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Old 03-03-2014, 01:33 PM
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Default 2002 yamaha wr426 low compression

     

i have an 02 wr426 and have been having a lot of problems with it lately. im trying to get it running by spending as little money as possible. i did a compression test on it and only got about 50 psi. the manual doesnt specify doing this test so i imagine i should be getting somewere around 180 psi. i also did a leak down test and got 100% leak and heard air coming from the airbox which i know tells me that it would be a leaking intake valve. i pulled the head back off, cleaned and buffed off all the carbon build up, made that squish area spotless and filled the intake and exaust ports with water...of coarse nothing leaked out of the valves so i am to guess that theyre good. i also pulled the rings off, measured and inspected the end gaps and clearances and theyre also good. again i cant really test them because the manual doesnt specify a compression and leakdown test, also this bike doesnt have a crankcase breather so i have no way of knowing if the air would be leaking past the rings. i have also replaced all top end gaskets, timed the motor perfectly and got my valve clearances within spec. basically i have all my bases covered and still am not finding an answer. so IF THERE IS ANYONE OUT THERE who has the same bike and has faced similar problems could you PLEASE help guide me in the right direction so i may get this god forsaken bike running again. i would greatly appreciate any help at all
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Old 03-03-2014, 02:32 PM
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When you did the leak down test did you make sure the piston was at TDC and all the valves were closed? What happened to your crankcase breather? It came from the factory with one. Are you running stock cams or do you have an auto decompression cam?
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Old 03-03-2014, 02:39 PM
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If you have auto decomp you won't get a good PSI reading. Same question with the leak down test as how the cam's are positioned will change things.
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Old 03-03-2014, 03:38 PM
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yes i still have manual decompressor, and yes i know for a fact the motor is timed perfectly ive timed this motor over a doezen times, and idk if the bike has a crankcase breather. i dont see one on the motor anywere, the manual doesnt specify one and so i dont think it has one cuz it has 2 rubber hose breathers coming off of the top of the valve cover, one that directs back to the frame cuz the oil is stored in the frame, and the over vents oil overflow to the ground
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Old 03-03-2014, 05:01 PM
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2002 Yamaha WR426F WR426FP Parts, 2002 Yamaha WR426F WR426FP OEM Parts - BikeBandit.com
Parts 21, 25, 23 are the parts for the breather.

Remember that "water" will not go all places that "air" will. I suggest that you do have valve issues.

Paw Paw
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Old 03-03-2014, 06:43 PM
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yea you're right. thanks for the tips. i guess im gonna just stop being a F@#Q jew about it and fork up the money to rebuild the top end, new gaskets, rings, valve lapping, springs, guides, re-shim it. if this doesnt help im just gonna push this POS off a cliff
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Old 03-03-2014, 08:10 PM
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You can't lap the valves on these engines. You have to have the seats cut and maybe replace the valves.

Paw Paw
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Old 03-04-2014, 09:43 AM
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If the ports will hold solvent without leaking past the valves (you used water, close enough) then there is no good reason to tear into the head. That's assuming that the valves aren't worn beyond spec, which the valves on that bike have a good history for durability.
Do you know how many hours/miles are on the bike? What else have you tried to resolve the no start issue?
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Old 03-07-2014, 09:49 PM
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ive done everything by the book as far as testing for cylinder leakage. and yea your right i ment to say cut valve faces not lap them. the last test i havnt done yet is valve clearances which i dont think i shoud have to change. spec is .10-.15mm on intake, .20-.25mm on exaust. right now my bike is sittin at .11mm on intake and .20 on exaust which is good i would imagine. but i have a feeling that perhaps they are just a tad to tight and are holding the valves slighty open even when on tdcc. however i had checked the valves last summer and they were at the same spec, bike ran like a champ up until now. so im just looking for other things i can check and test before i dump more time and money into this *****
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Old 03-08-2014, 11:09 AM
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If you got clearance there and the valves are free there is no way the valves can be held open.
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Old 03-08-2014, 12:09 PM
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Am I missing something here....
What is actually wrong with the bike? No start I'm guessing....
So in all your troubles in that topend have you thought to check anything else related to starting issues.....???

---------- Post added at 06:37 AM ---------- Previous post was at 06:35 AM ----------

So did you do the leak down test before or after you pulled the head apart??

---------- Post added at 06:39 AM ---------- Previous post was at 06:37 AM ----------

If your confident ( or sort of confident ) how about just check the other factors that would cause this to be a non starter...
At least for peace of mind you can rule out other factors...
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Old 03-10-2014, 11:47 PM
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its a no running issue. its got spark and fuel. just low compression. ive gotten it to fire a couple of times i just cant get it to stay running. and yes i did do the comrepssion and leakdown test before i pulled the head apart
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