Compression testing

[Phil C]

Hello everyone, Having recently read some posts on the twins section of the forum, I thought I might check the compression on my 1965 B40F. From what I can gather, the way I should do this is:  warm the bike up by taking it for a short run; with the ignition off, take out the spark plug; twist the grip to full throttle; stick the tester (see pic) in the plug hole; kick over the engine.  Is that right?  If so, two questions: 
1. Should the choke be fully open or fully closed?
2. What reading am I hoping for?
3. Anything else I need to know?

Thanks.  Phil.

[Blackthou]

Evening Phi,

Firstly I don't know about doing the test warm. I've always checked when cold.

Re throttle wide open is to let maximum air in for compression. If the choke is closed it will be an obstruction.

Tappet / valve clearances must be correct.

The compression reading depends on the piston in your bike. I tested my C15 the other day, as I'd just done a top end overhaul and the reading was 132psi or about 9bar, roughly about 9:1. There is a formula I've read somewhere that corrects the inefficences and ups the reading slightly, either way I was happy with mine.

A screw in compression tester with an O ring seal is much better than the stuff it in type unless you have a glamorous assistant to stuff it in hard for you while you're hard at it.

[Phil C]

Hmmm, thanks Rick. You say the required reading depends on the piston. The person I bought the bike from said the owner before him had had the cylinder rebored and a new piston. I don't know any more than that. Is there a rough figure I should be hoping for?  Phil

[chaz]

A screw in compression tester with an O ring seal is much better than the stuff it in type unless you have a glamorous assistant to stuff it in hard for you while you're hard at it.

I had to do mine cold as I ran out of fuel, as you say, on the left cyl holding the throttle open, compression tester (same as this one above)  and balancing , while kicking produced 110psi, then trying to do the same holding the tester in the right cylinder, right hand on throttle, left hand on tester, or vice versa, a true balancing act, without the assistant,
I suspect a single with left plug hole easier than a right hand plug hole.

[Blackthou]

From what little I can find I would think your reading would be just over 100 for the lower compression piston up to 130 for the higher compression.

Do you have any info as to your compression ratio?

[Phil C]

My owners' manual, which I hope us the right one for the year, says 7:1.

[Blackthou]

That would indicate around 100psi. Do check your tappets though before you start.

[Phil C]

Okay thanks. What kind of reading would give cause for concern?

[Blackthou]

Probably about 70-80 but if the bike runs fine, just enjoy.

[Phil C]

Thanks very much.