YZ 465 loss of compression = death?

genus727 · Jan 28, 2008

This 1981 Yamaha YZ 465 is my first 2-stroke and I just finished a top-end rebuild (piston was seized by previous owner). I got all excited and took it out to the beach (Oceano, CA) to mob around and test it out. It started out great, but after 15 minutes of good riding (at higher-throttle) the engine just dies.

I'm at first concerned that I hot-seized it, but I can still kick the bike through it's stroke. The only thing is that it's now about 5x easier to kick the bike, like there is very little compression. When I try and kick-start it I get no life from the engine. Only the occasional backfire, which indicates to me that the electrics are still good.

I inspected the plug and it looks good.
Any idea about what I did to this bike? What would cause the compression to go like this? I don't see how I could have burned through or cracked a piston in 15 minutes of hot-riding.

2strokerfun · Jan 28, 2008

Something caused it to sieze the first time---maybe too lean jetting, or an air leak. And there is a good chance that whatever caused it to seize the first time caused it to do the same thing again. It is not abnormal for a piston to free up and move after it cools down following a seizure. But you probably have some damage now causing you to lose compression.
You could be lucky and just have something simple like head gasket cause it to stop running and loss of compression. But you'll have to get out the wrenches and check it out.
Just my 2.4-cents (inflation and all).

genus727 · Jan 28, 2008

I don't think it seized on me, because as soon as I came to a stop I tried to kick it to get started and the piston moved fine... minus compression. I'll check the head gasket.

ats666 · Jan 28, 2008

It could have got alittle too warm and stuck a ring. Pull the jug and have a look.

rmc_olderthandirt · Jan 28, 2008

Riding in soft sand can put a pretty big load on the engine. I would have advised breaking the engine in a little more gently.

Did you check the end gap on the piston rings before you installed them? You do this by inserting the ring into the cylinder by itself and then using a feeler gauge to measure the gap. There needs to be some, and the proper amount would be found in the repair manual or perhaps in the papers that came with the new rings.

If you didn't check the end gap then there is a high probability that it was too tight. My experience is that the rings are supplied a bit large so that you can make them fit: you can make a big ring smaller by filing the ends but you can't make a small ring bigger.

Like the others said, it sounds like it siezed on you. I would say that it is about 50/50 if a "sieze" actually locks the piston. Half the time it just ruins the rings and you lose compression without it locking up.

Take the head off, look inside.

Rod

jarheadyzfreak · Jan 30, 2008

Dont It Suck

My yz490 did this last year because i went to lean on the jetting. It seized but kept runnig. It bogged down with no power or snap and died if it idled for any time. The piston rings actually sunkk into the piston and it had no compression. Definetly need to pull the top end back down. sorry

zaneyzrex · Jan 31, 2008

yep , im betting ya holed the piston. ya ALWAYS go thru the carb when ya rebuild to make sure it clean and make sure your jetting is on before beating on the bike.
good luck and thats an awesome bike had one myself. loved that bike but what a bitch to start.

genus727 · Feb 1, 2008

The Carb was definitely clean. I went through it... essentially a brand new mikuni. I just don't know how to jet it perfectly.

Jasle · Feb 1, 2008

Some things to think about...read above in the jetting sticky. it tells about jetting.

check your plug color. is it white/tan/black...

I'm betting on a stuck ring. Was the arrow facing forward? I've seen backwards pistons run for a while then barely hang and nick a bit off and then have low compression. Basically you need to tear it down and see whats up then report back so we can figure out the why of what.

2strokerfun · Feb 1, 2008

genus727 said:
essentially a brand new mikuni. I just don't know how to jet it perfectly.

That might be the problem. Using new carbs is risky until jetted correctly. Like Jasle said, look at your plug. Should be about the color of wet brown cardboard box. Not white or off white and not black.

genus727 · Feb 1, 2008

Yeah. I looked at it. It was white.... definitely running hot.

2strokerfun · Feb 1, 2008

Well, I think you've found the problem. Too lean. Have you looked at the piston, cylinder and rings yet??

BigRedAF · Feb 1, 2008

I'm pretty sure that a backwards piston on a 465 would hang a ring in that huge exaust port...

If you have little or no compression and your using a stock cast piston I would think that a holed piston isn't out of the question. Those bikes pinged as deliver from Yamaha. Combine that with questionable jetting and deep sand riding and you have a recipe for trouble.

It's easy to pull the head off that bike so take a look and see whats up.

I had a 1980 YZ250G, great bike. Your 1981 has the same look, I hope you get it going soon.

genus727 · Feb 2, 2008

Arrow towards exhaust, right? That's the way I put it.

genus727 · Feb 2, 2008

Do I need a new piston? Do I need to re-hone the cylinder?

zaneyzrex · Feb 2, 2008

genus727 said:
Do I need a new piston? Do I need to re-hone the cylinder?

ya wont know that till ya have it apart and see whats wrong. if ya holed the piston then ya get to tear it apart and clean the bottom end out (lots and lots of tiny pieces of alum. floating around in the bearings now)

let us know what ya find.

chevyss_98 · Feb 4, 2008

yikes :S, the only time i had that problem was when i had a late 70's YZ and basically it did same as you did, power died, and when you tried to start easy to kick, and backfired

turns out one of the circlips disappeared, the wrist pin had scored the crap out of the cylinder, and then the rings got hot, then a good chunk of the top of the piston went right out the exhaust lol

like Jasle said, i will also bet on the stuck ring situation from the piston not being in proper orientation, as soon as you loose one ring, your compression goes to crap, and if theres a few scores in the cylinder then its even worse compression

worst case you might have to get the cylinder bored out a bit, and re-sized, i ended up taking it to an amazing local guy who machined it for me, and fitted a new piston and rings, after that it was perfect

personally, i would tear it back down and hope for the best, if it just died then it may not be too bad

color of plug should tell the story tho

goodluck

(also if not watercooled pull head like i did on mine, much easier)

genus727 · Jan 17, 2011

Just thought I would close the loop on this thread. I finally pulled the head off and yes. The piston had seized and the rings got stuck in the cylinder grooves. Gonna get a new piston. Honed the cylinder and it looks good. Thanks all for you help (a few years ago).

YZ 465 loss of compression = death?

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