Motor revs on it's own after warmup idling

[MilkJuGGz]

Hey guys,

The bike runs perfectly fine, is jetted correctly in the normal operating ranges, and has good throttle response.

I've had this happen twice on me... both times while letting the bike idle on it's first startup of the day. After about a minute of idling (yes, I know it's a long time), the engine will start to freely rev, like I'm holding the throttle 1/4 open. Then it seems like it wants to rev even higher. The only time this has happened is from letting it sit at idle for longer periods of time. I have to hold the kill switch till the engine dies; just pressing it cuts the engine out but it starts revving up again. My throttle is not sticking in any way (I just cleaned the carb).

What the heck!? No air leaks that I know of, and the bike runs perfectly fine under normal conditions.

This is in an cool desert condition. I'm running 93 octane pump gas mixed at 35:1 Mobil MX2T. Pilot is 40 (stock), airscrew 2 turns out, MJ is 380, and needle at #3.

I have my bike set up to idle for only about 3-5 seconds and then the engine dies (under normal riding conditions). However, SOMETIMES I'll come to a stop and my engine is gently idling quietly and can stay that way for a long time. What is up with the sporadic idling?

Could it be the reed valves? On a recent topend I noticed one set of reeds was reaching it's bending limit and did not completely touch the reed cage's surface at the tips.

Thanks a lot in advance!

~JuGGz

 

[Shift-MX]

You answered your own question. If the reeds do not fully close that is probably causing your problems.

 

[evenslower]

I'd be suspicious of an air leak somewhere. The reeds will only allow as much air & fuel through as the carby slide allows them to. There must be air getting into the system from somewhere else.

 

[jmics19067]

check the needle and seat in the carb also. Possible that the needle is sticking slightly not letting fuel into the carb and a resulting lean surge?

 

[500 dirtrider]

Sure sounds like either running out of fuel or an air leak. How about crank seals or a base gasket, they will do it. I would shoot a little solvent of some sort around any place where it could be leaking and listen for an idle change up or down. But BE CAREFUL if it's flamable, even WD40 will flame up if it hits a spark! How about crud in the tank clogging the screen? Just my 2 cents!

 

[MilkJuGGz]

Ok I was testing the bike again today, and after warming up for a little bit, I started off nice and slow. After a few minutes of slow riding at low rpms, I noticed the engine revving high when pulling in the clutch, which scared me. I immediately killed the engine. I DID have the choke on the whole time I was riding - forgot to take it off. The bike wouldn't start after that. Pulled plug, looked slightly wet and dark, then popped it back in. After a few minutes of wait she breathed to life normally, and I rode around without any problems whatsoever with the choke turned off.

Could a rich condition cause this as the engine heats up? Seemed like the engine might have also been flooded, but that could have happened from me kicking it w/ throttle slightly open w/ out any good spark?

It sounds like an airleak, but I don't think it is, because this never ever happens unless the choke is engaged and the engine is in the process of warming up.

~JuGGz

 

[jmics19067]

maybe your choke knob is your air leak?

 

[Reesknight]

Try cleaning out your gas tank as well as the carb and petcock. Sounds like it could be junk in your gas causing it to run lean. Just a guess, of course, but something to try.

 

[DEANSFASTWAY]

I know its an older Yam but on the newer ones they have that funky check valve in the gas cap and one time it stuck and my friend had a little problem down a straightaway in full throttle . . If youre Wr has a original round slide carb maybe the shide is worn out ? I have one of these WRs in my garage .One time on an older KX the water pump impewller spun loose and didnt circulate and the motor got hot and revved up like that , It took another minute or so and started puking out coolant . Does your YAM loose coolant? Id check the base gasket and ieven if its not leaking I like to smear a little sealer at the base of cylinder . On microsprints with Alcohol the gaskets actually corrode sometimes and then leak .Or like Rees said clean out your carb & tank screen there could be some smegma in there .One time I had a 88 YZ 250 that the P/V R/S bushing and seal was worn out and leaking and was pulling air through the crankcase into the cyl and leaning it out .If I put my finger over the breather hose the idle came down a bit .That bike was a good bike but had a lotta lotta time on it . GOOD LUCK

 

[MilkJuGGz]

Thanks for the advice guys. I'll try cleaning out the fuel tank. The carb looks brand new (just took it apart a few days ago, except for choke).

I don't see how it can possibly be a gasket that's letting air in since this never, never, never happens under normal operation. Or maybe I'm just in denial.

If all else fails, I'll just stop letting it idle for long periods of time w/ choke engaged. It gets warm enough before that anyways.