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GS750 Jetting

mcycle-nut

mcycle-nut

Forum Mentor
Past Site Supporter
Charter Member
First of all, I have a 2-valve GS750 bored .5 mm oversize with a Jardine 4 into one exhaust, cheap pods, and Boyer Brandsen electronic ignition. Head has just been rebuilt save for new valve guides, (they were good). Carbs are hospital-like clean. After reading up jetting, I've set it up as follows;

Pilot fuel screws - 1 1/2 turns
Side air screws - highest rpm
Jet needle 5DL36 - clip at bottom, no spacers
vent lines - removed
Mains - 112.5
Carbs were bench synched (pretty close!) and with vacumn gauges

Results were;
1/3 throttle - sooty black
4th gear at 35 mph - sooty black
Full throttle - lean

My thoughts;
115 mains and replace the cheap pods with K&Ns.

Any other thoughts? :-k
 
Try 1 turn out on the pilot fuel screws (underneath). Since your minimal throttle test shows rich at 1 1/2 turns, try going to 1 turn. If you get good reads/performance at that point, good. If you're lean at 1 turn, then you know the sweet spot is between 1 and 1 1/2. Be sure the side air screws are set using the highest rpm method after this adjustment. Be careful they are set correctly.
If 1/3 throttle shows rich, then the jet needles will have to be lowered. Usually, with pods/pipe, the stock jet needles will only work in the richest position, IF, the factory e-clip position was #3 to begin with. I've seen 750 info that says the factory position is #3 while other info says #2. One position difference matters quite a bit. You're showing rich so...
I'd place the clip in the 4th position and re-test. I hesitate here a bit because you say the plugs are sooty black at 1/3 throttle. I can't see the plugs but many times one position leaner isn't enough for sooty plugs, but, my past experiences show that I've never seen a '77 to '79 750 with your mods run well with the jet needles e-clips any "leaner" than the 4th position. It's hard to believe your bike would require the e-clips be in the 3rd position, even if they may have been in the 2nd position from the factory unless there's some other factor involved, such as worn parts or weak spark. Bottom line, I'd try the 4th position from the top. Keep in mind your VM carbs MUST be re-synched every time you change the needle positions.
As for the mains, your stock ones were 102.5. Even with cheaper pods I think 122.5 or 125 is a close guess. 112.5 isn't nearly enough, as you've seen. I've seen 750's run 130's even but I'd guess the 122.5 or 125's in this case. Again, I can't see HOW lean the plugs were at your full throttle test.
Hope this helps.
 
Thanks Keith. I was hoping you would offer your input. :-D
 
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I just hope it's good advice.:)
Not easy jetting over the internet but I give it a try.

By the way, something I just thought about that may/may not matter.
When adjusting the jet needle e-clips, make sure you re-installed the two factory plastic spacers in factory order. The thicker spacer (ring) goes on top the e-clip and the thinner spacer under the clip. Many people reverse these by mistake and others actually leave them out for whatever reason, such as when using jet kits. Either way, you'll run much richer than you intended if you install them in any way other than factory order.
 
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