Piston problem

[claybear]

So I just picked up my XS for 5 bills, mint looking bike, everythings all original, fires up on first kick, but there's a problem. The left side has 0 and I mean 0 compression, I checked the valves, they're working fine, it's blowing air out, of that cylinder, but it doesn't feel like the piston is coming all of the way up. When I asked the guy I bought it from, he said that he was riding it one day and it just lost power, said there wasn't any noises or anything. When I listen really close there sounds like a very light ticking coming from the top end. I just want an opinion, If it were the rod that was broken there should be some noise associated shouldn't there be? could it possibly be the wrist pin had broken? I just don't understand how it can feel like the piston is still moving some, but not all the way up, and why there isn't any real noises.
Also is it really worth it for me to continue this project? I'm not sure what a crank assembly costs, but I'm pretty sure it isn't cheap.

 

[5twins]

I would start watching for a spare motor. Sometimes you can pick them up for as little as $30 or $40. I got a near complete parts bike for $50 (missing rear wheel). In the mean time, I'd pull that one apart to see just what's wrong. Maybe it isn't as bad as you think and only needs a piston.

 

[remmy1101]

If you know that the valves are closing then I would say it sounds like a hole in the piston... you might be able to bring it to a shop and have someone look into the spark plug whole with a scope just to make sure.
I don't think you should buy a new engine if theres no head damage from debris flying around in the engine if the piston blew apart just rebuild the one you have.

 

[gggGary]

+1 sounds like a holed piston, fairly high on the list of culprits is a sticky advance mechanism. (If it's a points model)

 

[claybear]

Is that pretty common for the pistons in these to just blow apart or get holes burned through them? I would think if it were an early detonation thing that it wouldn't have a very large hole in the piston, I thought they were usually just pin holes through them. This shows how much I really know about motors though. I thought I knew something about them, but I'm finding myself asking more and more questions every day.

 

[5twins]

If it's a points model then over advanced timing is the usual cause. Either it was set wrong, advances too much due to a worn ATU, or the ATU has come loose and the timing slipped.

 

[remmy1101]

Well your post is a little confusing just because you say 0 compression when you might just mean that one cylinder is not running.
Did you use a compression checker? if so what psi is that cylinder pushing? I had a problem with my latest bike not firing on one cylinder and despite me being a vet at cleaning carbs it was a dirty carb! something must have got in after I cleaned them or some unseen crud broke lose.
anyway it might not be your piston at all and you might not even need to buy anything unless you know the compression psi is much to low to run this bike.

 

[claybear]

Well your post is a little confusing just because you say 0 compression when you might just mean that one cylinder is not running.
Did you use a compression checker? if so what psi is that cylinder pushing?QUOTE]

I did use a compression tester, and the compression was at absolute zero.

 

[remmy1101]

Like I said bring it to a shop or buy a scope mikesxs part Part #35-9096 to check for a hole without tearing it down.
Maybe you could try sticking a small rod or something down the spark plug hole to confirm the piston is moving the full stroke.. maybe compare to the other side.
Also where do you live? a xs650 engine just went on new hampshires craigslist. Honestly it looks like the one from my last xs... hmmm

http://nh.craigslist.org/mcy/1904605707.html

 

[Sundie]

If you've holed a piston, you'd be spitting massive amounts of oil out the back.

I'd bet on blown head gasket.

 

[claybear]

Yeah, found the problem, the very cheap way. Just took a coat hanger and put it in the spark plug hole, turned the motor over to top dead and felt around. Hole in the piston... Could run the coat hanger right through it, so I know what I'll be doing next weekend when I get the parts! KInda wanted to do rings and replace some gaskets anyways.

 

[remmy1101]

new top end = piece of mind