vm carb air screw adjustment

[greg78gs750]

well, guys, IT RUNS!!!!!!!! fired right up last night. even took her for a spin around the block. work on my 78 gs 750 so far includes:

valve adjustment
realgasket
airbox sealing
carb rebuild w/ all orings, float height adjust, bowl gaskets
new exhaust gaskets
clutch springs/scuffed plates
oil pan gasket
sprocket cover repair
silverstar headlight
new plug wires, newer caps
coil relay mod
soldered connections
reg rec ground to battery
general cleaning and polishing.

my question is this: i need to tune my carbs. my fuel screws (on my vm carbs, by the way, you cv people dont confuse me!!!!) are 1 turn out, per stock settings found here. my air screws, i started at 1 1/2 turns out. but when i turn them to do the "highest idle" method, i hear little perceptible change. do i need to be more patient? how quick will things respond to a turn of the screw? will it be a dramatic change? cause if so, something aint right, still. though, it went around the block lickity split and runs smoother than it ever has. that cold 3 pipe of mine even warmed up, right off idle!!!so......

thanks to everyone on this great site!!!!!

now i gotta order those progressive shocks and springs. and tires. and brake lines. and those rotors sure need turning, i gotta be able to stop, honey......

greg

[greg78gs750]

ok, another question too. i should probably start a new thread, but i hate cluttering things up with my ignorance....anyway....i need new tires. my question is, what size? specifically, on the back? currently i have a 130 90 17 that the po put on, thing is, it is so wide that he had to "modify" the torque link connected to the back brake caliper to squeeze it in there. what is stock? again, this is a (mfg in 12/77) 78 gs 750, with alloy rims and the single disk up front. pics would be helpful here, maybe, but i gotta go to work soon and i have dial up. i will post some later.

also, (this is the true ignorance part) um, what do the numbers mean? the tire numbers, i mean? 17 is the diameter of the rim, right? in inches? then the 130 and 90 are in mm? one is height and one width? width of the tire at its widest? height from the edge of the rim? they really mix english and metric measurements? i know, i am stupid. sorry.

thanks guys!

[Nessism]

If turning the pilot air screw doesn't make any difference in the idle rpm and the bike is running well, you are done. Just make sure the screws are all set the same.

Alloy rims? Do you mean mag wheels or actual aluminum rims with spokes?

The early 750's with spoke wheels take an 18" rear. More specifically, 3.25-19 front and 4.00-18 rear. You mention 17" rear so I'm confused.

Regarding tire sizes first number is width (old bikes used inch measurements but modern are all mm), this is often followed by the aspect ratio (ratio of height to width), speed rating (H is up to 130mph and V is to 150), and lastly is wheel diameter in inches.

All GS bikes other than the 450 take 19" fronts, get yourself a 100/90-19. Rear is a little more difficult. Not sure what the spoke rear wheel width is. Is it stamped on the rim anywhere? If it's a 2.50, you can go to 130/90 rear, if it's only a 2.15, you should keep to a 120 max (my opinion).

Hope this helps.

[Dan Ruddock]

It is common practice on this forum to count fuel screw turns out and adjust air screws to get idle mixture correct. I believe this is the wrong way to do it. I believe the opposite is correct, count air screw turns to a fixed amount and adjust fuel screws to adjust idle mixture. I could go into a long explanation as to why but for now just let this roll around in your heads. Dan

[tomm]

From work on my own 78 GS750E . . .
Front Tire: 100/90-19
Rear Tire: 120/90-18 (I wouldn't go any bigger. I have sufficient clearance, but not much extra.)
Spent weeks trying to fine tune my carbs. Had similar problems. Cold #3. Air screw not making a difference.
You mentioned o-rings, but not boots. Did you replace the boots on both sides of the carbs?
I found fuel screws at 1 to be a little lean. I started at 1-1/2 and tweaked from there based on examining the plugs (1/8 turn at a time). I think two of the carbs are open a bit more and two are at 1-1/2, but nothing over 2.
I had to look closely, but I could clearly see the tach respond to the air screw changes. Started at 1/4 turn increments and then 1/8 turn. Usually had to wait about 10 seconds between changes for engine to stabilize. And sometimes go back and forth.
Then it was back and forth between fuel screws, air screws, plug chops, fuel screws, air screws, plug chops, etc.
But it wasn't until I had everything single leak sealed, vent hoses routed correctly, etc, that the tuning actually worked as expected.
Probably took me a year to finally get it where it is now. Kept riding the whole time though.
If you're happy with it, don't worry about it and ride. Tweaking is for winter.

