Wacky High Idle Problems!

[VermontNewbie]

This is on my Yamaha Maxim 700 AS WELL AS my GS 400. Both have cleaned well jetted carbs, both have new carb boots that do not leak. The problem is when I set the idle to 1000 to 1300 ish and I rev it, the revs stick at 3 grand. The choke is also off on both. I'm at my wits end trying to solve this. When the idle is set below 1000 it doesnt happen at all! Someone shed some light!

[VermontNewbie]

To the top for some help!

[KEITH KRAUSE]

I'm not sure. Sounds mechanical. Just a little less idle adjustment solves the problem? But I assume the bike won't idle then, correct?
Try this first. Warm the bike up. Let it idle at about 1,200 rpm and don't rev' it. Then adjust the mixture screws to achieve the highest idle possible. Hopefully your mixture screws will turn OK. Turn the screws slowly and when the rpm's stop rising, stop turning. After all the screws are set, re-set the idle to about 1,100 rpm's with the idle screw knob.
If that does'nt do it, considering that you say the carbs are clean, etc, I would synch the carbs manually and follow that with a vacuum tool synch.

[Anonymous]

high rev problems at idle

had same problem with gs 550 found out the air jet screws needed to be set at 3 1/2 turns out on the carbs on I have. was a little rich so I turned them back 1/2 turn idles and runs great

[KennyJ]

Sound to me like the pilot jet is too big. Thus when you transition from the idle circut to the pilot, you are running too rich. I could be all wet, but my 85 550 was this way with being only 1 size too large.

Kenny

[KEITH KRAUSE]

Sound to me like the pilot jet is too big. Thus when you transition from the idle circut to the pilot, you are running too rich. I could be all wet, but my 85 550 was this way with being only 1 size too large.

Kenny

There is no transition from the idle circuit to the pilot jet. There is no "idle" circuit. Your bike idles on the pilot circuit. The pilot jet allows fuel in through a main passage. A second passage is regulated by the mixture screw. The screw allows fine tuning of the mixture.
I don't see how too large of a pilot jet could cause this problem. :?

[KennyJ]

Ok, I thought I might be wrong on this one (I was going back to my experience on the Weber). Now let me jump in a little deeper...

The pilot jet is the fuel supply for Idle up to main jet transition. Granted the idle circut provides the fuel for the idle circut, but doesn't the screws actually adjust it (lean it out) further for idle? Does it really matter what size the pilot jet is when it comes to idle? From my teardown of the CV's on my GS, I think I could run any size pilot (well it would have to be a minimum size on up) and get it to idle with the screws.

I personally had a similar problem, and a guy at work had the exact same one on a V-Star (after he popped in a K&N and jet kit). The Yamaha mechanic said it was a common sympton of rich jetting. They rejetted his and it was fine, I needed to do a complete carb clean anyways and just replaced the jets and mine worked also (though mine could have been other problems, since it did many things to mine).

So what do you think about the pilot/idle theory? When I talk about transition, doesn't it transistion from the idle mixture screw circut to the pilot circurt, once the transfer holes/slots are uncovered by the butterflys? I am curious, for personal reasons only...I like to know the workings of mechanical things.

Thanks
Kenny

[KEITH KRAUSE]

The pilot jet allows fuel to go through a main passage into the carb throat.
This main passage has a secondary branch to the carb throat. This second passage is regulated by a mixture screw. This screw allows you to fine tune the total amount of mixture entering the carb throat. It's limited in how much effect it has. It's range is perfect for the factory. It can be adjusted to compensate for "minor" intake changes.
The pilot circuit allows the bike to start and idle. As you open the throttle, the needle circuit immediately kicks in. The pilot circuit has an overlap effect with the needle circuit from just slightly opened throttle to about 1/4 throttle. As the throttle opens, the effect of the pilot circuit tapers off at the same time the needle circuit's effect increases. There are many charts out there to show you this transition.