weird - compression loss

Can any of you push the kickstarter through TDC without the decomp?

I've just noticed that mine will go through when the engine is cold, but not when it is warm. I think it may be time for a new piston and ring. I figure that the compression returns when the engine is hot because of expansion of the parts.

But, the weird thing is that is starts perfectly, runs perfectly, the front still lifts easily and there is no sign of any smoke. Which has me wondering if it's a valve issue. I submerged the bike a few weeks ago, so maybe a valve is gummed up and staying open after the engine sits cold for a week. The decomp cable adjustment is right.

First thought was that I've scorched the piston/ring/head when I submerged it, but the compression returning once it's warm has me baffled.

A new piston was put in 6 months ago, and it's had an easy 6 months of cruising.

Any thoughts?

Mine was doing the same thing. I have the valve cover off right now. Both exhaust valves are dead on the money in tolerance, but 2 of my intake valves are a little tight (about .005 standard out of tolerance). Maybe we are due for new rings? I haven't done mine in about 6 races (MX). You say you have 6 months on yours, but you could be due. Where's Grayracer? He'll know.

I'm sorry, what??

Oh, um, compression, well...

Check and correct the valve clearances first and see what happens. I've seen some really old, tired engines act that way because of rings losing their tension, but no big YZF's that I know of.

Heh heh, see I told ya he'd know what to do... :naughty:

Thanks lads.

Valves were reshimmed about 25hrs ago, but I shall check. First things first I guess.

If it is the valve clearances, any thoughts on why compression seems normal at operating temperature?

ok, search revealed a few common themes. Here's my novice view of most likely scenarios for me...

1. Worn ring. Since compression returns when engine is warm, I'm betting on this one.

2. Valve clearance out of spec. Re-shimmed not so long ago. Will check this first.

3. Stuck valve. Need to do a leak down test for this.

Damn. Looks like I'm out of action for a few weeks while I tear this down. Never done it before. I'm scared.

I got faith in ya. Looks like i'll go ahead and do the same thing.

Might aswell pull the whole bike apart and go crazy with the grease gun while it's out of action.

Good luck to us!

Search here, sudden loss and regain of compression has been experienced by many. For me, if I get any carb cleaner down into the head then I have zero compression. However after a few kicks it fires right up and is 100% back to normal. Other people have had similar experiences (search here). If you consistently have no compression then either 1) make sure you're turning the gas off, or 2) do check the valves, if one or more are tighter than spec then it can hang open which is not good. But typically I get this problem if too much of anything liquid goes down into the head (carb cleaner, gas, etc..).

Thanks for the input, sirthumpalot. I'll add to this thread as I get closer to understanding and fixing whatever is wrong.

I did search and there are many threads on this issue. Most ask questions and experienced owners give answers, but the original poster rarely returns to note the source of the problem, which is a bit of a shame. From reading the various threads, I am guessing that rings are the major culprit, so most take a while to have the work done and forget or don't bother to post the outcome.




You don't specify the year/model/number of hours you have, but if it's one with Ti valves, consider this as you check the clearances. Ti valves in the big YZF go for a very long time needing nothing. After the first adjustment that requires more than .05 shim change, however, it's not always certain how long it will be until it needs it again. If the nitride coating has worn through, clearance will close up much more rapidly than during the time before the first adjustment, and this would be a good indication that you should replace the valves, and 25 hours is plenty of time to tighten one up under those circumstances.

Thumpalot may be onto something concerning fluid in the cylinder, especially if you don't turn your gas off when you leave the bike. That's like leaving a fuel pump running, and it can also seriously contaminate your oil.

Hey guys ive been reading this thread and 1 thing doesnt make sense to me. If liquid gets down in the cylinder wouldnt that bump UP the compression instead of lower it due to hydraulic affect? Or am i thinking in the wrong direction here? :naughty::naughty::D

Hey guys ive been reading this thread and 1 thing doesnt make sense to me. If liquid gets down in the cylinder wouldnt that bump UP the compression instead of lower it due to hydraulic affect? Or am i thinking in the wrong direction here? :naughty::naughty::D
Oil would, but small amounts of solvents like gas will wash the oil off of the rings, allowing compression to sneak past. Under these circumstances, the leakage occurs mostly over the top and around the back of the ring through the ring groove.

Of course, if there was enough of any fluid, then yes it would increase compression while it was in the chamber, taking up space.

I’m still having weird compression loss problems. My bike ran great a few days ago right up ‘till I parked it. I went to go for a ride yesterday but it wouldn’t start. I kicked for a while, got a few coughs and then ran for 15 seconds and quit. When I tried to restart there was almost no compression. There was compression up until it started. There goes my hot engine tight valve idea. I’m back to the gas washing down the cylinder walls theory, but this also happened 2ce while I was driving. I find it hard to believe that an engine running fairly well could run rich enough to wash down the walls enough to lose compression and quit. :naughty:

If the rings are failing woulding you notice your bike smoking like a choo choo.

If the rings are failing woulding you notice your bike smoking like a choo choo.

Both times I was going fairly fast when the bike quit and wouldn't have noticed, and maybe synthetic oil burns cleaner?

grayracer, sorry mate it's an '00 with about 30 easy hours on it since a new piston and ring.

Like bushy, I would also expect to see some smoke if the rings were failing. There is the tiniest trace of smoke when I crack the throttle wide open for the first time each ride, but I have to be standing next to the bike and looking real hard to see it. I would think that it's negligable.

As for fluid in the cylinder, I don't think that's an issue for me. I never use carby cleaner or starting fluid and I always shut off the gas, though I do leave gas in the float bowl. But in any case, I have done that since owning the bike and am only now noticing a loss of compression. Likewise, my oil is changed after almost every ride, two at most.

I asked about this over at DRN too, and one bloke says that this was a common experience for him with his 00 and 01 yzfs. He put it down to oil washing off the cylinder walls and regarded it as normal. But again, I am quite sure this is a new phenomenon for my bike.

I will check the valve clearances and change the oil out first (this will be it's fourth oil change since submersion). If no change, I'll pull her down and give the top end a thorough checking.

Thanks again everyone.

Sorry, but i must add to the confusion.. I was riding my 04 450 and shut it off.. Went to kickstart it, still warm might i add, and it would kick right throught the TDC with out any resistance... i don't know what happened but finally it caught and has not done it what could of caused bike has only 5 tanks run through it and runs like a champ with no smoke or nothing...

Has anyone else had reports of spaceships in their area? That seems to be the simplest explanation. :naughty:

Hehe Frostbite.

Toobold, I experienced it a few weeks ago. Didn't happen again until now. My bike still runs like a champ too. I would make an effort to check it before each ride.

Watch this space.....

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