CSL

Member
Oct 30, 2000
86
0
Need help diagnosing a problem.

Problem:
About every second or third time out on the bike, the throttle will just take off and rev to the limit while I'm riding. I roll off the throttle but it has no effect. I have to save myself from crashing by clutching it and hitting the kill switch. This has gotten me into trouble on more than one occassion. I now have no confidence taking the bike out on technical trails (like Moab) for fear of shooting off a cliff.

I know that a piece in the throttle assembly was recalled shortly after the release of the bike. This has been replaced with the correct part. Actually, I have now replaced the ENTIRE throttle assembly and the cable to the carb. Basically, everything is brand new between my hand and the carb. Problem has not gone away.

I know that a vacuum leak can cause the bike to idle high. But can it cause an intermittent high rev that can't be controlled by rolling off the throttle?

I replaced the reeds last week and thoroughly cleaned the reed block and mounting surface while doing so. Problem has not gone away. I'm going to inspect the reed block surface one more time to make sure it's not warped.

Anything else I should be looking at? Maybe a spring malfunction inside the carb (carb has been thoroughly cleaned)?

What's the best way to track down a vacuum leak?

About ready to give up on this basket case before I become one...

BTW, I should clarify that I would not put this in the "surging" category. It is more like a full-tilt "pinned". With no way to control it except hitting the kill switch.
 
Last edited:

Rcannon

~SPONSOR~
Nov 17, 2001
1,886
0
biglou said:
I think there was a recall on this back in 2001. There are threads, let me see if I can find one.


I dealt with this recall on my YZ last year. The recall was regarding "sticking" throttles. I dont recall it being an issue of the throttle opening by itself. The fix was a new upper wheel and a new cable , if needed.
 

CSL

Member
Oct 30, 2000
86
0
Hey guys,

Thanks for the comments, however, I have already taken care of the recall issue (replaced throttle assembly all parts including the cable wheel).

I'm looking for something more hard to find...

Any help would be greatly appreciated!

BigLou? You going to Reno again this year?
 

Detonator

Member
Jul 7, 2003
241
0
CSL...a number of us have been through this before with that vintage of YZ. Go to the search function and type in "throttle sticks wide open," and scroll down to a post started by Mark Sims.

It should take you step by step through the process. I revived the post to make it sort of a resource, since I nearly bought the farm when my bike revved to the moon after a jump landing. Hope it helps.

Will
 

jeffh

Member
Jun 2, 2002
11
0
CSL, My 03 kx250 stuck WFO on the trail and I went down the hard way!!! It had stuck 2-3 times since new but I hit the kill,restarted & it would run fine. Over the course of 5-6 more rides it stuck at will during which time I had a qualified mechanic...
-Inspect & clean carb (3 times)
-Install PC carb vent & PC filter seal
-Silicone seal boot connection to airbox
-Replace throttle cable, housing & tube
-Check cable routing
-Check for air leaks in bottom end
I called one of the Midwest's top two stroke tuners and he was sure it was "a piece of dirt or grit in between the slide & the body & that it could be like finding a needle in a haystack".Made sense to me.
The factory "wild guessed" the Throttle Position Sensor may be faulty.Defaulting wide open? Scary thought.
After 32 years of riding I was not enjoying it, wondering when the RIDE may be on. I decided to try a new carb before trading in.
After a 10+ rides with new carb all seems fine.
It has changed how I ride & think on a bike.
Good luck & I hope you find the fix.
 

Highflier

Member
Nov 27, 2001
20
0
I would spend the $30 or so and buy the Motion Pro throttle assy. The Yamaha can and will hop the cable over the wheel in the stock housing. Definitly, check the throttle return while steering the bike full left and right on the stand, as mentioned above, make sure it is snaping closed and not binding.
 

SN3AKYPETE

Member
Jul 22, 2018
1
0
Hey, I think I had the same issue as you on my new YZ250X . When I’d roll on the throttle the bike would run away ( rev out) on its own . The throttle tube was full returned , no binding . It’s as though there’s a vacuum leak only when the engine is under load ( when the engine is is creating the most vacuum ) Did you ever figure it out ? I’m worried it’s a leaking crank seal !
 

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