Compression Advice Please

[DaveB]

CB 750 K6 US Import - apparently built in Dec. 75 and has supposedly covered just under 16K miles in all that time. It was obviously not kept in a warm climate and there's much cosmetic work needs doing which I knew at the time of purchase. The seller in the UK (yes, I know, salesman) described it as, and I quote... "comes fully serviced with fresh oil, filters, plugs and points, and a front brake fluid change. It has a new battery, starts first time, and runs and rides superbly, in fact one of the nicest CB750s we have ever had.". I'm now wondering if I've bought a dud or at the very least paid over the ods for what I've really got.

Anyway, to the compression...

Test gear, eBay purchased compression tester (not good quality I think as I already managed without any effort to strip the thread where the gauge joins the extension on the spark plug hole).

I understand from the manual I should expect to see 150 to 175 psi.

I saw nothing like it.

Tested cold, no oil, nothing down the bores. Choke not on (open), throttle held full open, all spark plugs out.

No 1: 110psi.
No 2: 108psi.
No 3: 110psi.
No 4: 110psi.

Put everything back together and started the engine to make sure I hadn't broken anything. Then came here to read about compression and discovered that I should probably have put some oil down the bores (not sure how much) so with the engine quite warm now, so not sure if it's a valid test, I redid the whole thing only with a squirt of engine oil down each bore.

All cyclinders improved by about 10 psi so now reading around 120. Whether this was the oil or the warm up I have no idea.

The engine seems to me to run OK. It doesn't tick over well when cold but from what I've read this isn't unusual. It's working on all cylinders and so I'm wondering if these readings are anything to be concerned about or if it's more likely the gauge is rubbish. If there was a wild difference between the cylinders I'd know it's time to have the engine rebuilt but as they seem to all read the same, I'm not so sure what to do.

Thank you for any thoughts and advice.

[totty]

Typically the differences tell you if you have a fault and from that perspective it looks excellent, the values either tell you how worn it is or how well calibrated your gauge is - easiest way to tell which is to try it on other motors.
If there are no other reasons to suspect poor compression then it's probably fine.

[Dazza215]

Dave, You're getting double the compression of my bike and mine will not start as a result. I would test it again after putting a few miles on it.

[hairygit]

You could ask a local bike garage to test it for you, just to compare readings with your gauge. If you get low readings, putting a little oil down the bores helps knackered rings/bores seal better, so if a test with oil shows a big increase in compression it shows worn rings/bores. But do try testing with a known good gauge before panicking too much.

[royhall]

I was only getting 90psi when I tested with a new gauge. After a bit of panic I tested the gauge on my modern CB1300 and it only read 95psi. The gauge is reading way low. If you have another bike check that and compare the readings before doing anything drastic. I now know with my gauge anywhere around 95 psi is good, should probably throw it in the bin and buy another make.

[florence]

the important thing is that you are getting consistent readings across the cylinders and it runs well. I wouldn't give it another thought.