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77 GS750 Main Jet Questions

gbw

gbw

Forum Mentor
Past Site Supporter
Bought my bike used a few years ago with K&N pods and stock exhaust. It has 102 (or maybe 102.5...I just can't remember even though I was just looking at them a few days ago...getting old sucks!) main jets in it.

Then I put the stock airbox back on. I see in the manual that I downloaded from BassCliff's site that stock main jet for the 77 GS750B is supposed to be 100.

1st question: I've searched the forum and read many threads and have seen difference references for the stock main jet size. Can anyone confirm that 100 is in fact the correct stock main jet size?

The reason I ask this question is that a few years ago I bought a carb rebuild kit from Z1 and it came with 102 (or 102.5) main jets in it as well. As a result I have two sets of 102 main jets. That just seems weird to me.

2nd question: Would running 102 (or 102.5 if that what is it) cause the bike to run noticeably rich?

3rd question: Should I buy and install 100 main jets?

Its been a couple of years since I've really ridden this bike but I'm trying to get it back on the road. I seem to recall it runs rich and fouls plugs but I thought that was due to poor spark and I spent a lot of time troubleshooting the electrical system. I'm wondering if having 102/102.5 instead of 100 main jets could be causing or adding to my issues.

Really appreciate any help. The 100 main jets aren't expensive (~$6 each) but I'll have to order them and wait for them to arrive before I can move on to the next step of getting this bike back on the road.

Thanks for all your help!
Greg
 
102.5 may be correct for the 77. It had slightly different things in the carbs. (needle wasn't spring loaded, etc) if you're going back to the box, I'd stick with those for now.
Fuel screw setting needs to be checked as does the air screw. Where are they set? Float setting? (also different from later models)

Are you still using P&C ignition? If so, get rid of that contraption. Dyna S will make life soooooo much easier.
 
Thanks Josh!

Actually, the bike came with a Martek electronic ignition. I have my questions about Martek since they have been out of business a long time. But it seems to be working pretty well.

In troubleshooting last year Wazz sent me a stock P/C ignition and got the exact same results with the P/C as the Martek.

The only thing that really bugs me about the Martek is it doesn't seem to allow the mechanical advance to work. Its just a tight fit and the advancer does not turn like it does with the P/C ignition. Maybe the Martek doesn't use the mechanical advance. I'm just not sure. At some point I'll put a timing light on it and see if it advances.

This is also something that I wonder how much this is adding to my issues.

Of course my biggest issue as I recall was charging system related. I'll start a separate thread on that once I get the bike started and running again.
 
100 was stock but if your going to use a k&n filter in the stock box the 102.5 jets will be fine, they may be anyway. definetly set your floats to 26mm as any less and you will see fuel seepage around your float bowls, and have trouble getting your pilot circuit tuned (ask me how I know).
 
depending on the year, 100 and 102.5 were both stock sizes. It's a pretty minimal difference, to be honest, and was changed for emissions purposes for 1978 and 1979.
 
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