Compression testing

[timebombprod]

Quick question, will let me know what is the next job.


When doing a compression test do you only want to let the compression gauge move 3 or 4 times or do you let it go until it cant go any higher.


If it's the first one, hello rebuild, if it's the second one, I have great compression.


I have a feeling it's the first one and I am definitely not passing with flying colours.

Talk about 90psi.....I guess the upside would be once I do finish the motor, I will have a GREAT running bike, that i will not doubt.

 

[Nessism]

For a proper compression test the engine should be warm, the throttle held wide open, and you should have a strong battery so the starter can spin the engine with proper speed, cranking until the pressure doesn't increase anymore.

Needless to say, if the valve clearances are too tight you will loose compression so check those. If you find valves too tight they may also have some carbon on the valve faces so even after adjusting the valves you may get some leakage until some miles pass.

 

[timebombprod]

For a proper compression test the engine should be warm, the throttle held wide open, and you should have a strong battery so the starter can spin the engine with proper speed, cranking until the pressure doesn't increase anymore.

Needless to say, if the valve clearances are too tight you will loose compression so check those. If you find valves too tight they may also have some carbon on the valve faces so even after adjusting the valves you may get some leakage until some miles pass.

Valves are in good shape, the mechanic said let the gauge only go up 3 times, if it goes 3 times it goes to right under 90, if I do it until it stops it goes to 150, 4 is a little lower but it's close enough to the others.


My spark plugs are as always fuel fouled but dry

Battery is charged well starter spins great

 

[Grimly]

I let each cylinder compress four, maybe five times. The manner in which the psi rises can tell you something about the state of the rings / bore.
A bore / rings in good order should leap up to near the final figure very quickly, almost on the first compression stroke, and the remaining comp strokes are just for fun (or diagnosis, if the engine is a dog).

 

[timebombprod]

I let each cylinder compress four, maybe five times. The manner in which the psi rises can tell you something about the state of the rings / bore.
A bore / rings in good order should leap up to near the final figure very quickly, almost on the first compression stroke, and the remaining comp strokes are just for fun (or diagnosis, if the engine is a dog).

It will climb up to 150 but, takes about 4 bumps to get to under 90, didnt count the rest because I did let it go till it couldn't anymore first, then mechanic told me only let it go 3 or 4 times. It goes up at a decent pace.

 

[portdave]

I will add two things. Sometimes the tool you are using may not have the best seal or take longer to get a good reading. Try the tool on a car if possible. (I learned this the hard way until I got a nice used Snap-on compression tester)
You may also want to try the test a second time with a few drops of oil in each cylinder to see if you get better readings.

 

[timebombprod]

I will add two things. Sometimes the tool you are using may not have the best seal or take longer to get a good reading. Try the tool on a car if possible. (I learned this the hard way until I got a nice used Snap-on compression tester)
You may also want to try the test a second time with a few drops of oil in each cylinder to see if you get better readings.

I'm gonna rent a tester from advance auto if they still do it, if I get a better reading with some oil the piston rings are a little worn right?

Gor the test today I'll be having the throttle wide open, and all 4 spark plugs out, and the engine warm.

Gonna try to tune the carbs to run better so I can get the tester, car gave out yesterday so I have to ride the bike there.

 

[wymple]

I let it spin until it tops out. It's not going to top out any higher than it's capable of. When you start an engine you get 3 or 4 cycles in a nano second. That would mean nothing to me. If you can get it to 150 cranking it over, stop worrying unless you can pick up another 15-20 lbs by putting about an ounce of oil in the cylinder for a best case ring seal.