 6th, it currently just exactly holds it's own at full throttle with the factory jets and airbox. If I get any performance improvement, it should show as increasing speed on that hill. That's my 'dyno'.

Once I get the bike running smooth, I continue to bore the jets out until performance levels out or drops off on the 'dyno'. I will also utilize the Colortune to evaluate mixture, but that will only be on the stand, not on the road.

Then I order 2 or 3 sets of jets in the range indicated by the bore out testing & retest for power & mixture, using 'real' jets.

Then I will work on the midrange, if it needs it. Last time I took the airbox cover off, the bike idled fine and seemed to transition to rolling pretty well, so I am hoping I will not have to mess with the pilot jets (currently 20's.)

I think I am going to end up with the, .052 or .055 bore working the best, but that's why I am testing, ain't it?

That's the plan. If anyone wants to shoot holes in it, I am all ears.

[West]

Update. I realized that I actually had 110 mains, with the same .045" holes in them, which changes the calculations a bit

35 jet numbers, 15 thousandths, 2.33 jets/thousandth

bit .046, 1*2.33=2.33 jet sizes, brings it to 112.33
bit .049, 4*2.33=9.2 jet sizes brings it to 119.2
bit .052, 7*2.33=16.31 jet sizes, brings it to 126.31
bit .055 10*2.33=23.3 jet sizes, brings it to 133.3
bit .058 13*2.33=30.29 jet sizes brings it to 140.29
bit .059 14*2.33=32.62 jet sizes brings it to 142.6

Jet needles available

max dia dia at tip length
4h11 .1182 .079 21.4 mm

4f23` .090 23.6mm


First, I gave it a good run in the stock config & paid extra attention to how it ran. It's slow off the line, weak until about 4500 RPM, then it takes off pretty nicely, a decent, but not super strong pull until around 75 in 6th, when accelaration tapers off a bit, RPM about 6500. It'll keep going until around 85 or 90, when accelleration really starts tapering off, mostly due to wind resistance. Power band is 7000-9000 (redline 9000), very noticable dropoff on both ends.

Then, I took a drive without the airbox cover. It idled fine, but tended to stall, something it had not been prone to before, at all. The low end was pretty much the same, then it missed a lot at mid throttle, and at full throttle/higher speed it was really weak, but not missing. The power band is also lower, ending at about 8500.

So, I drilled out the 110 jets to 112.3 & tried again. Still missing at midrange, not as bad as before. Missing got better as engine warmed up.

Jets drilled to 119.3. Missing gone at midrange, top end about the same as stock.

I am not going any bigger on the jets until the pod filters get here (Old Bike Barn said 'in stock', which apparently doesn't mean 'in stock', but 'we can get them for you, which is going to take a while'), so I decided to play aound with the needles.

I raised the clip to the top position on the needles, (leaning it out). When I test drove it ,it was once more missing at midrange.

SO, I lowered them to the lowest position (richening the mixture). Missing gone, Not much performance change over the mid position.

I swapped the needles with the ones from my parts carbs, a 4H11 needle instead of the stock 4f23's. The 4H11's are thiner at the same depth as the end of the 4F23's, meaning a richer mixture. They are also a couple of mm longer. The taper starts at about the same depth on both, and the non-tapered diameter is the same. The result was a pretty good ride, power satisfactory, perhaps a tad better than stock, but the low end is still weak.

Next step was to swap out the needle jets, Y-0's with the slightly bigger Y-2's from the spare carbs. This should richen up the low end, since with the needle all the way down, the orifice is not on the taper yet, so the determining factor would not be the needle taper, but the needle jet annular size. Test drive tomorrow.

[West]

Well. THAT was a mistake. This morning the bike would hardly run. Wouldn't idle, and on the stand it was almost impossible to get the revs over 2K. I did get it there a couple of times and in the higher revs it was OK. It got worse and worse, and finally would hardly run at all. I pulled a plug - covered with carbon. Tried leaning out the air jets, only a slight improvement.

Pulled the float bowls (I am getting good at that), reinstalled the original Y-0 jets, and he fired right up & runs very nicely.

So, I learned something out of this: don't mess with the pilot jets. My guess is that pilot jet size is more determined by the displacement of the engine than tuning requirements.

Those pilots were from a 750 triple - 25% more displacement per carb than my 400.

So I fixed it with DDT.

Now I will have to wait for the pods to come in for final adjustments. I plan to do an open throttle plug chop to see if I can tell whether the current jetting is too rich or too lean before then.

I hope no one minds me putting my fairly tedious blow-by-blow story up here, but I find it helps me keep track of what I am doing and keep my thinking juices flowing.

