Cub compression test

Mon Jan 13, 2014 7:16 pm

Hi Folks, further to my question regarding the miss my cub has at the exhaust end, thank you all for your responses, advice and suggestions. I am borrowing a compression testing kit tomorrow just to test it so I know where I am working from here.

Can any one tell me what sort of pressure (psi/bar) I should expect out of each pot? The previous owner informed me he had new rings and shells fitted in 2007, but could not remember if the valves were done or not.

Thank you in advance
Regards
Ian Donald
Scotland

Re: Cub compression test

Mon Jan 13, 2014 7:31 pm

IanD wrote:Can any one tell me what sort of pressure (psi/bar) I should expect out of each pot?
Conduct the first test on each cylinder dry. Add a tablespoon of engine oil to top of each piston, rotate engine a time or two. Then take the 2nd, wet test. Record the test results of each test. Also, make sure you get at least 5 compression strokes per reading. Report back if using hand crank or the engine starter. Engine starter results will be higher - much better information wise.

Report the results. Much easier to analyze written results. Because there are a lot of variables.

Ideally you would like to see 110 -120 psi on each cylinder. But no more than 10% variance between psi from high to low per test.

OH. If the compression tester reads in PSI - report back in PSI. If not, bar, I will have to convert to psi to understand the readings.

Re: Cub compression test

Mon Jan 13, 2014 8:03 pm

IanD wrote
The previous owner informed me he had new rings and shells fitted in 2007
if by shells you mean what we call sleeves or liners, the cub does not have sleeves. So the Previous Owner might be mistaken or being misleading. it is bore in block. if you get dry compression readings of 80 to 90 PSI, the motor will run decent. 110 to 120 is new tolerance readings and ideal, but I doubt you will see that.

a lean carb will cause the miss you describe, as will poor ignition, either 6 volts with a marginal battery, or bad plugs/points. so there are a number of things to look at. tell us what you find for compression readings

Re: Cub compression test

Mon Jan 13, 2014 8:15 pm

I take shells as meaning rod bearings?

Re: Cub compression test

Mon Jan 13, 2014 9:03 pm

Ian, you will want good compression readings preferably over 100 lbs but it is important that the compression reading be somewhat consistent to remove a sticking valve from possible causes of the miss you are hearing.

Re: Cub compression test

Tue Jan 14, 2014 7:34 am

now that you mention it, I have heard the bearings referred to as the shells, by a friend of mine from down under.

Re: Cub compression test

Tue Jan 14, 2014 5:55 pm

Eugene wrote:
IanD wrote:Can any one tell me what sort of pressure (psi/bar) I should expect out of each pot?
Conduct the first test on each cylinder dry. Add a tablespoon of engine oil to top of each piston, rotate engine a time or two. Then take the 2nd, wet test. Record the test results of each test. Also, make sure you get at least 5 compression strokes per reading. Report back if using hand crank or the engine starter. Engine starter results will be higher - much better information wise.

Report the results. Much easier to analyze written results. Because there are a lot of variables.

Ideally you would like to see 110 -120 psi on each cylinder. But no more than 10% variance between psi from high to low per test.

OH. If the compression tester reads in PSI - report back in PSI. If not, bar, I will have to convert to psi to understand the readings.



Hello, thank you very much. I got the gauge etc but I did not get time to do the test, I will do it on Friday and report my findings.
Thank you for your help, its very much appreciated
Regards
Ian

Re: Cub compression test

Tue Jan 14, 2014 6:02 pm

Hello Folks
I have the gauge but I did not get time to do the compression test today, I will do it on Friday and report back.

Regarding my reference to "shells", please accept my apologies, this is what we call "main crankshaft bearings and big end bearings" here in the UK!

Thanks to all for your responses, very much appreciated
Regards
Ian Donald
Scotland

Re: Cub compression test

Tue Jan 14, 2014 6:27 pm

IanD wrote:Regarding my reference to "shells", please accept my apologies, this is what we call "main crankshaft bearings and big end bearings" here in the UK!

The term "shell" in reference to a bearing isn't heard much in the US. But I believe that even here it is considered the proper term for plain bushings that are split for installation. Crankshaft inserts are the primary use for that type bearing.