Q: Pilot FUEL screw- GS750B with VM26SS

[GS750]

A (hopefully quick) question about the pilot fuel screws on my GS750B with VM26SS carbs.

What should they be set at and what effect do they have on running?

I just bought the keyster kits because I have been having running issues for a while now and after doing my 850 wiseco kit they were more noticable.

All of my pilot fuel needles were busted (or cut down) and I needed to replace them. I attempted to get suzuki or mikuni factory parts but the majority are unavailable here in Australia and the keysters seemed to offer the next best solution.

Now I have followed the carb pages on here but they say to set at around 1 & 1/4 turns out, does that sound right?

I know that they are factory set, but what effect do they actually have?

Thanks in advance.

[TheCafeKid]

A (hopefully quick) question about the pilot fuel screws on my GS750B with VM26SS carbs.

What should they be set at and what effect do they have on running?

I just bought the keyster kits because I have been having running issues for a while now and after doing my 850 wiseco kit they were more noticable.

All of my pilot fuel needles were busted (or cut down) and I needed to replace them. I attempted to get suzuki or mikuni factory parts but the majority are unavailable here in Australia and the keysters seemed to offer the next best solution.

Now I have followed the carb pages on here but they say to set at around 1 & 1/4 turns out, does that sound right?

I know that they are factory set, but what effect do they actually have?

Thanks in advance.

The VM carburetors use both an air and a fuel inlet vs the BS series basicly using a screw that adjusts a pre-mixed volume into the pilot circuit and onward. On your VMs you'll find that you'll have to tinker with BOTH screws a little ways away from factory norm with your bore kit in there. To more directly answer your question, YES the fuel screw has an impact on how your bike will run. Try 1.5 turns to 1.75 turns to see if helps. From my experience, (and this is with the BS series, though it SHOULD transpose to the VMs as well i would think) the 850s and 750s (and even the 550s) were jetted pretty darn close to one another, with the only major difference being the needles, and the pilot AIR jet, which isnt removable anyway. Hell ive even swapped the needles out of a set of 750 carbs and slapped them on a 550 with no rejet, and it ran like a top and didnt foul the plugs. What are you current jet/needle sizes? Needle position? Air and fuel screw turns? These things will help us in guiding you in the correct direction, but again, I dont think, as long as everything else is STOCK, the bore kit is going to change things drasticly.

[GS750]

What are you current jet/needle sizes? Needle position? Air and fuel screw turns? These things will help us in guiding you in the correct direction, but again, I dont think, as long as everything else is STOCK, the bore kit is going to change things drasticly.

Thanks for your reply.

My bike:

850 WISECO BigBore kit
K&N Pods
4-1 eggsauced
Iridium plugs
Boyer Brandsen ignition
Pazon ignition coils


The carby settings were as follows,

Pilot fuel needles: Cut approx 3mm shorter than standard P.F.N's
Main jets: 112.5's
Needles: WERE 5F21's / 4th clip from the top of the needle, now Y64's / 2nd clip from top
Needle Jets: O-6's
Pilot jets: 15's

I synced the carbs with my morgan carbtune after installing the 850BB kit and running the bike in, and did the mixtures at idle with the colortune plugs, and re-checked my valve clearances and ignition timing.

My air screws were all at 2.75 - 3.25 full turns
Fuel screws were at .5 - .75 turns (but were cut down, not broken)

[TheCafeKid]

112.5s on a piped, podded and bored 750 is IMO probably WAY lean. I have a 77 750B with pods and a pipe on it, no bore kit yet, and im around 125-130 area. The fact that you have the airscrews out that far and it STARTS is a bit amazing to me because out on the air is leaner on those particular carbs. And how do you mean the pilot fuel needles are cut??

[GS750]

112.5s on a piped, podded and bored 750 is IMO probably WAY lean. I have a 77 750B with pods and a pipe on it, no bore kit yet, and im around 125-130 area. The fact that you have the airscrews out that far and it STARTS is a bit amazing to me because out on the air is leaner on those particular carbs. And how do you mean the pilot fuel needles are cut??

When i first bought the bike 6 years ago it already had genuine 850 pistons put in it (but I didnt know at the time).

