T140 Compresssion PSI Value

[Charlie]

Does anyone know what compression pressure you should get on an unmodified Triumph 750cc T140 (1977 model) with composite head gasket? I'm getting 110 PSI on both sides after about 5 kicks. Is this normal?

All thoughts gratefully received!

[pogotri500]

Hi there, I would say that if you reckon your getting an equal reading on both cylinders there's not a lot wrong. 110 psi  doesn't sound bad anyway so ride and enjoy. Cheers, Dave.

[Charlie]

Thanks!
Interestingly my compression guage registers green (good) from 130PSI to 170PSI - I suppose it's designed for these new fangled engines that don't last like real bike engines.

[L.A.B.]

I cannot find any compression figures for the T140 but would agree with pogotri500 that as both compression readings are the same then there isn't likely to be much wrong, and the original pistons would likely have only been 7.9:1 compression ratio.

The test should be done with the throttles fully open.

You can check the piston ring condition by squirting some thick oil through the plug holes onto the bores, and repeat the test, if the readings increase noticeably then the rings could be leaking?
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Does it feel as if there is a reasonable amount of compression when kick starting?

Is the performance OK?

[Charlie]

I've done some more tests and had these results:

- Add a squirt of oil and the PSI rises from 110 to about 115 on both sides.
- Doing the test with throttle open and closed makes no noticeable difference

Having thought about the physics of this I think the ideal PSI value is easily calculated - surely it's just the compression ratio x atmospheric pressure, i.e. 7.9 x 15 PSI = 118.5 PSI. If you subtract a bit for living above sea level, bad weather, losses past the rings etc, I reckon a PSI value of 110-115 is pretty good. This logic seems to work on my car too - 9:1 compression ratio and 135PSI when I measured it last night.

Any physicist classic bike enthusiasts out there who'd like to comment?

[L.A.B.]

I've done some more tests and had these results:

- Add a squirt of oil and the PSI rises from 110 to about 115 on both sides.
- Doing the test with throttle open and closed makes no noticeable difference

Sounds fairly healthy then.