[KEITH KRAUSE]

Wow! I just saw this thread and it may be just because I'm tired and about to sign off, but it seems like you're complicating things.
I don't understand why you're re-jetting before you have the new filters installed, unless you're just playing with it. I don't quite follow your plug test method either.
I'm also not a fan of running a stock exhaust with pod filters. They contradict each other and funny things happen to performance sometimes.
I haven't jetted a 400 but I'll take a poke at it since you asked for suggestions. Just guessing of course. Don't know your elevation either.
All basic tuning must be done first. Valve clearances, compression, spark/timing, fuel flow, clean carbs, floats adjusted, mixture screws set by highest rpm method, vacuum synch, bowl venting, no intake leaks, etc, must all be checked first or your re-jet will be more difficult.
If your stock mains are 110's, I'd try 130's.
Raise the jet needles two positions richer.
Go up 2.5 on the pilot jets in addition to possible richer mixture screw adjustments.
Go up a size on the pilot air jets.
Keep the needle jets stock.
Remove any float bowl vent lines and leave the ports open to breath.
If you have throttle response issues, you'll also have to enlarge the vacuum passage to the diaphragm chambers and possibly lighter piston assembly springs too.
Warm up the bike, adjust mixture screws for highest idle, then vacuum synch.
Test/get plug reads by chopping off at full, 1/3 and minimum throttle positions, to see what the mains, jet needles and pilots are doing.

[West]

Wow! I just saw this thread and it may be just because I'm tired and about to sign off, but it seems like you're complicating things.
I don't understand why you're re-jetting before you have the new filters installed, unless you're just playing with it. I don't quite follow your plug test method either.
I'm also not a fan of running a stock exhaust with pod filters. They contradict each other and funny things happen to performance sometimes.
I haven't jetted a 400 but I'll take a poke at it since you asked for suggestions. Just guessing of course. Don't know your elevation either.
All basic tuning must be done first. Valve clearances, compression, spark/timing, fuel flow, clean carbs, floats adjusted, mixture screws set by highest rpm method, vacuum synch, bowl venting, no intake leaks, etc, must all be checked first or your re-jet will be more difficult.
If your stock mains are 110's, I'd try 130's.
Raise the jet needles two positions richer.
Go up 2.5 on the pilot jets in addition to possible richer mixture screw adjustments.
Go up a size on the pilot air jets.
Keep the needle jets stock.
Remove any float bowl vent lines and leave the ports open to breath.
If you have throttle response issues, you'll also have to enlarge the vacuum passage to the diaphragm chambers and possibly lighter piston assembly springs too.
Warm up the bike, adjust mixture screws for highest idle, then vacuum synch.
Test/get plug reads by chopping off at full, 1/3 and minimum throttle positions, to see what the mains, jet needles and pilots are doing.

Thank you very much for the advice, Keith. First, all of the pre-tuning maintenance you suggest had been done before I started, without exception.

Second, yeah, I am just playing with it right now, with the air box cover off, which seems to be the major impediment to airflow in the system. I wanted to get a better idea of the dynamics of the problem and whether it 'felt' tractable with the steps I was considering taking. So far it does. I plan to continue this process (currently on hold, as I have done as much as I can without the pods) when the pods are mounted.

That last 'plug test' wasn't a test - I just pulled the plug and it was so sooty it would have fouled pretty quickly. Don't need to do a plug chop when it's that far off. So as to your advice to keep the stock needle jets - I figgered that out, the hard way.

All that said, here is where I stand. I am currently running a 119 (approx) main, the slimmer 4H11 needles with the clips in the lowest (richest) position, the stock needle jets and pilot jets. Bike starts well, runs well, slightly better than stock, occasional missing in mid/high throttle range. I did a open throttle plug chop today, ran it full out uphill with open throttle for 3/4 mile, killed it & pulled over. No sign or sound of preignition, but maybe if I had run it like that for a while....The plugs when I pulled them were actually smoking, and they are definitely running lean at FT, the electrodes were burnt clean to the metal and no deposits were visible at all. There were also little trails of smoke coming out of both pipes - both that and the smoking plugs I attribute to burning off the extra carbon that was deposited earlier when it was running so overrich. Later when I get around to doing fine tuning I will do plug chops with new plugs, of course.

I am doing the tuning with the pods only mainly because of budgetary constraints. I bought them because they cost little more than a new air filter element, and at the time I was unaware of the significant changes they would make in the machine's performance. Now that I know, I am dealing with it. The other factor is that it seems, particularly on the GS series, is that the nature of the airflow is changed much more radically by the upstream configuration on these bikes than downstream. While I would like to put a MAC 2 into 1 on the bike (The only pipes available for the GS400), right now it's $184 that I don't have - it'll just have to wait.

So far, I think it's working out pretty well. Based upon the 119's still running lean, your estimate of 130's looks to me to be spot on (of course). The stock pilot seems to be OK, but going a bit richer on the pilot to see how that works out is definitely in the plan. These carbs don't have a pilot air jet. I am setting the air screws mostly by highest rev, I also have a colortune available, but I am trying to not overuse it, and I have pretty good ear anyway. I picked that skill up when I learned to tune my twin SU's on my old Volvo 1800.

