vm carb air screw adjustment

[tkent02]

You are backwards, open up a carb and see where the two progression ports are. The fuel screw controls the port on the high vacuum side of the slide. The air screw feeds air to both ports. I am right about this and when I tune these carbs they respond accordingly. When I turn the fuel screw it changes the idle drastically. Dan

Okay explain it more thoroughly so it makes more sense than what the Mikuni manuals say.
Not that I don't believe you, just don't quite understand how to do it your way yet.

Sure the fuel screws make drastic changes in idle, but don't the airscrews fine tune the idle mixture more accurately?
Don't want a drastic change, just want to get the idle mixture perfect.
And tweaking the fuel screws by riding around with the throttle just barely cracked open and adjusting the screws to get the nicest throttle response will get it running very nicely indeed at small throttle openings.

Spending the next few days dialing in a GS1000e with pods/pipe so the timing is good here.

[Dan Ruddock]

As you can see in this diagram there are two ports, one ahead of slide and one before the slide. The port ahead of the slide (the one with the fuel screw) is the one for idling and the other supplies most of the fuel when the slide starts to lift up exposing it to vacuum. The air screw will richen or lean both idle and part throttle but the fuel screw mostly controls idle. Yes the fuel screw can effect part throttle but mainly because you will have to adjust the air screw to get it to idle. I suggest adjusting the air screws out 1.5/2.0 turns out and adjusting for a strong idle by turning the fuel screws. If part throttle is rich or lean adjust the air screw count to compensate. You will have to re adjust idle mixture after doing this. One thing I like about doing it this way is you don't have to turn the fuel screws in all the way to get a reference point which has a nasty habit of breaking off the screw tip. Dan

[greg78gs750]

As you can see in this diagram there are two ports, one ahead of slide and one before the slide. The port ahead of the slide (the one with the fuel screw) is the one for idling and the other supplies most of the fuel when the slide starts to lift up exposing it to vacuum. The air screw will richen or lean both idle and part throttle but the fuel screw mostly controls idle. Yes the fuel screw can effect part throttle but mainly because you will have to adjust the air screw to get it to idle. I suggest adjusting the air screws out 1.5/2.0 turns out and adjusting for a strong idle by turning the fuel screws. If part throttle is rich or lean adjust the air screw count to compensate. You will have to re adjust idle mixture after doing this. One thing I like about doing it this way is you don't have to turn the fuel screws in all the way to get a reference point which has a nasty habit of breaking off the screw tip. Dan

so can i adjust the fuel screws with the "highest idle" method, just like i would with the air screws? or do i need to chop every time?

i am def. running a bit lean, took it for a rather spirited around the block last night, she pulled hard, harder than before the work, all the way up to redline in all gears, and up to 105 before i knew what was even happenning.....8-[ the plugs at the end where all WHITE, i am looking for more of a cinnamon, correct? i know i was on all the circuits, just trying to get a starting point here. thanks, guys. i am so stoked.

[tkent02]

That still doesn't sound right to me.

The idle mixture (all mixtures) is mostly air, only a tiny bit of fuel. By adjusting the airflow the idle mixture can be adjusted many times more precisely than by adjusting the fuel.

The air screw does almost nothing when the throttle is open much at all, why would air go through that tiny passage when there is a much larger opening under the throttle slide to go through?

Adjusting the fuel flow for a slightly varying throttle is done by the pilot circuit, what we adjust with the fuel screw is how much fuel per volume of air, it compensates and brings in the correct amount of fuel when the throttle is open a little more or a little less, that's what it is designed for.

To change just a tiny bit of airflow with the airscrew to adjust mixture when such a large volume of air is going under the slide seems like it would be ineffective at best.

The pilot circuit works less and less as the throttle opens, by the time the throttle is about 1/4 open the pilot circuit is doing next to nothing, the needle and needle jet are metering the air at this point.

Never had a problem with breaking off the screw tips, although I have inherited a few carbs with this problem. Some guys are hamfisted I guess.

The screws only need to be closed all the way when they are first put back together, after that you don't need to close the screws, just move them for best running at low throttle. Actually the screws don't need to be closed to get a reference anyway, you can see the screw tips sticking up into the bottom of the main bore.

I will try what you are saying tomorrow, but I doubt I will get it any better than the crisp throttle response and fifty miles per gallon that I get with Mikuni's recommended method.

[Dan Ruddock]

Yes adjust your fuel screws to fastest/strongest idle. Sounds like you need a bigger main jet or adjusting the clip on the needle. Try the main first and if the bike has more power you are going in the right direction. Dan

[hampshirehog]

I agree that proper air screws first / fuel meter screws second is the proper way to do a VM carb adjustment. However, when the slide starts to open with a bit of throttle the pilot circuit quickly gets swamped. When I've been lazy and just twiddled the airscrew to get a decent slow run I've always found it ok.
Effectively the airscrew and fuel meter work in series with each other (though it's not a proportionate relationship) and they affect each other. You can keep your mixture the same by winding one in and the other out. What you do achieve by doing air screws first / fuel meter screws second is a truer carb balance. eg if you've got one airscrew out 1 turn and another out 2 turns to get the best mixture and then you do a balance you have to drop one slide slightly lower to compensate so your carb balance won't be spot on when you open the throttle, though you probably won't notice it. I'm also not so sure that Mikuni machining tolerances were close enough for it to have any significant effect.
Having said that, I do prefer to do things properly and 'play safe'

[greg78gs750]

love this forum. i opened up a can o' worms. good stuff, learning, grokking.....

[80GS750]

wow i just learned a ton!!!

i am running way lean on my pilot circuit....

the air screws are all the way in, she idles, but gets better with more choke...

so i should adjust my fuel screws out for best idle, then tweak it with my air screws...

sounds like my plan for tomorrow!