Compression test question.

[The Shipley Squirrel]

Dear all,

I have just done a compression test on my ’49 flyer which is in ‘standard’ trim it has not been rebored and has the original pistons.
I honed the bores and fitted new old stock rings (thank you Eddie Shermer) when it was rebuilt, and while the bores were a bit worn they didnt look too bad.

I’m getting approx 90psi on one side and about 95psi on the other, this is with throttle and choke fully open and kicking it over as fast as my heart and lungs will allow – which is not very fast these days !
My question is this, is this a resonable figure to expect or should I be thinking about a rebore and pistons ? The test was with the engine cold and no oil squirted down the bores so I imagine that figure would go up a bit when the engine is hot.

Does anyone else have compression test figures that I can compare to ?

Thanks, Mark

[BRIAN MARSHALL]

I have Tom Ward’s old pocket books, and in one of them he lists compression test results, which are VERY variable. From those lists, I would say that 90 to 95 psi is acceptable, if a little on the low side for a late Shipley engine. Some engines gave results as high as 140 psi, but were also listed as having serious detonation problems that were not remedied by retarding the ignition. He had to fit decompression plates to these engines in order to stop the detonation/pinking. One can only wonder WHY some engines were like that, and also wonder if that is how they left the factory, or a consequence of owners’ attempts at tuning.

[Barrie]

If this engine has not run a couple of hundred miles or so since the rings were fitted , I would not be to worried by your results , they are really not a true representation as the rings should be run in first

[Tman]

Yep, run it in and then see how it performs. If it runs well, then Bob’s your uncle, whatever the comp gauge may say.
With variable old bikes, the seat of your pants can be a more accurate assessment tool than gauges.

[Stan Thomas]

So instead of “Made to Limit Guage” – should Scott’s slogan have been:

“Made by the seat of our pants for the seat of your pants”?

[The Shipley Squirrel]

Thank you all for the replies.

As I have’nt done that many miles on the bike yet I have decided to leave it as is and see how it settles in.
I’ll tweak the carburettor and ignition settings along the way to get it running at it’s optimum and think about a rebore next year.

Mark

[BRIAN MARSHALL]

If it runs well I would leave it alone. There is an old saying, “If it ain’t broke, don’t fix it”….

[Tony Kitching]

There’s an even older saying, “if it ain’t broke, keep fixing it until it is!
Tony Kitching

[gammakeith]

In a fit of enthusiasm I just tested my late Shipley flyer. I’ve not tested it before but got a stonking 190 and 200 psi on a gauge that I have used for many years. That’s about the same compression as my big Ducati v-twin so pretty massive for a 2 stroke! The engine has not done many miles since it went through the hands of our esteemed Chairman so I can only assume that nothing major is wrong? No obvious signs of any detonation (except for when my forks collapsed and pulled the brake on without me noticing) though I do wonder if that may be contributing to my overheating problem?

Keith

[BRIAN MARSHALL]

I suppose that modern fuel will allow higher compression ratios without causing problems, and Tom Ward’s investigations were done in the days of low-octane ‘Pool’ petrol. 200 psi does seem very high for a two-stroke !

[gammakeith]

I will check the calibration of the gauge and report back as it seems too high to me as well 😀

Keith

[gammakeith]

I tested my compression gauge and it was reading roughly right when I tested it against my decent tyre pressure gauge at 70 psi. Thus, it does seem that my flyer has very high compression. It also felt quicker than my Dads Speedtwin so I suspect it is making a bit more power than stock…..

Keith

[BRIAN MARSHALL]

If you want to borrow my gauge for comparison let me know…

[gammakeith]

Thanks! That sounds like it is worth a try.

Keith

[Bob Mather]

Hi,
One atmosphere at sea level is 14.7 psi. At 7 ; 1 compression ratio, 7 x 14.7 =102.9 psi which should give some indication of what to expect.
Regards,
Bob Mather.

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