Pilot fuel screw ( different from an air screw)

[Bopi]

I've got a 78 750c with VM26SS mikuni's on it. I've read up on all the carb sync procedures and will begin that process soon but.......
I've got a needle screw at the bottom of the carb and can't get any info on how to set it or what it does exactly. The pilot air screw is located on the side but this is something else. I seem to remember a mention somewhere of a pilot fuel screw ( the location would be right) and of course the book only lists a passing sentence about a factory set adjustment and to not fool with it. I'm not sure if this is what they were referring to but... if it was a factory preset, it wasn't sealed. The rebuild kit came with a replacement screw and I tried to match the existing adjustment which i believe was 1 1/2 turns out but again I have no way to verify that this is correct as it is not listed in the haynes manual.

Any one have any ideas on this one.

[Bopi]

Ok so after a search and much reading it seems that the generally correct setting for a pilot fuel screw for this model is about 3/4 turns out. This from a previous post. I'm guessing that once the screws are preset, I should leave them and move on to the sync procedure. Once preset, is there any further adjustment for these screws?

[49er]

Have you just bought this bike? Did you clean the carbs before attempting to sync them? If you cleaned them, you should have removed these mixture screws and cleaned them and all the other passages with compressed air. The manuals say not to change the settings on these screws to comply with emmisions laws that appeared in the US around that period. If you dont remove them, you can't clear any partially gunked up passages. You will then have poor performance at idle and up to around 4000rpm. The 3/4 turn setting was what my 850 carbs were set at from the factory.
Following on from your bench sync, you need to warm your engine to normal running temperature then adjust your pilot air screws until your engine peaks in rpm. Re-adjust your idle and continue through the carbs.
Now you need to road test the bike and do plug chops at different RPMs to check the plugs for richness/leanness by their colour. When the fuel/air ratios are correct, the plugs should look a medium to light tan colour.
Check the engine at 3500, 7000 and 9000rpm. You should hold you engine at those speeds for a short period, then kill the ignition and coast to a stop.
Remove the plugs and read them. The adjustments are made for 3500 to the pilot mixture screws and air screws, at 7000 it requires the needle position to be raised or lowered, and 9000 needs a change of main jet size.

[KEITH KRAUSE]

If your bike is stock, try about one full turn out from lightly seated.
I would use the original pilot fuel screws unless they're damaged.
Set the side air screws using the highest rpm method.
Vacuum synch.
When checking the jetting circuits, it's not really about rpm, it's about throttle position.
Starting with new or good condition/cleaned used plugs, ride the bike around town a few miles and be sure it's completely warmed up. Now ride where you can keep a fairly steady cruising speed of about 35 mph in 4th/5th gear, level or uphill, a mile or so. For testing the pilot circuit, you want a minimal throttle opening. Now chop off. Chop off means to quickly shut off the throttle, pull in the clutch, turn off the key and coast to a safe stop with the clutch lever pulled in. Check the plug reads. Use a piece of fuel line that fits snugly over the plug ends to help you remove and re-install the last/first few threads of the hot plugs. A rag helps too.
Do what the plugs say. If lean, turn the pilot fuel screws about a 1/4 turn out more. If rich, a 1/4 turn in. Just being approx', depends on the reads.

[Bopi]

Well thanks guys for the response as this topic has been beat to death already but i'm just getting around to it.

Perhaps i should have included more info as per "how to post a thread" sticky so here are the details I omitted.

Yes the carbs were rebuilt in Dec.

I purchased the bike in Nov and ran it for a bit just to see if it would before tearing it down.

The teardown was mostly appearance mods but the carb rebuild was a matter of coarse since the bike had sat for a long time before I got it. I hadn't made any adjustments after the rebuild, just presets. I did replace the fuel screws but still have the originals so i can swap them out and may do that. ( I've got a teeny screw driver )

I've done the research from previous posts and I believe i have a handle on the carb sync methods described in those posts (Keith) which are reiterated in the above replies (thanks) but the pilot fuel screw is seldom mentioned . Anyway, the Haynes manual also doesen't mention them so obviously i have turned here for advice.

(Again though, once i preset the fuel screw, Is there any way of telling if the preset is correct. What should I notice with a correct or incorrect setting. The highest rpm and subsequent vacuum gage sync method applies to the pilot air screw and not the pilot fuel screw so ...
If the PO had the fuel screw adjusted incorrectly and if the manual doesn't include that info, i'll have to wing it at 3/4 - 1 turn and see. Is the fuel screw adjustment compensated for by the air screw adjustment or is the fuel screw preset independent of the air screw. Will the exhausts be belching black smoke ( joke) due to too rich a setting that can't be turned out by the air screw or can the air screw smooth out any inconsistencies in the fuel screw setting.)

the above paragraph was written before rereading your post

So many questions..so little time

Hmmm, I have to reread some of this stuff and it'll take a minute. (tic tic tic)

Ok so having reread your post, a lean or rich condition is dependant ( at idle to 1/4 throttle) on the fuel screw and thus should be set accordingly(+- 1 turn) regardless of the air screw settings . I should do this first as a preset and then recheck it later to insure the settings are correct . I shouldn't have to re-sync the carbs after (if) later changing the fuel screw settings. And of coarse mid-range and full throttle settings rely on needle position and jet size respectively. And above all "read the plugs" Does this all sound about right.

