XL800

[richweise]

I had saw running OK--but now it will not start. Carb is good, spark is good, compression is NIL. I have owned this saw since 1968 and it really does not have that many hours on it.

Could it be crank seals? Seized rings?

Help me--how hard is it to replace seals?

Rich

[925fetish]

Could be either,but I would say seals.

[lesorubcheek]

Yank off the muffler and take a look at the piston Rich. If the seals were bad and that was the cause of the low comp, you should see obvious damage on the piston. You'll also get an idea if its just stuck rings by looking here too. If they look dry and carboned over, but no obvious scoring, then it may just be stuck rings.

Dan

[MCS]

Crank seals won't kill compression. Remember, when the piston is at bottom dead center everything is open - transfer ports and exhaust ports. What ever is trapped in the combustion chamber when the piston closes the ports is what gets compressed. This saw does not have a compression release so that is ruled out as a cause.
Dan has the best idea. Pull the muffler and look around. Leave the body in place and just pull the cap and baffle. The studs may not come out of the cylinder on a saw that is 40 years old but you never know.
You will have to pull the jug unless you get real lucky in your inspection through the exhaust port. As long as the saw was run with proper oil and the correct mixture you should be able to salvage the engine.
Let us know the results.

[lesorubcheek]

Exactly as MCS said.... bad seals cause air leak, air leak causes lean combustion, lean combustion causes excess heat, excess heat causes piston to go kerplunk. Seals alone have no direct effect on the comp, BUT they indirectly can wreak havoc by the damage it can cause running the saw with an air leak. This kinda damage is usually evident on the exhaust side of the piston. Maybe, just maybe, it'll end up being just stuck rings.

Dan

[MCS]

A lean mixture could very well have been the cause of this failure. The saw has low hours so unless something has caused the crank seals to deteriorate I doubt if they are worn enough to cause excessive leaking. I think it is extremely important for anyone running a chainsaw, especially these old ones, to be able to hear the high speed miss that occurs at full throttle with no load and a correctly set carburetor. This is the only short term clue we have that the high speed is set properly.
Oxygenated fuel, gas with ethanol, will cause an increase in combustion temperature so that, coupled with a lean mixture will bring a saw down in short order.
What about the cylinder cooling fins?

[925fetish]

You're right I've got seals on the brain.I've had to put four sets in this week.I think this new gas is wreaking havoc on them turns them to mush sticks them to the crank,and rips the lips off