[greg78gs750]

ok, sorry, yes, aluminum mag wheels, not spokes. the rear is marked 17xmt2.50. i think i will go down to the 120, from what i understand wider actually can make the handling worse (though maybe this small increment doesnt much matter?) but the 130 is s-q-u-e-e-z-e-d in there, like i said. the po got creative with some vise-grips, from the look of things.

i didnt change the boots, but they are in ok condition, pliable enough. i will probably fix them next winter. maybe. they are ok, though.

you say "you are done". ha. this scares me and elates me at the same time. i dont believe it. but it IS running nice, at least from the 10 minutes i ran it last night.....

i am FREAKING STOKED.

i might try the fuel screws out a little more. i am considering getting a colortune, just for giggles. i will fiddle with the screws a bit more, but dont worry, it aint gonna keep me from riding it. it DOES ( i should mention) have 102.5 main jets, stock is 100. but from my understanding, these came lean stock, so i dont think that will cause me any headaches. i thought it ran pretty good before the work i did, so im sure it will be good enough after.

i plan on some chops to make sure im not too lean, dont want to kill the thing after all. though the main jet chops.....intimidate me, since WOT on this thing is fast. i am rarely at redline anyway, pretty conservative rider.

ok, thanks all for the responses, great help as always, lets ride already!!!!!!!

[greg78gs750]

btw, i know redline isnt necessarily related to which jet i am on....throttle position, not rpms.....

[greg78gs750]

oh, one more thing, the rims were not original. put on by the po. maybe why the size in back (17) is different.

[hampshirehog]

I believe the opposite is correct, count air screw turns to a fixed amount and adjust fuel screws to adjust idle mixture. I could go into a long explanation as to why but for now just let this roll around in your heads. Dan

That is the proper way to do it and I usually go that route. However, to fiddle with the fuel meter screw, the one underneath the front of the carb, you need hands made from asbestos, otherwise you get burned, and the world's stumpiest screwdriver.
In practice, setting the fuel screws 'about right' and tweaking by adjusting the air screws is a much easier operation and in reality is probably just as good.
As it only affects slow run (and a tiny, tiny part of the rest of the throttle openings) if your slow run gets sorted by either method it doesn't really matter.

[Steve]

...to fiddle with the fuel meter screw, the one underneath the front of the carb, you need hands made from asbestos, otherwise you get burned, and the world's stumpiest screwdriver.

or...you could get the special screwdriver from Motion Pro. About $30, but could be worth it.

.

[Dan Ruddock]

Yes it does matter more than you might think. The air screws affect mostly part throttle cruzing and the fuel screws have the biggest affect on idle but both will bleed over on to each other a certain amount. If the air screws are too rich your fuel mileage will be poor or worse yet too lean will cause overheating. I adjust all the air screws to a fixed amount till I get part load mixture where I want it then I adjust the fuel screws to get a good idle. Yes it is the hard way to do it but it seems the most difficult way is quite often the best way. Dan

[ddominik]

Pilot Screw Adjusting Tool

Thanks Steve

I gotta git me one of those!!!! .... been cooked one to many times.

Dom

[Greg O.]

How do you like your hand cooked? Rare, medium, or well done?:-D

Greg

[tkent02]

Yes it does matter more than you might think. The air screws affect mostly part throttle cruzing and the fuel screws have the biggest affect on idle but both will bleed over on to each other a certain amount. If the air screws are too rich your fuel mileage will be poor or worse yet too lean will cause overheating. I adjust all the air screws to a fixed amount till I get part load mixture where I want it then I adjust the fuel screws to get a good idle. Yes it is the hard way to do it but it seems the most difficult way is quite often the best way. Dan

Except the air screws are the idle mixture screws, and the fuel screws affect fuel flow above idle, up to where the slide cutaway and the needle jet and jet needle take over.

Idle air does not do much except when the throttle slide is closed.

Fuels screws have a huge effect on throttle response and smooth running in the first 1/8 or so of throttle opening.

The way Mikuni says to adjust it works pretty well.

[Dan Ruddock]

Except the air screws are the idle mixture screws, and the fuel screws affect fuel flow above idle, up to where the slide cutaway and the needle jet and jet needle take over.

Idle air does not do much except when the throttle slide is closed.

Fuels screws have a huge effect on throttle response and smooth running in the first 1/8 or so of throttle opening.

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