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'99 YZ250 Stutters at mid throttle

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The bike starts easy, idles fine, and generally accelerates fine.  But when riding at a constant speed, the bike stutters real bad.  I've tried to lean it up a little, but to no avail.  Could this be reeds?  Would it be the pilot jet?  It usually runs fine WOT, but recently is starting to stutter even at WOT.  I've changed jets, clip position, air screw position, but I'm stumped.  I ride my sons CR250 2T and it runs great.  Why can't my YZ??

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I don't know, but add crank seals to your list of things to check out.

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15 hours ago, Kinger317 said:

I don't know, but add crank seals to your list of things to check out.

Bite your tongue, this was supposed to be an easy fix!  Actually it makes a little sense, the bike seems to run rich, and is kind of cold-blooded.  It blows a lot of blue smoke on when cold until it is up to operating temp.  So this could be tranny oil leaking into the crank case I guess.

Edited by Marc Fonger

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Check / adjust the float height, a too high level will richen up each metering circuit.

 

Inspect the nozzle in the bottom of the venturi where the needle slides in,

if it's worn oval shaped, it will cause an overly rich condition that makes any jetting / clip height change un-responsive.

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I'll check the float level today and let you know.  

 

How do I check the crank seal?  Or should i just plan to replace it?

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If you don't remember when they were last replaced,

it's probably a good time to install new ones.

 

A worn out RH side might makes for a rich running engine,

the danger is a worn/damaged LH side which if severe enough could cause a seizure by sucking in air.

 

You can test them by doing a crankcase leakdown test.

Edited by mlatour

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I bought this bike about 18 months ago, so I went ahead and ordered both cranks seals and the shift shaft and oil seal.  i'll tear it down later this week and put it back together.  i did check the carb tonight for float height, it was off a little, and there was a piece of hair or something in the pilot jet. But after test driving it, no real change in how it runs, but the idle is noticeably higher. The bad thing is when I took it for a test ride I lost 5th gear. . . . . .sunuva!  Also noticed an old oil leak opened up in the tranny.

I think I can get the oil leak fixed tonight :) with some redneck engineering.  Not sure what is up with 5th gear, maybe because the shift axle is bent, from a recent chain wrapping experience (which is also what caused the oil leak).

 

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Check the case where the shift shaft comes out (behind the shift lever) to make sure it's not cracked.

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On 7/2/2017 at 1:59 PM, Marc Fonger said:

The bike starts easy, idles fine, and generally accelerates fine.  But when riding at a constant speed, the bike stutters real bad.  I've tried to lean it up a little, but to no avail.  Could this be reeds?  Would it be the pilot jet?  It usually runs fine WOT, but recently is starting to stutter even at WOT.  I've changed jets, clip position, air screw position, but I'm stumped.  I ride my sons CR250 2T and it runs great.  Why can't my YZ??

Stutters at constant speed, yet it accelerates fine.  That's got me curious. 

Is there a specific throttle position or RPM range where it stutters???

Just a guess, but I'm wondering if your PV is stuck open and this stutter issue is at low RPM only.

You might as well check the reeds anyway.  Also remove the exhaust system and check for blockage.

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I rotated the reeds the last time I had the carb open as one side had a little gap that I hadn't noticed before.  The stutters seems to be worse mid RPM range and about 1/2 throttle. But is always present when trying to hold a steady speed on flat ground.  I road about 20 miles a couple of weeks ago with my son.  I had a heck of a time keep the bike from stuttering, so I was constantly speeding up and slowing down. His bike (CR125) had no issues.

 

I have not checked/cleaned the exhaust or PV in a while.  Have cleaned the PV twice since owning it, and there is a lot of spooge, so I'll check when I tear into it to do the crank seals and shift axel/shaft.

 

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Since you tried jetting, etc.  It could be a PV malfunction of some kind. eg. one side valve open and the other shut. Or both stuck open.  Give the top end mechanical health a good check over. 

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***UPDATE***

 

I replaced the crank seals, both sides and she runs much better.  Remarkably reduced the smoke from the exhaust.  She also seems to have more power.  Here's a list of what I've done recently:

 

1) Turned the reeds

2) Adjusted the float height

3) Replaced wetside crank seal (no apparent damage or wear but since there is almost no visible smoke, I'd say it needed to be replaced!)

4) Replaced the dry side crank seal, the fly wheel had some black sludge, probably premix that had been drawn in from a leaky seal

5) Replaced the shaft shaft (now have 5th gear back!)

 

The bike does run quite a bit better, but I wouldn't say it is "fixed".  I did not tear into the top end, I did take a peak through the exhaust port and the piston and cylinder appear to be in good shape (I put a new topend in it 18 months ago).  The linkage and PV seem to work, but I did not open it to see if PV itself is working.  Maybe I'll tackle that one next week.


Thanks everyone for your suggestions and help.

 

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6 hours ago, Marc Fonger said:

The bike does run quite a bit better, but I wouldn't say it is "fixed".

A full top end check would be worth it.  Just put a new piston kit in it anyway. With a new plug of course.

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