Lost Compression

You could say I am knew. Actually I was registered about two years ago shortly after the site was started by Brain. I have my bike back and thus some questions. I was tooling on it getting ready for the GNCC race this Sunday in Indiana. Kicked it over and while it was warming up it shut off (not uncommon) but when I went to kick it over I noticed that I lost the compression. It is not quite like holding in the compression lever but I can kick it over without using the lever at all. Any thoughts?



Hum.... you might want to check the compression cable slack... something may have hung, causing cable to catch and leaving the valve just slightly open.. this is the only thing I can think of.. any other suggestions are welcome... also,, just to be sure, check your plug.. I mean, check its torque in the head.. sometimes plugs back out or just do not get tight enought and allow compresion out of the plug.... other than that... you might want to scream help!!



For your sake I hope I am wrong, but it sounds like one or maybe both exhaust valves have aquired some rust on the stems form sitting so long. When the stems get rusty and you kick it over, your cam opens the valves and the rust binds in the guide and the valves stay open. Thustly, no compression. Try some WD40 down the old spark plug hole. If that doesn't work, pull the cam cover and pull the valves closed. Or......your exhaust valves could be bent from making contact with the piston. That is why I said "I hope I am wrong". Go easy

More news. There was still a little compression and it acted like it wanted to start so I kicked it a couple of times and it started up. It sounded fine so I shut if off and now the compression is back. Hmmm? The compression lever is a little stiff to pull but has never given me any trouble. Do you think it is ok to race this weekend? What else should I look at before riding it?

Maybe something was lose, or stuck (that engine hasnt circulated lube for a while) and after it ran for a few minutes it lubed itself up and rammed a few things back into place. Who can say?

Indy, This is quite a coincedence!!!! I went riding today and when I went to kick it over I noticed that I could kick it all the way through whitout using the compression release lever. I had lost compression.... This has never happend to me before. I just stood there thinking oh my god what is going on??? Well, I got the thing started (without compression) but after riding it for a few hours I noticed that I had all compression back, I mean alot!!!!! I dont know what's going on but you're not crazy this is what happen to me. If anyone knows why this happends let us know.

I had the same thing happen to my WR a couple times, I will bet the bike was a flooded, The oil gets washed off the cylinder, and you lose compression. I would pull the Hot start out, and kick it through a couple times, and the bike would start and run ok. I could shut it back off immediately, and the compression would be there.I wouldn't worry about it too much.

I've been thinking about this one. After reworking my head I weas reassembling it and when I was putting the compression release shaft in I noticed yamaha left the shaft sharp on the edges. If the shaft were to rotate too far during engagement the sharp edge could grab the bucket and leave the compressor shaft engaged, the return spring is kinda wimpy. I polished the edges of my shaft(sic) and put it together.

Is the proper free play in the cable when the compression goes away?

Is the decompression lever loose in this state?

SFO, Thank you for making me look at my compression lever. You are right(in my case). The compression release lever WAS getting caught between my Hebo clutch resivoir and my Yamaha hand guards(which I just installed). The cable doesnt have much slack in it so routing is real important. Im glad to find out before I caused damage to my valves....(hopefully)

Thanks, Dan

Create an account or sign in to comment

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

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now