looking for baseline settings for vm carbs

[Tola]

Just rebuilt my VM carbs on my '77 gs750. I counted turns on both pilot jet and air jet screws but they were all over the place and didn't line up with factory marks to start off with. Yes, I've searched for baseline settings and found various recommendations .75,1.5, 1.75 turns out for fuel screw and anywhere from 1.5-3 turns out for the air screw.

Where do I need to be for a starting off point for both screws, mainly the fuel screw on bottom since I think 2-2.5 is a good point for air screw. I have a factory gauge to balance carbs so that will be next.

Stock airbox-

Thanks!

 

[Nessism]

The baseline setting is in the rebuild tutorial...http://www.thegsresources.com/garage/docs/vm_carb_rebuild.pdf

Surely you found this during your search. Are you disagreeing with the recommended settings?

 

[MisterCinders]

The baseline settings for various GS models can be found here.

http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html

The '77 GS750 offers a bit of a trick question here, so you need to determine what size mains and pilots are in your carbs.

The GS750s from 1977 through 1979 all use VM26 carbs. I think the carb bodies are identical across those years, but they may have subtle differences in their passage sizes. Even if they differ, everyone treats them as interchangeable.

The specs for 1977, however, are different than later years. They call for much larger pilot jets (22.5 and 27.5 instead of 15) and larger mains (105 instead of 100). The recommended float height for the early models is much higher though (26mm instead of 22mm-24mm). I think the carb bodies are the same and Suzuki just rebalanced the specs with lower float heights and smaller jets.

Either way, after 30+ years of POs and Clymer (which calls for 23mm - 25mm float heights), who knows what specs your carbs are set for? Thus, even if you haven't switched to pods and a pipe, you need to check the jets you have in place to set the float properly.

As far as screw turns, under all versions, the base specs are Fuel Screw 7/8 - 1 turn out, and Air Screw double that, then adjusted to highest idle. You may find, however, that some PO changed some of the jetting according to Clymer but not all (e.g., set the floats to 23mm but did not change the pilot jet from a 22.5), and then used the air/fuel screws to compensate (that PO might have set the fuel to 5/8 turn and the air to 2.5, to accommodate all that fuel in the mix).

HTH

 

[Tola]

Thanks, all the main jets on my bike are 97.5 which is way off I guess but then again maybe they aren't the original carb's?.Would 97.5 make sense for later years of the 750? Need to take carbs apart again!

Nessism, not disagreeing. There just seems to be conflicting info out there and each bike seems so different.

Thanks again-

 

[tas850g]

My mains are 102.5 and airs are 15 for the 79 850. I have the vm26's.

Bike is stock and my fuel screws are about 3/4 turn out from seated and the air screws are about 2 1/2 turns out from seated. That will get most bikes in the ball park.

 

[Tola]

well, I guess according to chart 97.5 is right for the earliest 77's so that's good. Glad this bike was free!!

 

[MisterCinders]

well, I guess according to chart 97.5 is right for the earliest 77's so that's good. Glad this bike was free!!

You should also check the pilot sizes, if it had 97.5 mains, they are probably 27.5s. If any PO used a K&L rebuild kit, however, you will have 15s for pilots.

Also, check the needles in those carbs. I think there is an oddball early 750 that did not have adjustable needles like most VM carbs. If you have a non-adjustable needle, it may also be thinner than the typical VM needle, requiring the smaller mains.

If your bike has stock exhaust and airbox, you can use those mains, needle and 27.5 pilots, but you will want to set your float height at 26mm, a bit higher than the typical VM26 float setting.

If you are changing to pods and a pipe, you will suffer even more than others with VM carbs. You have no Dyno kit, your needle won't adjust, and your stock jets differ from everyone here that might guide you through re-jetting.

 

[Tola]

Not doing Pods or 4-1, I've read enough to want to stay away from that hassle. Bike was given to me so goal is to get it running well and if I like it then I'll make it look pretty, run it for the summer and try to sell it and move up to a GS1100 which fits me perfectly but this years budget has gone into my Triumph. Never appreciated how nice a single carb 750 is to work on:lol:

 

[chedd]

tola,iv'e just finnished re building the carbs on my '77 750,the pilot's are22.5,mains are105.standard airboxfitted.she did stumble a little above 6,000revs so lifted the float hight from 25mm to26mm seem's to have sorted the problem.hope you get her on the road soon!

