Feral Compression Test

[Gavin]

Please knock yourself out with advice for the following dilemma:

After a complete rebuild and before firing up an A10 I decided on a compression test. 80PSI left and right left me more than a tad disallusioned. Yep, throttle wide open and plenty of repetitive kicks. So I decided to put the spark plug in the opposite cylinder, and behold 140PSI. Same both sides as I swap the process. Yes Throttle wide open.

Yes head and barrel planed, aneiled head gasket, torqued to 24ft lb. barrels honed, new rings. Baffled owner.

Thanks for your thoughts.

[berger]

try putting some miles on the engine and bed things in

[muskrat]

 

[Roger (Doomtrainbarx)]

Head gasket issue ? - torque setting should be 30-32 - try retorquing to that figure.

[RDfella]

Run the engine - if only for a few minutes - then tell us what the figures are. The 'try one plug' is an odd diagnostic process; can only assume the reason it affects figures is because it alters the way you're kicking it and thereby affecting the speed the engine is going over TDC

[Jules]

personally I'm not a great believer in compression tests, in all my years working on IC engines I dont think I've ever based an issue diagnosis on a compression test, there is always something better to help determine the cause IMO.......just what is it that you want to diagnose, what issue are you chasing??

[BSA_54A10]

You won't get a proper reading till the rings have bedded in so don't bother till you have put some miles on it
Pouring oil down the plug hole to get a wet compression test is a sure fire way to end up with glazed bores.
This is a big probel that people who are not thinking strait have when they re-ring
The temptation is to start the bike and adjust the carb bfore you take it on the road.
However as the rings are yet to bed in  this adjustment can take quite a while and while the engine is stitting there idleing unloaded, you glaze the bores

So riding gear on start the bike and off down the road
If it won't idle , hold the throttle open
Afte r 10 minutes ( or longer ) return home then adjust the carb and do a compression test if you must .

[muskrat]

G'day Gavin.
To add to Trevor's reply.
To get a true reading the motor must be spun over at least idling rpm or greater. A kick starter wont get the revs needed. Either start the motor on one cylinder and test the other at full throttle  or use a paddock starter (roller) with the spark leads off, the other plug out and throttle wide open.
Doing this allows for the cam to operate as designed and a true reading given.
Cheers

[Triton Thrasher]

G'day Gavin.
To add to Trevor's reply.
To get a true reading the motor must be spun over at least idling rpm or greater. A kick starter wont get the revs needed. Either start the motor on one cylinder and test the other at full throttle  or use a paddock starter (roller) with the spark leads off, the other plug out and throttle wide open.
Doing this allows for the cam to operate as designed and a true reading given.
Cheers

There’s no “true reading.”

What you get with a compression gauge and a kickstart or electric starter is a figure to use in comparison to the other cylinder,  or to previous tests, or in reference to similar engines.

[muskrat]

G'day TT.
Yes at any rpm the test can be used as a comparison.
There is a "true" reading but not until the cam starts to work it's magic. Depending on the cam our A's would need to spin at 4-5000 rpm. The reading at those revs would be much higher than cranking speed.
Cheers

[Triton Thrasher]

Depending on the cam our A's would need to spin at 4-5000 rpm

Does anyone on God’s Earth do that as a BSA compression test?

[muskrat]

G'day TT.
Have done on the A7SS racer. Starter rollers geared up got her up to 3500 so still not "on the cam". Can't remember the exact figure but it was well over 250psi Wish I still had them.
My rHonda just on the starter motor and about 30 revs (15 compressions) shows over 190psi.
Cheers