400 Engine problems, NEED HELP!!

Ok guys here is the story, I bought a 99 Yzf400 about 1 month ago, rode it twice the first day, about 2 hours in the morning & 1 hr that night, on the second ride the bike died, turned out that it bent an intake valve. I fixed the valve checked the clearances, one little nick in the piston so I left it alone. Got the bike running again took it up to the mountain it ran for about 5 - 10 minutes up the road and then 'clink' it died again! This time it broke a valve and chewed up the piston and head. Let me say here that I was sure I had the timing right, the bike would start, idle & rev just fine, it rode pretty good but started to develop a miss just before it died. That isn't the end of it, last night I just put it back together again, new piston, different head with a couple new valves, checked all valve clearances again, triple checked the timing, got it running again, took it back up to the mountain and the same thing happened, valve broke, piston and head screwed! What is wrong with this thing. Anyone have any ideas???? Is the crank somehow bad, could the timing chain be stretched out, could the tensioner be bad?? Please help this bike has now cost me over $3000 in purchase price and repairs and it is broken again. Thanks in advance. :applause:

You bought a used high strung motocross bike, with god knows how many hours. You are doing incremental replacement of high wear parts and reaping the rewards of said half assed work. Not to be mean dude, but here are your options.

1. tear the engine completely down and R & R everything. I would suggest paying someone to do this as it seems you have bad luck replacing obviously worn items, ie the piston, other valves, the springs, the timing chain etc.

2. Part out the bike and buy a NEW one.

You bought a dog tired used, nearly 7 year old race bike.

Does anybody have any constructive ideas? I am looking for help, not ridiculous suggestions and insults on my mechanical ability. didn't your parents ever tell you ' if you don't have anything nice to say don't say anything at all.' The bike was put back together with all settings and tolerances just as the manual specifies, I am not just winging it and guessing at things.

I don't want to pour any more money into this thing until I have a direction to solve this problem. Why a bike can run fine for 10 minutes with no problems and then just like that, snap a valve, I don't know.

Thanks for nothing MotoGoalie.

I don't want to pour any more money into this thing until I have a direction to solve this problem. Why a bike can run fine for 10 minutes with no problems and then just like that, snap a valve, I don't know.

Thanks for nothing MotoGoalie.

I just gave you your answer. If you don't like it, change a valve out or something. :applause:

It bent a valve becuase the cam chain was too stretched out.. why didnt you replace it the first time?

I talked to my local Yamaha dealer today and they seem to think that it is also because of a stretched out chain.

Silly sully :applause:

Did the same valve break, or was it a valve on the same cam that broke - intake or exhaust?

It seems to me that the chain would have to be extremely stretched to cause a valve to hit a piston unless the tensioner is at it's limit and not taking up the slack. If the timing marks are lining up when you put the engine back together it can't be too bad. The fact that you can change the WR exhaust cams 1 tooth without the valves hitting means there's a bit of cushion there.

You installed a different head, did the head come with cams or did you use the cams that you had? I've heard that the stock cam sprockets can slip on the cams. Your marks would be lined up but the lobes would be in different positions. If you have aftermarket cams with slotted sprockets, they may be off. Checking the cam timing with a dial indicator is the only sure way to tell.

It may also be possible that a valve is sticking or a bucket is hanging up. Since the engine runs, the initial timing must be close - no valves hitting. If your cam chain and or tensioner is letting the cam timing float around that could cause the problem. If the clearance closes up too much when the engine gets hot it could be the problem, but you say the clearance is correct.

I've been having a strange intermittent compression loss problem with my 400, usually happens under full load. I have new cams and chain but the problem is still there. I'm thinking it must be a bucket hanging up in it's bore and holding a valve open. Maybe a bit of metal or dirt, or maybe the top end isn't getting enough oil at high RPM causing the buckets to dry up.

I haven't bent or broke a valve yet but the engine has died and had zero compression when I tried to restart. After kicking for a while the compression comes back 100% and the bike runs fine. Once when it quit the engine would only turn over a bit, I could feel a definite stop like a valve meeting a piston, but it gradually loosened up and the engine restarted.

Check the sprocket on the crank.... I have seen this before on a 2000 426... Everything looked good up top and worked great, but the sprocket on the crank was wore out and was also missing a tooth... Unfortunately it ment a new crank.... The chain tensioner went bad and allowed the chain to slop around....It was strange because it didn't wear excessivly on the cams..

Did you check the timing when you pulled it back down? Are you sure you are getting oil to the head? quick check is to fire the bike and crack the top bolt loose on the oil line to make sure you have oil at the head...This is not a guarantee but a little more assurance....

Check to crank... :applause:

Thanks 'mkporn' and 'frostbite' for your suggestions. To answer some of your questions: They seem to be stock cams and the valves that are breaking and bending are intake valves and not the same valve. I can't check the timing when I pull the bike back apart because the bike is seized with a broke valve head so I can't rotate the motor to check the marks. There is definitely oil going to the head. I will check the crank sprocket though. I am now thinking that I will probably just take my losses and part the entire bike out rather that sink more money into it. It is a shame because the rest of the bike is in great condition but I just can't bring myself to spend another 500-600$ to get it running again and have to hope that it doesn't do it again. Taking it to the dealer is out of the question, they want $3000 to put it back together with new Yamaha parts. Thanks for your help guys, if anyone knows where I can track down a complete running motor from another yzf or wr please let me know. I just missed one on ebay this morning, it went for $1000. I bid $960. Crap!

Bring it down to Spokane and I'll give you a quick hand in figuring it out.....That is if you get into the area very often....Where at in BC are ya?

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