I am not running the machine much right now , just some tootin' around town, a long road trip in this config would probably eat a piston.

As of this writing, the project is in abeyance, I think I have learned as much as I can without installing the pods. I should get them next week, and I will post an update.

Thank you once more for taking the time to respond with your valuable input.

[KEITH KRAUSE]

OK. Good luck with it.
Just to add a few thoughts, based on your last reply.
Be sure the bike is completely warmed up before testing.
All bikes air intake is effected greatly by installing pods. The exhaust SHOULD be changed to a freer flowing one. What comes in must come out. It's always best to match them. Sure, you can jet for just pods, but the stock exhaust WILL limit the power gain potential of the pods. This combo can make jetting difficult.
1/3 throttle test to see how the jet needles are doing. Marking your throttle housing and grip is best. If it was my bike, I'd give the stock jet needles a chance before swapping. But since the others are in already, you could run a quick 1/3 test to see if they can be a good fit.
I assume the plugs are standard NGK's. Should be. Gapped correctly.
The CV carbs have pilot air jets. Also primary air jets. Some are screwed in, some are pressed in. CV carb jet kits commonly include larger pilot air jets. You should see these jets at the filter side, alongside the carb throat. Your pilot air jet should be the one on the right (?). Shoot some air or cleaner into it and you'll see it come out the pilot jet or mixture screw passage (screw removed).
Be sure when tuning by the highest rpm method that you use the factory recommended idle rpm. Not any higher.
If the mixture screws end up less than 3/4 turn out or more than 2 turns out with stock pilot jets, you most likely need to replace the jets.

[West]

OK. Good luck with it.
Just to add a few thoughts, based on your last reply.
Be sure the bike is completely warmed up before testing.
All bikes air intake is effected greatly by installing pods. The exhaust SHOULD be changed to a freer flowing one. What comes in must come out. It's always best to match them. Sure, you can jet for just pods, but the stock exhaust WILL limit the power gain potential of the pods. This combo can make jetting difficult.
1/3 throttle test to see how the jet needles are doing. Marking your throttle housing and grip is best. If it was my bike, I'd give the stock jet needles a chance before swapping. But since the others are in already, you could run a quick 1/3 test to see if they can be a good fit.
I assume the plugs are standard NGK's. Should be. Gapped correctly.
The CV carbs have pilot air jets. Also primary air jets. Some are screwed in, some are pressed in. CV carb jet kits commonly include larger pilot air jets. You should see these jets at the filter side, alongside the carb throat. Your pilot air jet should be the one on the right (?). Shoot some air or cleaner into it and you'll see it come out the pilot jet or mixture screw passage (screw removed).
Be sure when tuning by the highest rpm method that you use the factory recommended idle rpm. Not any higher.
If the mixture screws end up less than 3/4 turn out or more than 2 turns out with stock pilot jets, you most likely need to replace the jets.

Got it. I know I am taking chances with not replacing the exhaust, but c'est la vie. Besides, you learn more when you screw things up. If there was a factory jet kit for this bike, I could have bought it, dropped it in & learned nothing. Where's the fun in that?

Warm up before testing. Sometimes the warm up ride (or inability to do so) IS the testing....

Throttle grips are marked.

I plan to test with the stock needles once the pods are fitted, I was just getting a feel for how the various components I had on hand affected things.

I should have said that these carbs do not have replacable pilot air jets. I know where they are. They have been cleaned out like the rest of the carbs. The carbs are Mikuni BS34's.

Factory recommended idle RPM. Factory is 11-1200, I usually set it 10 1K for running, but use 1100 when tuning,

Mixture screws are currently right at 2 turns.

Thanks, Keith.

[KEITH KRAUSE]

If there was a factory jet kit for this bike, I could have bought it, dropped it in & learned nothing. Where's the fun in that?

Well, you would be out riding. That's fun.

Again, good luck with it. Jusy be sure the bike is fully warmed up before testing. No point doing it any other way. I go a good 5/6 miles/10 minutes minimum before testing. Works for me.

[West]

Well, you would be out riding. That's fun.

Again, good luck with it. Jusy be sure the bike is fully warmed up before testing. No point doing it any other way. I go a good 5/6 miles/10 minutes minimum before testing. Works for me.

I make a good crack & some wise guy has to point out the obvious.:?

I will take your advice to heart. As a matter of fact, I took the bike out this evening & did notice a change in performance that only showed up after a good 10 minutes of riding.

I do love to ride, but the edge is taken off somewhat for me if I feel that the machine is not running as well as it could be. You know, there's that special feel to a machine when it's giving you all it can. That's what I am looking for from Gustav. He's a little bike, so maybe what I am shooting for is for him to run like a raped chimpanzee.

(edit) BTW, I have two slides & diaphragm's, breather holes drilled out, ready to drop in as part of the testing.(/edit)