Bike idles with idle screw completely backed out...?

[noiseguy]

I've been dealing with floating idle on my bike ever since I bought it. 1990 with 127.5 / 40 jetting, idle jet screws are open and backed out 4 turns. Typically it idles fine but starts floating when it gets hotter... typically just after exiting the interstate and around town. I've been controlling this by backing off the main idle adjustment screw until it would just barely hold idle when cold... seemed to work.

Now, however, I've backed off the main idle adjustment screw to the point that it's no longer contacting the throttle cam at all. Bike idles around 1200 when cold and starts floating when hot. Seems like it's gotten worse.

Carbs were cleaned yesterday, synch'd a few years ago (haven't done it in awhile), valves are in spec. No other obvious running problems. Little o-ring is in place on diaphram cap. Intake boots appear fine visually / to the touch.

All this seems like a vacuum leak of some sort... I can't think how the bike could idle so high with the throttle plates shut otherwise. Plan to spray the boots with WD40 this weekend and see if I can find a leak around the intake boots. Otherwise I'm stumped. What am I missing?

[RossLH]

Check the float heights.

[Funderb]

is there still throttle cable free play when you've backed the idle stop out? (is the cable holding the butterfly open?)

it seems like your b-flys are sticking open, which seems unlikely, or you do indeed have a vac leak, could be any number of the carb orifices that are supposed to stay plugged. try spraying the pivot points for the throttle, and maybe even around the head to jug seal.

[bigfatcat]

Sounds like you have a chronic (though minor) flood situation, with a vacuum leak that manifests once the motor gets hot.

Remove the intake pipes and check the o-rings and mating surfaces to the cylinder heads - replace o-rings and clean up the surfaces of any corrosion if present. This area must seal perfectly or : vacuum leak(s).

Carb float needle valves should be in good shape, and float heights adjusted to put the carb bowl fuel level at the gasket surface.

Must have good seal at carb-to-intake pipe boot as well - the rubber not deformed by crooked clamp, and the rubber molding seated in the groove on the carb throat.   Don't overtighten the clamps.

Consider replacing the intake pipes/boots if your bike still has the originals.

I prefer carb cleaner  for the vacuum tests - more definitive response than wd-40. Doesn't take much.

[noiseguy]

Throttle cable has free play... it's not holding open. Carb float needles are in good shape... were replaced a couple years back when I first bought bike.

I didn't check float height after carb cleaning... I'll take a look at that. It was fine last year.

I didn't realize there were o-rings under the carb boots... I'll check those as well. Assume a correct size nitrile o-ring will serve as a replacement.

How tight should the carb boot clamps be? The clamps are aligned on the boots but I've really no idea how tight to set these... they are probably over-tightened if anything. 

[bigfatcat]

I didn't realize there were o-rings under the carb boots... I'll check those as well. Assume a correct size nitrile o-ring will serve as a replacement.

How tight should the carb boot clamps be? The clamps are aligned on the boots but I've really no idea how tight to set these... they are probably over-tightened if anything.

The o-rings seal the intake pipes where the pipes bolt (jis head screws) to the cylinder heads. OEM are not expensive -why substitute and risk potential problems of fit and durability ? (my opinion is that these o-rings should be considered wear items  to be routinely replaced about every couple years). See the ronayers.com parts fiche for your year.

For the boot clamps (between carb and motor) I just do it by feel - very snug but not jam tight, though probably there is a torque value specified.

[salamander]

I didn't realize there were o-rings under the carb boots... I'll check those as well. Assume a correct size nitrile o-ring will serve as a replacement.

I have an extra set of the o-rings for the intake boots, if your interested.

[noiseguy]

Thanks for the offer. I pulled up the parts fische... on several of the sites they give the o-ring dimension so that you can just buy them. ID 39.7mm, T 2.4mm. That translates to a 1 9/16" ID o-ring that is 3/32" thick.

They are not leaking, BTW, based on testing them with carb cleaner, but they are crusty. So I'm replacing them. Also replacing exhaust gaskets. I'd left the old ones in after some exhaust work so went back and replaced them with new.

Bike's float valves were worn so they are being replaced now as well (they leaked after cleaning). Floats were set about 1.5mm too high based on static bench measurement, so will adjust that too. Replacing in-line filter... my tank is a mess and I have to run the cone-o-stone to keep the carbs reasonably clean.

I don't think any of this will fix this issue, but it's enough changes that I'm going to need to see where the bike lands after I'm done.

[utzguy]

I will be interested to hear what you figure out as mine is acting very similar to what you describe. Checked all the usual suspects and still no joy. I'm also getting pretty bad mileage, low 40's mpg.

[noiseguy]

So... Christmas passed and bike's back on the road again... with the same problem.

Since posting, I've replaced the o-rings from carb boots to head. Replaced carb needle valves. Checked and adjusted static float height. Straightened out warped airbox. Still idles with idle set screw completely backed out, and idle floats after complete warm up.

I'm preparing to pull the tank, hook up a spare fuel reservoir, and hose the carbs down with carb cleaner top to bottom until I find a vacuum leak, b/c I'm at a loss to figure out what's going on with these carbs / engine. I can't see how this thing can idle with the throttle air valves shut unless there's a vacuum leak somewhere.

What's irritating is that the bike runs fine otherwise... there are no dead spots or weird behavior at other speeds.

Ideas?

[piresito]

Have you checked the choke circuit? May be a long shot, but it kindy sounds like a "allways on" choke...doesn't it?

[noiseguy]

I'll double-check the choke, but I think it's OK... seems to snap back and have enough dead play in the cable. Perhaps something going on internally...

[adidasguy]

The choke lever clips onto the choke plunger on each carb. One plunger could be off the rail and staying open or mid-way.

[noiseguy]

...One plunger could be off the rail and staying open or mid-way.

That's what I was thinking. Looks like I'll be pulling the carbs off again this weekend and double-checking things.

[twinrat]

you may need to pull the brass plug above the mixture screw and reset it say ,2 1/2 to 3 turns out.

[yamahonkawazuki]

you may need to pull the brass plug above the mixture screw and reset it say ,2 1/2 to 3 turns out.

yup tis what im thinking. as they say in the hood, "somethin aint jivin"

[noiseguy]

you may need to pull the brass plug above the mixture screw and reset it say ,2 1/2 to 3 turns out.

Already did this. Currently 4 turns out. Helped midrange, did not affect idle problem.

[adidasguy]

Isn't 4 turns out about 30% richer than normal?
And without reading all of the tread again, insure there is about 1/4" of play in the throttle cable.