Settings for pilot fuel jets - GS750

[jim-k]

I am rebuilding a set of 1978 GS750 carbs, the bike won't come off idle under load unless the airscrews are less than 1/2 turn in - then it runs OK and from 1/4 throttle on, the bike runs great. While it needs choke on a cold start-up, it is sputtering and makes a popping sound that disappears once it is warmed up. Plugs show it is running rich I suspect the pilot circuits are clogged.

Can anyone advise on the factory pilot fuel setting. The manual states that the pilot air screw should be one turn out and not to mess with the pilot fuel screw......I will check to see how they are set before disassembling but wanted to know how close to factory they are - not sure if these carbs were monkeyed with before I purchased the bike.

 

[Nessism]

Unfortunately, there is very little information available from Suzuki on setting the various adjustable screws on the carbs over the years. General rule of thumb for VM carbs is to start with the fuel screws at about 3/4 turn out and play with the air screws until you get the highest idle setting, about 1-1/2 turns out. Unfortunately, sometimes it's hard to tell any difference in the idle when tweaking the air screw settings (at least that's the case for my VM carb 550). Frankly, I'm not sure what to suggest other than doing some plug chops to judge whether the mixture is rich or lean. No easy solution on this other than some trial and error and checking the plugs.

 

[hampshirehog]

From memory the very earliest models had a pilot airscrew setting of 1.25 turns out and the later ones 1.75 turns out - I suspect that yours, being a 78, will be the latter. In practice, somewhere near here is usually good enough.

However, it sounds to me as if you've got an issue in the pilot circuit - either some muck in the system or your fuel meter screws have been wound in too much, and possibly snapped off (they are incredibly puny).

If your fuel meter screws haven't been disturbed (the yellow paint is intact) very carefully and gently screw them in, counting the number of turns on eash carb until they seat - each carb may be different.

Suzuki don't issue any info on how far out the fuel meter screws (the ones underneath) should be - they are apparently facory set to that particular carb. As Nessism says, a good starting point is around 0.75 to 1 turn out.

With any new VM bike where the fuel meter screws have been disturbed I normally use a colortune, set the pilot airscrews to the standard setting and tweak the fuel meter screws - not an easy job as you have to have hands of steel (you burn them on the motor) and the world's stumpiest screwdriver. That way if I have to muck about with the carbs again I know the manual settings are ok.

good luck
Wally

 

[jim-k]

good stuff

good stuff

thx for the great advice, will keep you posted......

 

[jim-k]

Problem solved.....

Problem solved.....

OK, finally gave up on the old carbs - bottom ends were pretty full of corrosion and could not get the pilot circuits cleaned by just removing the bowls and using solvent and copper wire. Did not want to disassemble and soak - don't have a synchronizer tool.

Found these on fleabay, still had the yellow seal paint on the pilots and airscrews - bowls were immaculate. Stuck them on last night and runs great - no more stuttering, plugs look much better also. I'll keep the old rack for spares.

Also put new S11's on and installed tapered steering bearings, wobbles are gone - feels much more secure in the turns.