Jump to content
Sign in to follow this  

2002 YZF 250 Another valve question, I think

Recommended Posts

Hey guys, I'm working on a 2002 YZF 250 I traded for. It was not running when I got it, when kicking it has a hard spot (not compression, harder) you can push though it and then it becomes easier. We did manage to get it to crank by rolling it off. It felt sort of like a clutch that is about gone, it would not pull on the bottom but pulled hard on top. Also heard a loud tapping sounded like the left side but cant say for sure so we did not let it run long.

It reminded me of old EXC 450 when I rebuilt it and got the timing one tooth off, ran very similiar. So I tore it down this morning, I pulled the plug and it still had the hard spot, checked the timing and the exhaust was one tooth off, so I corrected the timing. The motor then pushed through very smoothly, put it all back together and when I put the plug back in the hard spot came back! I kept playing with it and figured out that when the decomp lever is pulled it is very smooth. I know this sounds like its just compression and I just need to suck it up, but it just honestly feels like its binding my none of the 450 I have rode are this hard to kick. Could it have bent a valve when out of time? If so (please correct me if I am wrong on this) the 5th valve is the decomp valve (I think) and by pulling the lever would lower the valve then in theory it should not be making contact with the piston when the lever is not pulled. Sorry for the long post guys, but jjust trying to get all the details. Thanks in advance for the help, any advice is appreciated.

Cody

Share this post


Link to post
Share on other sites

I would highly recommend you don't run or kick the motor until you pull the head and inspect all of the valves! Yes you could have easily bent one of the valves.

A bent valve will leak, a leaking valve will rob you of power, burn up and then snap off taking the piston, head, and your wallet with it.

I would pull the valves and inspect them based on what the manual says. That's measuring them and checking for roundness. You'll most likely have to replace the one that got kissed by the piston (check that, too!). I would replace all of the valves, if not that, then replace all of the kind that got tweaked (intake or exhaust). I suppose you could replace just the bad one, I'm noit sure if that's a no-no or not, but my OCD would prevent it.

While you're at it, judging by the age of the bike you will probably want the valve seats cut. A new cam chain is probably in order. I would also throw on a manual cam chain tensioner for good measure.

While it's apart, check the piston, crank, and cylinder wall.

  • Like 1

Share this post


Link to post
Share on other sites

Thanks for the input, I'm planning on tearing it back down this week. It still just bothers me am I incorrect thinking when the decomp lever is pulled it should be harder to kick (in the idea a valve is messed up, not due to compression) or am I wrong on it opening the 5th valve when the lever is pulled? Thanks for the help again!

Share this post


Link to post
Share on other sites

Before you read on and get bored, make sure you read my comment at the bottom to convert your decompression nightmare into an auto decompression dream that you will be able to start while wearing flip-flops.

It sounds like you do not understand how the decompression system works and because of this you do not know how to start your machine. Now worries, I am trying to help not insult. I had the same thing when I bought my 2002 YZ250F. The guy I bought it from did not explain it to me good enough so I read the service manual. The decompression lever does open one of the exhaust valves a little bit. Not much more than is needed to vent the high pressure from the compression stroke. The idea is to relieve only a portion of that compression.

The procedure is to find the hard spot in the compression stroke, take your foot off the kick starter (at least until you get the hang of it), pull in the decompression lever and rotate the crank about ten more degrees only, let off the decompression lever, let off your foot off the kick start thus letting the kick start to go back to the top of it's stroke (not the top of the piston stroke, but the top of the kick starter stroke), then give it a good long hard kick. Just so you understand what is going on here is that let's just say for example purposes only, you would normally get say 220 psi of pressure from a full compression stroke. It is tough to kick this kind of pressure so you let some of it off to get a fighting chance to kick it over. This time, with the decompression lever pulled in for part of the stroke, you get for example purpose only, say 160 psi. This is a lot easier to kick over. Again the pressure I stated are example only and not actual.

All that being said, I am pretty sure that the valves would not make contact with the piston if the cam was only off by one tooth. Does it turn freely with the decompression lever pulled in versus not pulled in? You did not mention if the bike pulled well at the bottom after you corrected the valve timing issue. I can't hurt to check everything out though I guess.

2002 was the last year the YZ250's used the decompression lever to aid in starting. In 2003 they came up with auto decompression. It works by having a small plunger near the exhaust lobe of your exhaust camshaft. It opens an exhaust valve for a predetermined amount of time during a certain part of the compression stroke. After the machine starts, the plunger get moved inward away from the valve lifter using a build in mechanism which uses centripetal forces of rotation. The best of it is that the rest of the camshaft is identical for 2003 and 2002 and up. All you need to do is to install a 2003+ exhaust camshaft be it OEM or aftermarket, like Hot Rods, etc. Well you should check the valve clearances too but a lot of people doing the conversion, including myself, checked and did not need to re-shim the valve.

