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02 yz250f. the neverending saga of BS

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02 yz250f ran awsome then the next time it wouldnt start. Kicked it for 2 hours doing different things and eventually had the hot start and choke on and got it to start. I had been messing with the idle speed on the previous ride so I blamed it on slow idle so it wouldnt start. (It fires every time you kick but wont run) but that wasnt the problem. Took it for one more ride and it ran fine. Then the next time I went to start it it was impossible so I checked the valves for like the 3rd time. Middle one had moved like one thou (was way over shimmed already) so i blamed the valves. Got it apart and put gas in the ports and my middle intake and both exhaust leaked like crazy so I replaced the problem valves (dont judge me the seats were good and it worked) and they all sealed up.no evidence of blowby on piston. Put it all together and still nothing. It gurgles every kick. Tried new plugs cut the plug wire and put the boot on again. Im going to burn this thing to the ground any advice is apreciated

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Kinda sounds like carb issues if it starts. Pull the carb and check jetting. Go to the jetting sticky on here for a baseline. Your piston rings could be shot too. One day I was riding, the next I found out the seco d ring frozen to the piston....

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I forgot to mention I pulled the carb off to and cleaned out the needle and pilot circuit. Carb was spottles inside. Rings werent stuck but the compression still isnt crazy good. No numbers just by feel. I feel like it should still start though

Edited by BloodBMcG

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Well, in my experience the next proper thing to do would be to do a compression test to see how well the rings, and your valves, are actually sealing. I recently picked up a 2003 that when I did a smoke test on, I had smoke getting past the piston, and rings, and also the intake valves. So, new piston, rings, & new valves & seats. That is the price of these high-performance off-road machines.

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I have a comp tester but of course it doesnt fit these dumb motors. Will have to find one. All I want to do is get it going so I can sell it as I bought a yz250 (which blew the crank bearings on my second ride) is this piston really that bad?IMG_20140528_231942.jpg

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What did you do to check to see if the problem was electrical? Do you have spark?

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I have spark. It fires while kicking but wont run. Is It me or do those rings look broken? I checked if they were stuck or not and looked for sighns of blowby but it almost looks like the end of each ring is broken? How much gap should there be uncompressed

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Only have 100psi compression. From what iv found I should have 180-200

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QU. "Rings are stuck..."??  first indicator  won't seal properly if they don't rotate in the piston lands.  Also, look at scoring on piston skirt??  second indicator   How does the cylinder look??  need pics.... indicator.   How much experience do you have with these??  Did you read the FAQs??

 

When an engine overheats (as in when the radiator has run dry) the aluminum parts swell up. 

I just re-built my son's '02 (after all, it was 12 years old) and we replaced piston, rings, pin, clips,... you get the idea, we did a top end on it.

You gotta do that every so often on the RACE BIKEs.  They are high maintenance and highly stressed engines.  Book says piston every 50 hrs of operation. 

Edited by ronbuell

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Well the cylender is mint not a vertical mark to be seen even the cross hatch looks nice. Never been overheated since iv had it anyway (3 years). rings werent stuck at all, im not an idiot.

The "scoring" on the piston cant even be felt. I blame it on the steel oil filter on the old bikes.

But you did sound pretty smart there for a minute

Edited by BloodBMcG

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Also you can't just look at a piston & know whether it is "good' or not. You need to measure the piston in various spots around it's circumference and then measure your cylinder internally, then subtract the smaller piston measurements from the larger cylinder measurements to get your clearances.

 

A manual for your bike will detail exactly where & how to measure the piston & cylinder, and will give you specifications so you can determine whether you are still within spec or not.

 

Just looking at the skirt on that piston, I personally would not re-run it regardless of what it specs out at. You can see significant wear on the skirt. Also, if you look in the cylinder down below where the piston stops, if you see any black carbon or residue, that is blow-by and is a good sign the piston/rings are not sealing the bore.

 

Do I get a cookie for recommending the compression test?

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Haha sure. I would have done one before anything but I couldnt find one that fit. Ended up cutting up a sparkplug and soldering it to a piece of pipe and hooking it to my tester. My valves were like a jacuzzi so I jumped the gun on those being the culprit

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And if I may recommend something else.. After you get the new piston & rings installed do another compression test and record those numbers somewhere so you can reference them later. Also, buy an hour meter and install it immediately. That way you can track the actual hours on piston & rings. When you hit 30, then 40, then 50 hours you can do a compression test to see how much compression you have lost. You might be able to stretch it more than 50 hours but you at least need to know, and record, the compression approximately every 10 hours of ride time so you know when it is time to replace the piston & rings. Since you have a custom tool for it now, no reason not to use it.

 

This is the practice I just started after losing a piston skirt on my 2 stroke that cost me a lot of money to repair. Scary thing is that even if I had been checking compression every 10 hours there would have been no indication that the skirt was going to separate.

Edited by Kawabuggy

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Im doing exactly that. On my yz 250. Got a full rebuild and hour meter coming. But as soon as this thing runs its going for sale ( after I break it in)

I refer to this bike as boat anchor#1 and my yz250 is boat anchor #2

Edited by BloodBMcG

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So turns out my piston was worn just past the skirt limit so I think it was just piston slap making the rings not seal. We will see when it gets here

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Well ot runs great now but it makes a loud ticking noise from the valves. One valve is pretty loose, the one I replaced. Could this be causeing my ticking noise? Im planning on shimming it but the store is closed now

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what is a loose valve going to hurt? Other than performance. Im not reving it to the moon. Just starting it

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