I thought too that maybe one of the missing pieces from the slide might have finally lodged in a open valve. I'll try the air test today. I had planned to do a top end rebuild anyway, but I had hoped to get the summer out of it first.

 

I tried the air test and the problem is definitely on the exhaust side. Hopefully it's nothing serious. Thanks for that tip. I can't believe I never thought to try that before.

Edit: I just pulled the valve cover and the exhaust valve is opening and closing properly and isn't too tight.

 

I've been reading that these ATVs are known for shearing off the woodruff key and throwing the timing off. I'm going to check tomorrow and see if that's the case with this one.

 

Well, the detents in the piston may indicate this is an "interference"
engine, so the valves will likely hit the piston if the timing chain breaks
or one of the sprockets free-wheels.

Anyway, here is my 86, complete with SS gas tank, LED lights, new
piston, and lots of new electrical parts.

 

I wish mine still looked that good.
I checked and everything lines up so it isn't the woodruff key. It looks like it's time to tear it down.

 

I just got done tearing it down and here's the carnage. The piston is ruined and the head took quite a beating from the piece from the carburetor being hammered into it. The compression loss was from a piece of the head being damaged and holding the exhaust valve open. The valves and cylinder look fine though and I have a spare piston and head from my parts bike. Unfortunately the parts bike got water in the top end somehow (likely before I bought it since it's been under cover the whole time I've had it) and the valves and seats are rusty. A valve job is likely in order.
I see why it was burning oil since I bought it too. The oil rings were stuck.

The damage. In the first pic you can still a piece of the carburetor slide jammed into the head.




After cleaning the old head up.



Rusty valve seats in the parts bike.


Does this look ok?

 

Nice clear pix.

Where are the O-rings in the head gasket?

As to how much wear can be tolerated in the rod's small end, I do not
know what the spec is, but how much wiggle does the wrist pin have?

 

The o-rings stuck to the cylinder when I removed the head. The wrist pin fit tightly so I think it's ok. A friend of mine has an old Black and Decker valve grinder so hopefully I'll have the valve seats ground tomorrow.

 

Your gasket is supposed to have 3 O-rings. 2 of the 3 on mine were
damaged (photo).

 

It had all 3. I had replaced it a little over a year ago because of the o-rings failing.