'81 GS750E OEM carb jetting/carb questions

[ToddStyles]

I've got an '81 GS750E that I'm trying to get running. The current carbs off an '83 GS750 but I'm not sure what exact model. (The CV slide caps have a black crinkle finish.) The bike came with K&N pods and I don't have the original air box. I saw all that to say, I'm not sure what the OEM jetting should be so I'm not sure where I need to be to compensate for the pods. The carbs that were orignally on the bike had 115 mains, but my carb kit had 112 mains. The '83 carbs actually 125 mains and that is what I used when I rebuilt the carbs. The bike idles great, starts on the first hit even without the choke. But... the bike falls on it's face around 5-6K and won't rev past that on the road. Idle and just off idle throttle position seems responsive. Sitting on the center stand, it will rev past that but I've only hit up to 7-8K. The plugs are very black/sooty which leads me to believe I'm running too rich, but the bike doesn't appear to be smoking.

I suppose my next move will be to replace the 125 mains with the 115 and try that. I know the carbs need to be synced, I just don't any sync tool yet.

Any suggestions? Am I headed in the right direction with the main jets?
thanks.

[DimitriT]

The main jet comes into play near WOT. At 5-6k RPM you are running on the idle jet and the needle level/jet. It sounds to me like you need to raise that needle if you are running pods. Sooty plugs on the center stand may mean your idle mixture is too rich, or your idle jet is too big or your float level is too high.

[ToddStyles]

RE: '81 GS750E carb jetting/carb questions

The main jet comes into play near WOT. At 5-6k RPM you are running on the idle jet and the needle level/jet. It sounds to me like you need to raise that needle if you are running pods. Sooty plugs on the center stand may mean your idle mixture is too rich, or your idle jet is too big or your float level is too high.

Thanks for the tips. I believe I can use some small washers to raise the needle. That will be easy to try. The float level should be okay. I measured them with my calipers during the reassembly and made sure they were in spec. I have the idle mixture screws about 2 turns out. I can try screwing those in a turn. If that doesn't help, I have a smaller set of idle jets I can try.

Regarding the sooty plugs... it didn't matter if the bike had been running on the stand or I just shut it down after running it around the block. Would you still suggest it's a rich idle circuit and that I need to raise the needle?
thanks!

[DimitriT]

Regarding the sooty plugs... it didn't matter if the bike had been running on the stand or I just shut it down after running it around the block. Would you still suggest it's a rich idle circuit and that I need to raise the needle?
thanks!

It's all about throttle position (not RPMs). The needle doesn't contribute until about 1/4 throttle. Up to that point you are on the idle jet. I suggest you start with getting a good mixture on the idle jet (0 to 1/4 throttle). Once that is satisfactory you can then look at eliminating flat spots caused by lean mixture from about 1/4 to 3/4 throttle: the needle. Once your happy with that you can look at WOT throttle: main jet. You can do plug chops at different throttle settings to try and determine your mixture at the higher throttle settings by looking at the plugs - your just want to make sure you're not too lean above 3/4 throttle since you can damage your motor.

Personally I would start looking for an airbox. The whole system was really designed with a specific airbox in place and it is very tricky to get it tuned (as a street bike) right without one. You may get lucky and stumble on a combination of jets and settings which work for you.

[ToddStyles]

Thanks for the insight. I have several sets of jets so I'm going to try and get this dialed in without spending any more $$. If I can't, I'll just fall back and punt... if I can find an OEM airbox setup. I've been looking and haven't had much luck.

[ToddStyles]

I pulled the carbs off last night and made a small discovery. I used the slide and jet needle from my '81 carbs and the needle jet from my '83 carbs. I measured the '81 needle itself and it has a larger diameter than the '83 version. If I put the smaller diameter jet needle back in, that should allow more fuel... correct? If so, I'm going to try that before I attempt to raise the needle.
Todd.