 

[Tola]

chedd, thanks. Going to get float heights set to spec, guess my 97.5 jets were stock for the very early models. My father got this bike from a used bike/salvage shop and the guy there went over it and replaced one jet which was a 105 with a 97.5 so I may have one carb that was replaced. Doing front end now, adjust valves,timing then it will be ready to fire up, hopefully next week or two!!

 

[MisterCinders]

tola,iv'e just finnished re building the carbs on my '77 750,the pilot's are22.5,mains are105.standard airboxfitted.she did stumble a little above 6,000revs so lifted the float hight from 25mm to26mm seem's to have sorted the problem.hope you get her on the road soon!

Note that some '77 750s have 22.5 pilots and 105 mains, but other '77 750s have 97.5 mains and 27.5 pilots.

Again, I don't know if the carb bodies across the various '76 - '79 GS750 models differ, especially their internal passages. They are all VM26s, and the jetting specs seem to be different paths to the same point. In other words, the smaller jets in later 750s would offset the fuel increase from the lowered float height. If there are subtle difference to the actual carb bodies, you probably could not tell visually, absent x-ray vision. Those early '77 carbs might have deeper float bowls to account for that float height spec, for instance.

IMO, the best course of action would be to (a) inventory your current jet sizes; (b) compare them to BassCliff's carb spec chart and align them with the closest '77 750 spec line with whatever requires the least amount of jetting swaps. If you have to choose between changing pilots or mains to achieve this (e.g., you find all 97.5 mains and all 22.5 pilots), I would change mains before pilots. Then set the float height according to that spec line -- i.e., 26mm for the larger jets, or 23-24mm for the smaller ones.

Also, definitely check those needles to see if they are adjustable. Generally, the needles in the early 750s have the adjustable circlip notches, but one of those early models (maybe the Canuckistani B model?) may have shipped with a different needle. If you cannot alter the needle settings, you probably need to stick with the early jetting, since the pilot circuit still plays a role in the mid-range circuit, and the mid-range overlaps with the main circuit. If your needle circuit is locked into a single setting, you may have to stick with the specs for the other circuits to avoid weird issues transitioning in and out of the mid range.

 

[TheCafeKid]

The only differences in the 77' carbs were the needles and the jets. They use a different needle (it IS adjustable) and it is not spring loaded like the later models. Thus, it sits lower in the slide bore. It's also a little more blunt than those on the later models. Not much, but enough to make a difference. I don't really pay attention to the needle numbers as I have no idea what they mean (in reference to taper speed, thickness, sharpness, etc)

However, because of this fatter, slower needle, I would assume the differences in jetting are simply to compensate for that.
But, you need to figure out if you're using actual 77 750 carbs.
The main jet size sounds like that of a 1000C/E honestly, and would be interchangeable aside from jet sizes. The needle may not be the same, but can be made to work relatively easily. If your needle is set in a spring in the slide bore, I'd set the carbs up as a later model 750 (using 78 specs, perhaps)

If you're putting pods and or a pipe on it, there will need to be jetting changes.
And of course, I wouldn't bother with any of this until your carbs are cleaned and rebuilt and your valves are adjusted to spec, as this will all play into your jetting and testing...

 

[MisterCinders]

The only differences in the 77' carbs were the needles and the jets. They use a different needle (it IS adjustable) and it is not spring loaded like the later models. Thus, it sits lower in the slide bore. It's also a little more blunt than those on the later models. Not much, but enough to make a difference. I don't really pay attention to the needle numbers as I have no idea what they mean (in reference to taper speed, thickness, sharpness, etc)

However, because of this fatter, slower needle, I would assume the differences in jetting are simply to compensate for that.
But, you need to figure out if you're using actual 77 750 carbs.
The main jet size sounds like that of a 1000C/E honestly, and would be interchangeable aside from jet sizes. The needle may not be the same, but can be made to work relatively easily. If your needle is set in a spring in the slide bore, I'd set the carbs up as a later model 750 (using 78 specs, perhaps)

If you're putting pods and or a pipe on it, there will need to be jetting changes.
And of course, I wouldn't bother with any of this until your carbs are cleaned and rebuilt and your valves are adjusted to spec, as this will all play into your jetting and testing...

Cool. I knew there was some oddity to the older needles. The fatter non-spring-loaded is what I was thinking of, not adjustment. Sorry to be dumb on that.

Follow CafeKid's better and more knowledgeable advice on this. He is wise.