Compression Specs

In my quest to find out why my bike quit running, I did a compression check tonight. Since the engine is ice cold, I know these readings aren't the best, but I had 40 psi in two cylinders, 50 psi in one and 70 psi in another.

Do these readings seem low enough to cause the engine to stop running?

Yes.

I would think you would need at least 80-95 psi to run (poorly). I think with the numbers you have, that would indeed cause it not to start.

It won't run worth crap with that compression. The service limit minimum on mine is 115 psi.

I was fouling plugs in about an hour at 110 per cylinder. Time for some work.

Scoot Jockey wrote:

In my quest to find out why my bike quit running, I did a compression check tonight. Since the engine is ice cold, I know these readings aren't the best, but I had 40 psi in two cylinders, 50 psi in one and 70 psi in another.

Do these readings seem low enough to cause the engine to stop running?

Would add teaspoon of oil inside combustion chamber and re-test compression. Would also check valve clearances.

Good Luck!

Thanks everyone; I figured those numbers were ridiculously low. Looks like I've got some work ahead of me...

Did you have the throttle wide open when cranking it?

Yes, the throttle was wide open. Last night I checked the torque on the head nuts and low and behold they needed several turns to get to 42 pounds (heavy duty studs).

CRUD!!!

I think it's likely the head got loose during break in and that must have caused the head gasket to blow. Although tonight I am going to try the teaspoon of oil test to make sure the rings aren't at fault.

If the rings are the culprit, then the guys who did my machine worked ripped me off.

If its just the head gasket, then I'll be giving you guys at APE a call to order a 1075cc head gasket.

If it is the rings, then maybe I'll be sending you my cylinder block as well!

Just when I thought I'd be getting ready for riding season...HA!:laugh:

Were valve clearances within specs? If too tight, can easily reduce compression figures, and is a common fault.

Also a leak-down test could be very informative. If a tester tool is needed, rumor has it that AutoZone (and maybe some other auto-parts stores) has this tool and other tools available for loaning out to its customers.

The leak-down test will allow actually hearing where the compression is escaping from. If heard coming from the exhaust, it's an exhaust valve failure to seal. Or heard coming out the carb intake, an intake valve failure to seal. If heard coming from the crankcase vent, it's being lost by going down and around the piston and rings, or possibly through a damaged piston.

Good Luck!

That's good idea. I will look into getting one of those. I know Z1 Enterprises sells them.

I just finished adjusting the valve clearances, so I don't think that's the culprit. But I will recheck them.

I put a teaspoon of oil in two of the cylinders tonight and the compression jumped up to 150 psi in the cylinder that had 40 psi and 170 psi in the other.

If it is the rings causing this, I may decide to scrap this project and part this thing out. I'm not sure I want to work on this bike any more.

Scoot Jockey wrote:

I put a teaspoon of oil in two of the cylinders tonight and the compression jumped up to 150 psi in the cylinder that had 40 psi and 170 psi in the other.

If it is the rings causing this, I may decide to scrap this project.

Odds are REALLY HIGH that it's rings with those symptoms... in fact, I can't think of a way that it's not rings.

I took the top of the engine apart this weekend and the head gasket looked fine, as does the head. SO it was the rings...

I am done with this project. I will be parting it out on EBay starting with the 29mm smoothbores tonight.