engine compression for '66 CA77

JAI · Post by **JAI** » Sat Aug 18, 2018 9:34 pm

I'm a newbie and had my engine off the bike before realizing I needed to test the compression of the motor while the engine is warm. I tested both cylinders cold and both tested out at 150 psi. I assume that a warm test would yield a higher result. Per Mr. Silver, the compression should be in the 150-175 range when warm. Anyone have an opinion if I should still open up the engine to check the pistons and rings, or with this "cold" compression, should I leave the engine alone and reinstall it as is once the frame has been painted. Dont want to miss the opportunity to check now if necessary, but sure don't want to open her up if its not! Thanks!

G-Man · Post by **G-Man** » Sun Aug 19, 2018 8:24 am

Looks OK if both are similar. No need to pull the motor apart unless you are curious and prepared to spend money.....

Can you put your location in your profile so that we can see where you are? You may have someone local to you who you could turn to for help.

G

JAI wrote:I'm a newbie and had my engine off the bike before realizing I needed to test the compression of the motor while the engine is warm. I tested both cylinders cold and both tested out at 150 psi. I assume that a warm test would yield a higher result. Per Mr. Silver, the compression should be in the 150-175 range when warm. Anyone have an opinion if I should still open up the engine to check the pistons and rings, or with this "cold" compression, should I leave the engine alone and reinstall it as is once the frame has been painted. Dont want to miss the opportunity to check now if necessary, but sure don't want to open her up if its not! Thanks!

mike in idaho · Post by **mike in idaho** » Sun Aug 19, 2018 11:05 am

150 psi isn't all that bad, just using a gauge with a long hose on it will affect the numbers(more volume of air to compress with the extra volume of the hose). Another(cheap) gauge may read differently that the one you used. Altitude above sea level and barometric pressure differences (weather) come into play also. Valve clearance and timing chain wear are factors also. It's more important for both sides to be even than for them to match some arbitrary number. Keep in mind, a piston ring that seems to be leaking a small percentage at cranking speed will seal quite a bit differently while running, with combustion pressure forcing it out against the cylinder wall.

cknight · Post by **cknight** » Sun Aug 19, 2018 12:34 pm

Compression should be tested with the throttle wide open, or the carburetors removed. A cold reading of 150 should be fine. Did you check the valve lash clearances first? Regards, Chase

JAI · Post by **JAI** » Sun Aug 19, 2018 2:29 pm

Thanks for your comments guys. I did test with the throttle wide open, but did not check the valve lash before hand.......not sure what this is! I will probably just open the cases to replace the clutch cable and look at the clutch plates. Also, I will clean out the oil sump, replace the plugs and clean and set the points and timing. Anything else I'm missing before I reinstall the engine?

G-man, thanks for your comments on my other posts as well. They are very helpful for new guys like me! I am in Johnston, IA and have updated my profile. Thanks.

G-Man · Post by **G-Man** » Mon Aug 20, 2018 7:11 am

Don't forget the oil slinger on the clutch side of the engine. This is a mechanical filter and should be cleaned of caked-in debris.

Valve lash = tapped/valve clearance. I don't think you need to worry if you got 150psi on a cold engine.

You're welcome. When you get a chance, please post some pictures. We like to see what you're working with.

G

JAI wrote:Thanks for your comments guys. I did test with the throttle wide open, but did not check the valve lash before hand.......not sure what this is! I will probably just open the cases to replace the clutch cable and look at the clutch plates. Also, I will clean out the oil sump, replace the plugs and clean and set the points and timing. Anything else I'm missing before I reinstall the engine?

G-man, thanks for your comments on my other posts as well. They are very helpful for new guys like me! I am in Johnston, IA and have updated my profile. Thanks.

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