fuel screws setting

[tc862011]

so i went through the carb settings the other rainy day just to double check everything, i thought i had the fuel screws set to just under 1 turn but it was at 2 turns, i reset them to just under a full turn, it still runs great. it really didnt change much other then having to reset the air screws on the top of the carbs. my question is how much would a full turn with the fuel screws make on fuel consumpstion.

[Steve]

At idle and low-throttle settings, you would be running rather rich, which would upset your mileage. Would also tend to foul your plugs.

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[tc862011]

i never really fouled any plugs but, i have noticed idle and low throttle speeds seems a bit smoother now, it was pretty smooth before but now that i have moved it back to the normal setting it seems alot smoother on and off at low speeds and idle seems to be alot healthier

[OldVet66]

I take it you have VM carbs. Your Pilot fuel screws will most likely want to be near 3/4 turns out from lightly seated. You are at a good starting point. Go out and run the bike hard to clear the plugs and then drive home from about 15 minutes away at no more than 20-25 mph. Hit your kill switch and pull the clutch when you are home like you are doing plug chops (no idling) and read the plug colors. I would assume they will be a little dark, but the bottom line is you don't want them black or white, somewhere in between, and all the same color. Each cylinder will be a little different so adjust accordingly in or out in very small equal increments until you are satisfied they are right. Then you can move on to the air screws. With the pilot fuel needles adjusted correctly, low end behavior will be very nice, more power off clutch and smooth at low speeds.

[hampshirehog]

Your fuel screws and air screws run in the same circuit so wind one in and you need to wind the other in to compensate. Because the effectively counteract each other you won't affect your fuel consumption.

[tc862011]

yea i have done a few plug chops since and the plugs look good, they are all a little different but they are all in the range between white and black, runs the same as but the idle circuit feels better. when i readjusted them i went with a full turn out and then a smidge back turn so i wanna say they are right around 3/4 or a turn. i have put a couple thousand miles at the least since the rebuild of the engine and it just keep running better. its great to have this 34 year running so well. i get so many compliments on it, more from the guys that have seen these bikes brand new

[Steve]

Your needles will most likely want to be near 3/4 turns out from lightly seated. ...

Huh?? The needles are adjusted with CLIPS, not turns of a screw.

Your fuel screws and air screws run in the same circuit so wind one in and you need to wind the other in to compensate. Because the effectively counteract each other you won't affect your fuel consumption.

You sure about this? I don't do much with VM carbs, but my thinking is that if you add more gas with the fuel screw, then open up the air screw, you are keeping the mixture the same ratio, but you will be feeding more of that same-ratio mixture into the intake tract. Surely that has to affect mileage a bit?

If all you have to do is to add enough air to counteract the fuel that is being added, why are there recommendations for certain settings based on airbox and exhaust setups? Seems that we can start saying "set the fuel screws wherever you want, then set the air screws to double that amount", instead of "for stock setups, fuel screw about 5/8 to 3/4 turn ..." or "for pods and a pipe, start with the fuel screw at 1 to 1 1/4 turn out ...".

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[OldVet66]

Senior moment Steve, you know what I meant.

[Steve]

Senior moment Steve, you know what I meant.

Yes, I know what you meant, but I was not the one you were telling to adjust them by turning.

I only brought it up because some newbie might have seen that, taken it as 'gospel', then made a fool of himself asking how to turn the needles out 3/4 turn.

On the other hand, ... the pilot fuel screws in the VM carbs are rather sharp, almost needle-like, so could almost be called 'needles'.

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[OldVet66]

I went back and corrected it .

[hampshirehog]

You sure about this? I don't do much with VM carbs, but my thinking is that if you add more gas with the fuel screw, then open up the air screw, you are keeping the mixture the same ratio, but you will be feeding more of that same-ratio mixture into the intake tract. Surely that has to affect mileage a bit?

That is correct - about adding more fuel / air mixture - but that would mean a rise in the revs so you would turn the throttle stop down to counter when parked or just use less throttle on the move. Somewhere in there is the perfect setting but the adjustments down at those low carb settings are so fine compared to the coarseness of the rest of the carb circuit that we would never be able spot it.

[Steve]

That is correct - about adding more fuel / air mixture - but that would mean a rise in the revs so you would turn the throttle stop down to counter when parked or just use less throttle on the move. Somewhere in there is the perfect setting but the adjustments down at those low carb settings are so fine compared to the coarseness of the rest of the carb circuit that we would never be able spot it.

Thanks for the reply. I have always wondered how critical the fuel screw settings were, now I know to just make sure there is plenty, then fine-tune with the air screw.

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