I hope I helped at least a little bit. I think I spent about $125 on the conversion. Kicks over very easy now.

  • Like 2

Share this post


Link to post
Share on other sites
Before you read on and get bored, make sure you read my comment at the bottom to convert your decompression nightmare into an auto decompression dream that you will be able to start while wearing flip-flops.

It sounds like you do not understand how the decompression system works and because of this you do not know how to start your machine. Now worries, I am trying to help not insult. I had the same thing when I bought my 2002 YZ250F. The guy I bought it from did not explain it to me good enough so I read the service manual. The decompression lever does open one of the exhaust valves a little bit. Not much more than is needed to vent the high pressure from the compression stroke. The idea is to relieve only a portion of that compression.

The procedure is to find the hard spot in the compression stroke, take your foot off the kick starter (at least until you get the hang of it), pull in the decompression lever and rotate the crank about ten more degrees only, let off the decompression lever, let off your foot off the kick start thus letting the kick start to go back to the top of it's stroke (not the top of the piston stroke, but the top of the kick starter stroke), then give it a good long hard kick. Just so you understand what is going on here is that let's just say for example purposes only, you would normally get say 220 psi of pressure from a full compression stroke. It is tough to kick this kind of pressure so you let some of it off to get a fighting chance to kick it over. This time, with the decompression lever pulled in for part of the stroke, you get for example purpose only, say 160 psi. This is a lot easier to kick over. Again the pressure I stated are example only and not actual.

All that being said, I am pretty sure that the valves would not make contact with the piston if the cam was only off by one tooth. Does it turn freely with the decompression lever pulled in versus not pulled in? You did not mention if the bike pulled well at the bottom after you corrected the valve timing issue. I can't hurt to check everything out though I guess.

2002 was the last year the YZ250's used the decompression lever to aid in starting. In 2003 they came up with auto decompression. It works by having a small plunger near the exhaust lobe of your exhaust camshaft. It opens an exhaust valve for a predetermined amount of time during a certain part of the compression stroke. After the machine starts, the plunger get moved inward away from the valve lifter using a build in mechanism which uses centripetal forces of rotation. The best of it is that the rest of the camshaft is identical for 2003 and 2002 and up. All you need to do is to install a 2003+ exhaust camshaft be it OEM or aftermarket, like Hot Rods, etc. Well you should check the valve clearances too but a lot of people doing the conversion, including myself, checked and did not need to re-shim the valve.

I hope I helped at least a little bit. I think I spent about $125 on the conversion. Kicks over very easy now.

No insult taken at all:thumbsup: I'm more then willing to take all advice. Your right most fourstrokes I have owned have had auto decompression and had valves in a XR or the KTM RFS. (I believe these are called bucket valves?) I went ahead and checked valve clearances while I tore it down, I have not had it started since adjusting the timing so not sure if it runs better yet or not. I believe I will put it back together and try starting it using the decomp lever as you suggested before I tear it all the way down only to fin out its okay. If something was to happen and catastrophic failure was to happen (Lord forbid) I can still part it out and probably get my investment back but riding it sure would be alot more FUN!

If I can get it to start and see that it is going to run alright then I will invest in the newer cam, my wife will be riding it so I know she would rather just kick it and not have to worry about using the lever. Thanks for taking the time to write all the info out, I greatly appreciate it, I'll give a update of how it all worked out over the weekend.

Share this post


Link to post
Share on other sites

Well fellas I believe it "was" a main crank bearing. Checked the valves all in spec. Got it started rode for about 15 mins. then that was all she wrote bottom end locked and now the kick want budge.lol you win some you lose some. Havent had a chance to tear it back down to know for sure. I think we have just decided to part it out, itll take a little while but who know we may even profit a little. Thanks again for the help guys, I'll let you know for sure what happened when I tear it down until then just pass the word if you need parts I have them.:thumbsup:

Share this post


Link to post
Share on other sites
Well fellas I believe it "was" a main crank bearing. Checked the valves all in spec. Got it started rode for about 15 mins. then that was all she wrote bottom end locked and now the kick want budge.lol you win some you lose some. Havent had a chance to tear it back down to know for sure. I think we have just decided to part it out, itll take a little while but who know we may even profit a little. Thanks again for the help guys, I'll let you know for sure what happened when I tear it down until then just pass the word if you need parts I have them.:thumbsup:

That really, really sucks. Sorry to here that. Would you PM me if you do part it out. I might need a couple of small things for my 2002.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×