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another jetting question

  • Thread starter Thread starter horror1972
  • Start date Start date
H

horror1972

Guest
Hi,

I just put K&N pods on my gs750 '78.

Wondering who has done this and can tell me what size mainjet to use, how may turns teh air screw, how many turns the pilot fuel screw and what position the needle.

I know there's a search function, but I get really confused whit all the different stories.

I'm sure I'm not the first who put pods on the bike, so the jetting should be pretty similair to my bike.

I tried a 118 main jet and the needle on the 4th clip, but I need to hold the choke on the half of it ,the pilot screw 3 turns out and the air screw 1/4 out, so I guess it's very lean??

I need a good basic setup, and after that I will go to the dynojet testcenter, but I need to get there.. :)
 
horror1972 said:
Hi,

I just put K&N pods on my gs750 '78.

Wondering who has done this and can tell me what size mainjet to use, how may turns teh air screw, how many turns the pilot fuel screw and what position the needle.

<snip>

I need a good basic setup, and after that I will go to the dynojet testcenter, but I need to get there.. :)
Click to expand...

Instead of looking for the final answer, what you should be looking for is the method used to jet a bike after all the changes. This has been posted many times by many people. Your bike should have VM carbs, so you want to find posts by Keith Krause, because he has the VM carbs dialed and is very knowledgable on them.

Given the age of your bike, don't expect your Dyno people to be very helpful. Unless you simply want the raw test data to use for your own tuning work, then it should be OK.

Mark
 
Stage 3 kit from Dyno fixed me up, though I had to go one position lower on the needle clip because it ran too rich.
 
horror1972 said:
I tried a 118 main jet and the needle on the 4th clip, but I need to hold the choke on the half of it ,the pilot screw 3 turns out and the air screw 1/4 out, so I guess it's very lean??

I need a good basic setup, and after that I will go to the dynojet testcenter, but I need to get there.. :)
Click to expand...

Put in 127.5's for the main jets - wide open throttle. Try your needle on the bottom clip if 4 is poppin which is your mid throttle. Try putting your pilot fuel screws at around 1.5. Anywhere from 1.25 - 2 turns out for the pilot fuel screws. For the side air screws try 1.5 - 2 turns out. Play with it :)
That should get ya real close.
 
doctorgonzo said:
Stage 3 kit from Dyno fixed me up, though I had to go one position lower on the needle clip because it ran too rich.
Click to expand...
They dont make Dyno kits for the 750s with VMs. They have a 1000 VM kit, but I have yet to get an answer on how far off thier kit would be for the 750. I have had to hand jet my 750, based off Kieths suggestions, and I have to say he's pretty right on. If you ONLY have pods on it, it will be just a bit different than Coltrains suggestions, as his and mine are based off pods AND a pipe. My suggestion? Needle in the forth clip from the top, try a 122ish range main ( i think its a 122.5 no?) Pilot jets should not need changed from the stock 15s. Air screws out 2, fuel screws out 1. That SHOULD give you a good baseline setting. You still may be a tad lean, maybe have to go up to a 125 main, but you wont know till you can get some plug chops in. If you are still lean on the 122.5, maybe try 1.5 turns out on the air screws. Kieths suggestions may be different, and if so, id be more inclined to go with his.
 
I really wanna try that stage 3 jet kit for othe 1000s on my 750. There has got to be somebody who has tried this. The only thing I would need out of it is the needles. Thats alot of coin for a set of needles. I bet it works.

Ya know there are tiny little slight differences on the carbs for the 750 through 77 - 79. I remember reading that the air jets are different in 77. A couple other things too so we may very slight tiny jetting differences between the 77, 78, 79. I tried my needle on the bottom 5th richest setting with a tiny carb washer or spacer underneath to raise just a tad more and my bike liked it - I have pods and 4 into 1 on a 79GS750. Josh's bike (77GS750-cafekid) sounds like he uses 4th position down. I think that is because our carbs just a tad different.
 
coltrain said:
I really wanna try that stage 3 jet kit for othe 1000s on my 750. There has got to be somebody who has tried this. The only thing I would need out of it is the needles. Thats alot of coin for a set of needles. I bet it works.

Ya know there are tiny little slight differences on the carbs for the 750 through 77 - 79. I remember reading that the air jets are different in 77. A couple other things too so we may very slight tiny jetting differences between the 77, 78, 79. I tried my needle on the bottom 5th richest setting with a tiny carb washer or spacer underneath to raise just a tad more and my bike liked it - I have pods and 4 into 1 on a 79GS750. Josh's bike (77GS750-cafekid) sounds like he uses 4th position down. I think that is because our carbs just a tad different.
Click to expand...
The differences that I KNOW of in our carbs are in the slide arm assemblies. I have not cross referenced the jet/needle sizes, but i think they are the same. Where the 78 and 79 model VM26s have the brass plate and two nylon needle washers at the base of the slide actuator, my 77's needle simple has the clip, and slides up INTO the actuator arm just a tad. Indeed this could be the reason for the slight difference in needle setting as well, but im not entirely sure.
 
moz-screenshot-10.jpg
http://free-pu.htnet.hr/medo/carbs.jpg

There is a diagram there with all the specs through the years for 76-79 GS750. I think the 77's may have a P-1 needle jet. Thats what I might need to get my mid throttle correct. It is according to engine no. and since your running a 15 pilot jet you probably have the later carbs.
 
another jetting question

wow,

a lot of replies,

I ordered 128 mains, 125 were not on stock.
gonna try when they're arrive.

thanks guys!!
 