It was REALLY heavy on fuel, I was only getting 150-160km from 17 ltrs. My brother was following me on his bike and he said he could smell fuel really badly everytime I gassed the bike and when I was riding at 1/4 to 1/2 throttle. His eyes were burning from the fuel it was dumping.

We rode home and checked everything, floats etc, which other than the 125 mains, were all at the right settings.
I ordered the 112.5 mains (they were the only thing which I changed) and on our next ride my bike performed alot nicer and his eyes were not burning and couldnt smell fuel.

With the 112.5's installed it was instantly getting 240-270Km from 17 litres.

Most of my riding is done around 5 - 50mtrs above sea level and 60 - 95% humidity but I occasionally ride at upto 400mtrs ASL and 30 -50% humidity.

When I said that the pilot fuel needles were cut, I meant that they are definately shorter than the standard PFN's by about 3-4mm. They are not tapered to a needle, they are squared off on the ends like the air screws and there is definate file marks on the flattened ends.


I have found LOADS of dodgy work that has been done to this bike and my best guess is that the previous owner worked with what he had, rather than ordering new parts (no Orings on bottoms of barrels, cabys siliconed on, home made valve shims and extended camchain tensioner are just a start).
Im only speculating that he ground or filed the tips off the PFN's after installing the 850 kit.

[tas850g]

my stock '79 850 fuel screws recommendations are 5/8 turns out and air screws are 1 1/2 turns out, which is where i have them now and it runs reasonably well. It could and will be better once i carbtune it.

When i cleaned the carbs i counted 3/4 turn out with fuel screws but 2 to 2 1/2 turns out. At those setting i really did not run it much as i just got the bike and then adjusted the valve clearances and cleaned the carbs. It did run ok though with those settings.

I've been told that the vm26's tend to run lean, maybe the reason why the air screws were turned out more.

[renobruce]

the Pilot fuel screws effect mostly just the pilot jet, which is used at idle and less than 1/4 throttle. The fact that yours are only .5 turns out is because they have been cut.

The main jet which you changed should not have effected your performance at 1/4 to 1/2 throttle much. You are using the jet needle at that point.

You need to do some plug chop tests. Start with wide open throttle and check your plugs. Adjust your main jets accordingly. Then do 1/2-2/3 throttle. Adjust your jet needle. Then run at just off idle (45 mph in 4th gear) and check your plugs. If it's close, fine tune your pilot fuel screws. If way off, change your pilot jet accordingly. You will need to re-synch your carbs if you change your position of the jet needle.

[tkent02]

The air screws have an effect on mixture only when the throttle is closed or nearly so, the fuel screws control mixture until the needle jet takes over at around 1/3 - 1/4 throttle.

With mixed up parts like you have, you will have to adjust them individually to run the best. Go in on the fuel screws until the throttle response at very small throttle openings gets bad, and then go back out a little bit. Like about 1/8 throttle or less.

You could go the other way, set the airscrews all the same, turn the fuel screws for highest idle.

Either way, it will probably take a lot of tinkering to get it right in this case.

Probably all the cylinders will take a different adjustment, it's not likely that your PO cut all the screws identically, and those Keysters are not very precise anyway.

Patience is your friend.

[GS750]

A win at last!!!!!

After my 6 day saga with carbys I would like to say thanks to everyone who replied, and to the person who posted the VM carby overhaul guide.

Your suggestions and guidance made for an over all smooth experience in the rebuilding of my carbys.

I decided that since my carbys were all screwed with and nothing seemed standard, I would use the full Keyster kits. I found that they were better than I had believed after some of the things that I have seen posted about them.

The kits, I found, were on a par with the genuine Suzuki and Mikuni stuff.


After finding the only difference was the taper of the new slide needles, I decided to bite the bullet and just go ahead and use them since my original needles were very worn.

I spoke with a friend who is a retired motorcycle racer and he suggested to me that I use general purpose automotive paint thinners, and soak the carby bodys overnight, then use a high pressure hose to clean them, followed immediately by a blow with compressed air.

I totally stripped out my carbys, including the needle jets, and then soaked them and I am extremely pleased that I did so!

