'76-79 GS750 factory jet size variations in relation to engine mods

[Chuck78]

WELL NOW... Never looked into numbers before, but it seems as if maybe my engine is from a '79 GS750! hmmm... 27966, 20000+ are the last listings on that photo linked above:


VERY INTERESTING! Previous owner who only but 175 miles on bike in 2 years and knew nothing about it (was given bike to forgive friend of money owed to him), said "the moter's been board out" - who knows what else. Bike had been mostly well maintained until it sat, bearings all regreased, engine running pretty good other than dirty carbs, etc. Either they retained the original carbs from '77 (and maybe or maybe not changed the pilot jets) or else maybe bumped the main jets up a few sizes on the '79 carbs due to running a MAC 4 into 1 system and whatever overbore pistons (wiseco 10.25:1 844cc or GS850?) were installed? Interesting... Unfortunately I cannot find ANY numbers on the carb bodies to correspond to the chart on bikecliff's site. All they say are MIC and Made in Japan, no other writing cast into carbs unless I am just missing it.

[Chuck78]

[Chuck78]

That "mixture screw setting" in that GS750 VM26SS carb spec chart, is that the pilot fuel screw????? The air screw I would imagine is a little more fine-tuned than just a default "## turns" setting, right? My untouched from the factory (1979????) pilot fuel screws were still painted in place at the 7/8 turn out position. The air screws with stock airbox but on overbored engine with 4-1 exhaust were also just shy of 1 turn out, but had definitely been adjusted (not lined up with factory chiseled adjustment marks).

EDIT: this link http://www.gs-classic.de/tipps/verg_08.htm, although I cannot read German, seems to show part# 7 as pilot fuel screw and part#9 as pilot air (idle air) screw, and says that the later engine numbers that got the 15 pilot jet got 2.5-3 turns out on the fuel screw and 1 turn out on the air screw. This is the opposite of what you just told me rusty. more air than fuel seems to be what is needed, but as far as how many turns out to get that, I don't know air vs fuel... I tried an online website translator, and it still definitely seems as if 7 is the fuel and 9 is the air screw, and the fuel screw is more than double the turns out than the air screw according to this source.

I will probably try mine at 1-1/2 to 1-3/4 turns out on the pilot fuel screws and then 2-1/2 on the air screws I think. Sound like a good baseline for my mods? These K&N open element filters flow WAAAAAYYYY more than standard pods, so I may need to go larger on the jets to at least 125 and up the pilots a fair amount. All that filter area for two carbs worth of width, but only one carb is sucking at a time, so there is a lot more air on tap when each carb's intake valve is open! I'm VERY SURPRISED that NO ONE on GSR has seemed to have tried these filters yet! They fit the stock VM26's & VM28's on 76-9 GS750/850/(1000?) and 70's KZ900/1000.

[OldVet66]

Start your pilot fuel screw adjust at just a tad more than 1/2 turn out. Adjust them out from there so the highest idle on the air screws is approximately two turns out. Pilot fuel adjust requires air adjust until you are just about in the ball park. Tiny pilot fuel adjustments make a big difference so go easy until you start to zero into the two turn out on the air for highest idle. That adjustment range seems to accommodate fine tuning for low speed manners in the 1,500 to 3,000rpm range. I'm running stock #15 pilot jets and a Dynojet equivalent to a 132.5 Mikuni main jet 4-1 pipes and K&N pods on a stock GS1000E. Pilot fuel screws were initially 1/2 turn out and I had to tweak them out another 1/16 turn to hit the sweet spot for low speed manners. All bikes are different, but 1 1/2, 1 3/4 turns out on the pilot fuel screws is way too much. I'm getting a solid 41mpg when I flog it and 44mpg when I drive easier. Gas ain't what it was when these bikes were built, so the charts are nothing more than a guide IMHO.















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[Chuck78]

All very good info to know, thanks rusty & vette. The german site must have gotten it backwards.

A little over half turn out seems pretty lean to me, but I will give it a try. The VM rebuild tutorial says to start at 1 turn out, but has no instructions to adjust.
My factory settings were around 7/8 turn out. I woulda thought 1 or 1-1/4 turn woulda done it until the german site threw me for a loop. I have never seen or heard a real procedure for adjusting the pilot fuel screws, as the manuals say they are factory preset & are not to be tampered with.

If I have a whole lot more airflow than stock, I also would have thought that I would have to back the pilot fuel screws out more, no?

So basically you are saying I should set the air screws at 2 turns out, pilot fuel screws at 1/2 turn out, run bike, & then start backing out the pilot fuel screws until I get the highest idle? make sure the screw adjustments are uniform and then fine tune with the air screws??

[OldVet66]

No. 1/2 is a starting point on the pilot fuel screw. Then adjust the air screws for highest idle. Count the turns on the air screws. If they are less than two turns out at highest idle, tweak the pilot fuel screw out ever so slightly and adjust the air screws for highest idle again. You may end up doing this two or three times until you start to get the highest idle with the air screws at about two turns out. Then take the bike for a ride in slow traffic and see if you are smooth in the 1,500-3,000 range. If it stutters a bit and comes on fine when you roll the throttle a bit, it may be a tad rich on the pilot fuel screws. Here is the rub.The pilot fuel screws don't seem to work as you would think. If you turn them in now just a bit leaner, your stutter will probably get worse and have you scratching your head. Tweak it out a bit more. This also moves the air adjust for highest idle out a bit more. From what I can tell the pilot fuel needles move the air adjust range up and down the scale. When you find the point where they are both happy because they both work together, you will zero into where your bike needs to be set. My carburetors were virgin, with the factory paint on the screws and there was no damage to the pilot fuel needles that so many people seem to have. Your settings are going to be different than mine, but considering your motor modifications, it's probably a closer starting point than any of the 750 information.

[Chuck78]

Great, thanks a lot fellas. I was always uncertain how you were actually supposed to adjust then, since the factory service manual does not cover that, nor does Clymer's. Very good to know.

[OldVet66]

Just thinking about your motor mods, you could just use a Dynojet 3304 stage three jet kit. You essentially have a 1000. It sure makes things easier with their needle taper.