[KEITH KRAUSE]

Oh, I've made lots of posts about the pilot fuel screws.
Anyway, just follow my testing above for the pilot circuit. Works for me.
The PF screws are set at the factory using an emissions analyzer so it takes some tinkering to get them right. It just may take a little longer than having access to a tester as the factory does. If you test correctly and get the plugs to burn right, you're fine.
I'm out the door on vacation right now. Hope you get it tuned and enjoy the bike.

[Bopi]

Thanks for the help Keith and enjoy your vacation, I think I've got about everything I need for now.

Just a suggestion but with over 6k posts under your belt, do you think a comprehensive sticky regarding this procedure would help to slow down the steady trickle of questions regarding this topic.

[49er]

Oh, I've made lots of posts about the pilot fuel screws.
Anyway, just follow my testing above for the pilot circuit. Works for me.
The PF screws are set at the factory using an emissions analyzer so it takes some tinkering to get them right. It just may take a little longer than having access to a tester as the factory does. If you test correctly and get the plugs to burn right, you're fine.
I'm out the door on vacation right now. Hope you get it tuned and enjoy the bike.

Keith, the later models (80-86) may have been factory set by an emissions analyser. I doubt that the 77-79 VM's were. All my 850 FM screws were set at 3/4 off the seat. I was able to confirm this as the original paint was clearly visible across one edge of the screw head and the adjacent shroud on the carb body. Also, the screws are not sealed like they are on the CV carbs.
And don't even thing about the sticky proposition, it'll spoil your holiday.

[KEITH KRAUSE]

The VM equipped models were also tested and set using an em' analyzer.
Both the side air screws and pilot fuel screws underneath were set this way. The tester wouldn't be able to do it any other way. They then were supposed to place a small nick in the carb body on one side of each screw slot, then apply some sealant over the screw head. Sometimes I've seen no nick.
You CAN of course reset the pilot fuel screws close enough to factory settings, with luck, maybe even spot on.
Many years ago I went over to Mikuni Corp, about 15 minutes from here, and asked lots of questions. I was kind of surprised they allowed me in, but they did. A lead technician answered some of my questions which just happened to be mostly pilot circuit related. I also used to get the chance now and then to talk to a lead regional technician for Suzuki back then. This guy was pretty smart and he went around to the dealers in several states to train the mechanics. He also confirmed what I said above.
As for the sticky idea/ANY sticky idea, it's my opinion that something like that can obviously help, but, if all the info was right there all these discussions would slowly die.

[49er]

The VM equipped models were also tested and set using an em' analyzer.
Both the side air screws and pilot fuel screws underneath were set this way. The tester wouldn't be able to do it any other way. They then were supposed to place a small nick in the carb body on one side of each screw slot, then apply some sealant over the screw head. Sometimes I've seen no nick.
You CAN of course reset the pilot fuel screws close enough to factory settings, with luck, maybe even spot on.
Many years ago I went over to Mikuni Corp, about 15 minutes from here, and asked lots of questions. I was kind of surprised they allowed me in, but they did. A lead technician answered some of my questions which just happened to be mostly pilot circuit related. I also used to get the chance now and then to talk to a lead regional technician for Suzuki back then. This guy was pretty smart and he went around to the dealers in several states to train the mechanics. He also confirmed what I said above.
As for the sticky idea/ANY sticky idea, it's my opinion that something like that can obviously help, but, if all the info was right there all these discussions would slowly die.

Thanks for that feedback Keith. You have gone where not many others get to go.
My VM's certainly have a mark on the carb body for the the pilot air screw settings, but only paint marks for the pilot fuel screws. It seems a real coincidence that all my fuel screws were marked at exactly 3/4 turn off the seat. Is it not possible that the carbs for the US market were tuned more accurately than for our area ( no emission rules here at that time)? The air screws don't seem to have been sealed with sealant/paint here either.

[KEITH KRAUSE]

Yes, the US has stricter rules. I know the Canadian models also used adjustable jet needles for many years after the US received only non-adjustable needles in the CV carbs.
It's not unusual to have a bike with all four carbs adjusted the same straight from the factory. I've personally seen as little as 1/2 turn and as much as 1 1/4 turns on VM carbs that I believe were never touched. I've seen many with the carbs set differently from each other, though usually within 3/8 turn from the leanest to the richest.
One past member here swears his mixture screw factory settings were zero on at least two of his CV carbs. I suppose anything is possible. I've seen many carbs with no "nick" placed on them and some that apparently were never coated/sealed. The capping started with the CV's.

[andrewclaycomb]

I'm glad i found this, i have mine turned out 2 full turns and can't figure out whats wrong.