YZ400 sudden loss of compression, won't start

I just bought a '98 YZ400 and took it out for a shakedown day at the family farm. Supposedly has a totally rebuilt engine after the previous owner only drained the oil(didn't refill it) and then went for a ride. Anyway, all was going fine, bike pulled very strong, I stalled it a few times, my bad. Little hard to start but, again, probably just getting used to it after my XR400. Was running great when I stopped for lunch. Came back 3 hours later, kick, kick, kick..... - no luck. Tried towing it behind our trusty XR80. Got it to fire and run for a few seconds, then it stalled when I let off the throttle, no luck after more towing. More kicking revealed a seeming loss of compression. It's was almost impossible to feel TDC, like I had the compression lever pulled in the whole time. I noticed also that the plug cap had come undone but replacing it made no difference. My guess is that it somehow is related to the compression release mechanism as there was no apparent catastrophic event that would indicate a mechanical failure. Any help is welcomed. :)

They can get flooded easily, especially if someone twisted your throttle a few times while it was sitting. Once they're flooded the gas rinses the oil off the cylinder wall and then the rings won't seal to the cylinder wall... that's why there is no compression and you can kick it right past TDC.

Do this....

1. Hold the compression release lever in.

2. Hold the throttle wide open (don't pump it, keep it wide open)

3. Hold your finger on the kill button.

4. Now, while you've got those 3 things in that position kick it over fast 10-15 times.

5. After that, release the throttle, comp lever, and kill button and then start like normal.

Check when you turn your bars. i had this problem untill i figured it out. Watch where the cable hooks onto the engine and turn you bars. Mine would fall flat on it face on some turns(not fun). If that is it just move you perch in a little. Fixed my problem. Good luck

Dan

Hopefully the cable idea will be it, but I too have had a strange compression loss problem once on my '01. I shot carb cleaner into the carb while it was on the bike and then when I tried to start it there was almost zero compression. Strangest thing I've ever seen. Anyway as soon as it started the compression was back and it's been fine ever since. If the cable idea doesn't pan out then try changing the plug, turn off the gas and try to start it. Hopefully it will fire and then the compression will return.

This happened to me on my old 400. It went away on it's own, but I'm pretty sure one of the valves was stuck open (for no apparent reason). It did'nt look like a compression realease valve or cable prob. It sorted itself out, but I don't recall how.

Sorry, for no definitive solution.

Check and make sure your carb is attached securely to the engine.

Mine came off one day (engine side) and the bike would not start (obviously) and the compression felt really weird since the intake cylce was much easier since the engine did not have to pull the air all the way through the air box.

Stuck valve. Start there.

Blatham, this happened to me once. Try to start it by kicking/pushing the kick starter all the way through the stroke - to the bottom. Then let the kick start come up 2-3inches from the bottom. Immediately, try to kick it & see if it will start then. Otherwise, check the decompression cable for binding as above

Thanks for all of the helpful advice. The winner is "stuck valve". Plainly visible with the valve cover off, exhaust valve stuck open about 5mm. Any suggestions on what might have cuased this and the best approach to repair (competent mechanic self with limited time who enjoys riding more than working, or expensive dealer shop) would be welcome.

This happened to my 99 400 also. The "bearing" for lack of a better word. On the cam had lost oil and the aluminum had galled. This let it hold the valve open. I was lucky, I was able to trim and lap the bearing surface with scotchbrite. It is now .001 bigger than the other 2 journals. But hey, just better lubrication, right? I have been riding with the repairs for over 2 months with no ill effects. I adjusted the valve lash while I was in there. The root cause I believe was my throttle plate had cracked. I had to readjust the carb slide to prevent this from happening again. In short, don't get discouraged, spend the $38.00 and buy a new head gasket. That was the total amount of money spent for my repair. Good luck and make sure you have the shop manual before starting these repairs.

Chancer.

Don't fix it. There have been many mysterious stuck open valves and they usually go away while trying to start them. Besides, you never know what the shop will determine the appropriate fix is. Could cost tons. Tap the side of the outside of the head with something small enough not to hurt anything. If that doesn't clear it, tap on the top of the valvestem. If that doesn't clear it then consider pulling the head.

mwc

Discovered the final cause of the "stuck" valve. Aparently the bike sat up for quite some time (around a year) after the rebuild. Must have developed a sticky valve that ended up kissing the piston top. Major bend. Ouch. Rode it for all of an hour, now I am looking at major bucks.

Blatham, be careful that your decomp lever isn't to close to your clutch lever, as someone could inadvertantly "clutch" the wrong lever and cause what you have described. Move it out of the way and leave some slack in the cable and you should be alright. Good luck

After washing the bike, it's a very good idea to start the engine and let it run for a minute or two to allow and water that might have gotten inside the exhaust to evaporate. After stopping the engine, bring it up to compression which closes and seals the valves...not to mention allowing the valve springs to be at their least amount of tension.

Ouch, my advise sucked. Can I change that to 'Do fix it'?

mwc

p.s., Sorry about your valves.

The valve hitting the piston with accidentally operating the decopression lever is just one of the great myths MXA has perpetuated on the unknowing masses. There is no way the piston is going to hit the valve. There is only a slight amount of undue stress on the cam lobe because it isn't in contact with the bucket the whole time so it slams into it when opening. So don't be all paranoid about accidentally hitting the lever. Your unfortunate situation was just bad luck...Sorry

I just bought a '98 YZ400 and took it out for a shakedown day at the family farm. Supposedly has a totally rebuilt engine after the previous owner only drained the oil(didn't refill it) and then went for a ride. Anyway, all was going fine, bike pulled very strong, I stalled it a few times, my bad. Little hard to start but, again, probably just getting used to it after my XR400. Was running great when I stopped for lunch. Came back 3 hours later, kick, kick, kick..... - no luck. Tried towing it behind our trusty XR80. Got it to fire and run for a few seconds, then it stalled when I let off the throttle, no luck after more towing. More kicking revealed a seeming loss of compression. It's was almost impossible to feel TDC, like I had the compression lever pulled in the whole time. I noticed also that the plug cap had come undone but replacing it made no difference. My guess is that it somehow is related to the compression release mechanism as there was no apparent catastrophic event that would indicate a mechanical failure. Any help is welcomed. :busted:

Hey my 98 minty 400 just did this today.Road all day no issues.Stopped for 10 minutes and no compression.Packed up went home.Pulled cover and noted exhaust valve stuck open on rt side near compression release arm.Also noted small radial striations on edge of bucket.Warmed up head with heat gun and bucket popped right up.Know the hammered edge was holding valve open as i could remove all other caps with magnet but had to use pliers to get this one out.Well what is happening is cable to tight on compression release and when turn bars allows cam to tap edge of bucket causing a sharp burr that causes binding in bucket bore. ordered new cap and adjustment shims and will be back together by next weekend.Hope this helps out on some of the more mysterious causes of los of compression without any horrible failure or easily spotted cause.

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