TheCafeKid said:
Air screws out 2, fuel screws out 1. That SHOULD give you a good baseline setting. You still may be a tad lean, maybe have to go up to a 125 main, but you wont know till you can get some plug chops in. If you are still lean on the 122.5, maybe try 1.5 turns out on the air screws.
Click to expand...

Looking at this pic...
065_General%20carb%20exploded%20view1.jpg

Where is the "Fuel Screw"????? (as I will be putting my carbs back together next week...)
 
Hey those are CV carbs for most models 80 and older. Basscliff's website should help you out. There is a tutorial there. We are dealing with VM carbs which are on the earlier models.
 
Texatl said:
OK- sorry for the interruption 8-[ As you were.
Click to expand...
Tex your carbs dont have a "fuel" screw perse. They have an singular screw for adjustment purposes. Now, I just had this discussion with Steve the other day, as my 1100G has the same type of carbs, and I have never been really knowlegable on them, as i had never really messed with them, but apparently turning the adjustment screw will lean out or richen the overall mixture. the thing is, i cant remember which direction does what. He explained it as this. Know that turning the screw OUT adds more of something, either fuel or air, after that, it will be pretty easy to figure out which one. Some of the other guys might be able to tell us WHICH is added when turning the screw out...
 
another jetting question

Hi guys,

I'm confused :???:
I put 122 in it, as said the fuel and air screw about 1 turn out.
The bike runs, but only with choke fully open.
Tried fuel screw at 1 1/2 turn and air from 1/4 to 2 turns out, without any result.
is it still to lean, or what else can be wrong?
 
horror1972 said:
Hi guys,

I'm confused :???:
I put 122 in it, as said the fuel and air screw about 1 turn out.
The bike runs, but only with choke fully open.
Tried fuel screw at 1 1/2 turn and air from 1/4 to 2 turns out, without any result.
is it still to lean, or what else can be wrong?
Click to expand...
Opening the throttle bypasses the choke circuit, though there is some effect still.
I'm assuming by "the bike runs but only with choke" that you mean at idle or lower rpm's/smaller throttle openings? If you need choke to prevent stalling, then your pilot circuit is probably dirty/clogged. This also assumes the bike is otherwise tuned right which must be done first before you can accurately re-jet.
The valve clearances must be set between .03mm and .08mm, the ignition timing must be spot on, (if points) the points dwell set and the points cleaned, all electrical connections good and a solid blue spark, no intakes leaks, no exhaust leaks, CLEAN carbs with fresh inner o-rings and float levels set, clean pods/oiled properly if they take oil, etc. Then you can bench synch the carbs, re-jet, and then vacuum tool synch, then test. Basics.
Pods with a stock exhaust are a poor flow match in my opinion. Makes re-jetting difficult sometimes. You may have issues (hopefully minor) that can't be removed.
Some good advice given to you already. I'd probably try 3 to 4 full sizes up (15-20) on the mains for the pods. I believe your stock mains are 102.5 so I'd try 120's. Your stock jet needle e-clip position is #3, right in the middle of the stock 5 position jet needle. I think 2 positions richer (bottom position/5th from the top) is too rich. Either position 4 from the top or position "4 1/2" should work. If needed, position 4 1/2 is achieved by placing the e-clip in position 5 and placing an approx' .022" thick spacer directly on top the e-clip. Hard to say which position to try first, unless you go with the basic rule of always jet richer and work your way back. If K&N pods, I'd probably try position 4 1/2. If cheaper pods, I'd try position 4. Only solid 1/3 to 1/2 throttle tests will tell.
Remember, the thicker factory plastic jet needle spacer goes on top the e-clip and the thinner goes under the clip.
Also, the VM carbs MUST be vacuum synched everytime the jet needles/slides are disturbed. If you can't vacuum tool synch, at least bench synch very carefully. An unknown/poor synch will add to frustration and you may get uneven plug reads and less performance.
Try to make the stock 15 pilot jets work. Your pilot fuel screws underneath may have been moved in the past but try 1 1/4 to 1 1/2 turns out initially. Always set the side air screws using the highest rpm method.
 
another jetting question

Tried everything.
first re-adjusted yhe ignition with a strobe and bench synch teh carbs.
out the needle on the 4th and 5th pos.
tried a 122 and a 128 main jet, tried with both setups the pilot on 1/2, 1 , and 1 1/2, still no effect:
bike runs fine at idle, but when i open the trottle a little bit, it stops accelerating and even almost dies.

when the engine is hot, and I put the choke at the middle, it runs a lot better, even respons to trottle.

what the he** is wrong???
 
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