Compression test procedure

[OliverB]

What's your feeling about this?

I have always heard that you perform a compression test by removing all plugs, engine cold and throttle fully open. That's what I did this A.M. on my engine but the test kit of my son's said to perform the test on a warm engine. So which is it: cold or hot engine?

BTW, got 176 PSI on both cylinders, engine cold. Will repeat the test on a warm engine.

 

[HoughMade]

I've always heard cold as well...but the real question is whether when you are doing the compression test, you are comparing it to published specs. If so, you need to know what procedure the publisher of the specs is expecting you to use.

That is really, really high compression .

 

[OliverB]

Yeah, the numbers sure surprised me, too. My son just tested his '79, cold, and got about 150 PSI on each cylinder - with the same kit.

Another question, if you have a problem with a valve that has too little gap, when the engine warms up, doesn't the gap reduce even more? Might a compression test on an engine with a tight valve show good compression on a cold test but lower compression on a hot engine test because the valve might not be fully closing?

 

[HoughMade]

That is possible, but I would think the valve would have to be pretty severely out of spec for that to be the difference maker.

 

[OliverB]

Yep, I agree.

 

[OliverB]

Well, warmed up the engine and repeated the test. Same results, 176 PSI both cylinders.

I think I know why the compression is so high. My bike must have one of those rumored, rare experimental diesel engines that mysteriously went missing from R&D!

 

[OliverB]

The Yamaha service manual says "...warm engine, choke closed and throttle open. Between 140 - 170 psi with no more than 15 psi difference between the two."

 

[63-Coupe]

That is rather high. Do you know someone who could loan you a different tester for comparison? My neighbour borrowed mine to compare with his and there was a significant difference on the same engine, like around 35 lb. The only good thing for me was that when we tried two other testers it turned out his was the "Bad" one.

 

[Travis]

I've always read that a lot of carbon build up on the pistons can increase compression. Maybe try running some seafoam or boat "engine tune-up" through the carbs to decarbonize the valves and pistons.

 

[OliverB]

The same tester (a pro series) registered 150 PSI on my son's '79 XS400 engine but good point about accuracy of different gauges. A friend also has a kit and I will be trying his.

 

[MechNewb]

I didnt want to start a new thread so Ill ask in here. If im going to check out a bike for sale and i know it doesnt run but want to check the compression before I buy it, can I just use the kick start for the compression test since the elctric start doesnt work? Or should I use a car battery for the test and try to use the elctric start?

 

[Travis]

Yes you can do it with the kick start... just kick it hard about 5 times on each side. You're supposed to hold the throttle wide open. With CV carbs I'm not sure if you need to manually lift the slides or not.

 

[MechNewb]

Perfect. Thank you very much.