750 '77 carb main and pilot jet size question

[tmaenner]

Hi everybody,

I've got a '77 GS750, Mikuni carbs and need to re-jet the carbs.
The former owner took off the airbox w/o rejetting the carbs and of course with after-market cone air filters performance sucks...

Currently, the jets are 22.5 for pilot and 117.5 for the mains.
Based on some peoples' advice I was targetting two sizes bigger on the mains and one size bigger on the pilots.

What would you recommend?

Thanks a lot! Appreciate your help!

[SqDancerLynn1]

I believe stock jet was 105 so they have done some work. Have you taken a plug reading??

[tmaenner]

Thanks for your reply.
The plugs were pretty black.
I was also told that the carbs are a non-CA edition; maybe they have different stock sizes?!?

Thanks again

[Tarbash 27]

MY 79 GS 750 CAME WITH 102.5 MAIN STOCK. I PUT UNI PODS ON WITH STOCK EXHAUST. WENT TO 115 MAIN JET AND RAISED NEEDLE TO 4TH POSITION. SO IT DEFINATELY SEEMS LIKE YOURS HAS BEEN REJETTED. DO YOU HAVE THE STOCK EXHAUST? TRY GETTING SOME SMALLER JETS. THERE CHEAP.

[KEITH KRAUSE]

Just the "cone" filters, but stock exhaust?
Depends on filter brand. K&N...Emgo?
Good chance the stock #15 pilot jets will work fine with just richening the pilot fuel screws some (from where the factory would have set them). Try 1 to 1 1/2 turns out for starters. Then adjust the side air screws by the highest rpm method. The air screws effect all three jetting circuits.
If Emgo pods, try the jet needle e-clip in the 4th position (2nd from bottom). If K&N, try the clip in the bottom position with a .022" (or close to) thick jetting spacer on top the e-clip. Look at a diagram for your specific model/year carbs and make sure any plastic spacers that go above or under the e-clip are in place and installed in correct order.
Just as a guess I'd say 115 mains, but 117.5 is certainly ballpark.
REMOVE the two floatbowl vent lines and leave the ports open to breath.
A vacuum synch is a must to get good performance and plug reads.
Test each jetting circuit at the correct throttle position and go from there.
Wide open throttle for the main jets, 1/3 throttle for the jet needles and minimal throttle (4th gear at about 35 mph steady) for the pilot circuit.
Be certain the ignition is timed correctly and the spark is good before testing.

[tmaenner]

Really appreciate all your help!!!

Ouch, I haven't thought of the exhaust; it's 4in1 rather than stock, brand unknown.
How much would that change the optimum jet size?

Thanks again

[KEITH KRAUSE]

Really appreciate all your help!!!

Ouch, I haven't thought of the exhaust; it's 4in1 rather than stock, brand unknown.
How much would that change the optimum jet size?

Thanks again

With the 4-1 pipe, if it's good quality, I'd probably try 120 or 122.5 main jets.
I'd also try the jet needles with the e-clips in the bottom position, no jetting spacer. Best to start on the rich side. Test the needles at 1/3 and see what the performance/plugs say.
If the plugs are rich, lower the needles by raising the e clips. If the 1/3 test shows lean reads, you'll have to install a jet kit to make the bike run correctly. This would mean the stock needles aren't capable of being richened correctly.
All other jetting info in my first reply still stands, though I'll add clean carbs are necessary too. Good luck.

[tmaenner]

OK, sounds good; will try.
Thanks!