The amount of crap that I got out of my carby passages, and the old crap paint that had been painted on the outsides of the carby bodys (and partially inside) was very easily removed, and now my carbys finally look like they should!

My final decision on settings was to do a static synch of the carbs on the bench, which was a total surprise when I did them with the guages, but I will tell more about that later.


I used all the Keyster parts:
the Y64 needles,
the fuel needle and seat,
the 15 idle jet,
the fuel and air mixture screws and new springs, and
the gaskets

Due to the mods I have on my bike I decided to use my original 112.5 main jets instead of the 102.5's supplied with the kit.

I also purchased 16x VITON "O"rings from a local fuel injection specialist which I used to replace the "O"rings on the fuel links between the carbys and also on the choke control valves.


I ended up setting the fuel mixture screws (underneath the front of the carbys) to 1.25 (one and a quarter) turns out, and the air screws at 1.5 turns out and installed the needles at clip 2 (from the top) because they measured the same diameter there as my old ones did on clip 4, and that was at the point immediately at the top of where all the wear was on my old needles.

Well the bike started within about .25 to .5 of a second using the choke (I previously had to crank it for about 2 seconds before it would fire, with choke).

Did I mention that it is summer here in Australia (35c days and 28c nights).


Turned the choke off after about 3-4 secs and it idled pretty darn nice. A few little tweaks to the idle mixtures and I was happy.

Installed my Gunson colortune's and started it.......... I was right on the money for idle, and I checked the 1/4 throttle range which was also perfect.

Half and 3/4 throttle was slightly lean but I decided to take it for a run first to settle everything in and decided if I needed to alter anything.

My throttle range was extremely good from idle to around half throttle and it would pull hard in top gear on the hills, but after 1/2 throttle it was leaning off a bit as it wouldnt pull if I opened it up between 1/2 and full.

I didnt have the time to get anything else done today (or this week as I have big days at work) and nights are out of the question, so it will probably be 2 weekends before I can do some plug chops. At the moment I am thinking that I will have to go to clip 3 or 4 (from the top) on the needles.

Other than that, the saga is over.


I cant begin to reinforce how important it is to ENSURE YOUR CARBY BODY'S ARE CLEAN and the importance of NEW "O"rings when rebuilding the carbys.


I am a qualified automotive mechanic with 25 years experience in all facets of automotive work (bikes, cars, trucks, race cars included) but I found the thought of undertaking a rebuild and synch on 4 carbys pretty daunting, but thanks to all the help from the GS resources, the job was made extremely simple, and ANYONE could undertake the carby strip and rebuild!


THANKS AGAIN!!!!!

[SqDancerLynn1]

Their have been many instances where people couldn't get their bikes to run properly. Come to find out that carb kits were used and supplied with the wrong jets !! 90% of the time they are not needed. Except as you found - for the needles

[GS750]

Yeah the needles were buggered but the fuel screws were also "modified" by a previous owner (they were cut about 3-4mm shorter) and the air screw were also noticably damaged (had distinct grooves in them where they had been screwed home waaay too hard.

Anyways, she is all running nice now to 1/2 throttle and I will work on that next fortnight once I have some more time to spare, and refine that so its good across the board.

[renobruce]

Sounds like you are close! Do the wide open throttle chop tests first. Make sure your main jet is the right size. Then fine tune the jet needle. I agree that you need to raise it a bit.

[GS750]

this was my question before my old user got purged from the database

[chuck hahn]

Another point that has been brought to my attention while learning about my 750 carbs..although both my 77 and my 78 E model have the exact same carbs, they both were stock jetted differently AND require different mixture screww and air screw settings. This is determined by the engine number. Be sure to cross reference the engine number to be sure of the correct main jet size and screw settings. Someone before had posted the chart but i cant recall the thread name to refer you to...but its on here somewhere,

[chuck hahn]

Found the pertinent info..go to the carbs section and click on the sticky at top about settings per factory manuals and go to page 11..post # 43 just referemce the engine number on the flat area just above the clutch cover..I know it doesnt say a stock setting for the air screw, but it is generally recognized that a start point would be 1 1/2 turns out from lightly seated. of course the fine tuning process from that point will get her dialed in nicely.