Compression test question.

A place to discuss all matters relating to Scott Motorcycles.
Accessible to members and non-club members (you will need to register to post messages).....

Compression test question.

Postby The Shipley Squirrel » 08 Sep 2017, 19:42

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
The Shipley Squirrel
Club Member
 
Posts: 46
Joined: 19 Jun 2013, 23:51

Re: Compression test question.

Postby BRIAN MARSHALL » 08 Sep 2017, 20:52

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.
BRIAN MARSHALL
Club Member
 
Posts: 1812
Joined: 31 May 2004, 21:58
Location: East Midlands, U.K.

Re: Compression test question.

Postby Barrie » 09 Sep 2017, 08:30

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
Last edited by Barrie on 09 Sep 2017, 14:03, edited 1 time in total.
Barrie
Club Member
 
Posts: 215
Joined: 21 Jun 2015, 10:35

Re: Compression test question.

Postby Tman » 09 Sep 2017, 08:34

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.
Tman
Club Member
 
Posts: 167
Joined: 02 Dec 2014, 21:16
Location: Portsmouth

Re: Compression test question.

Postby Stan Thomas » 10 Sep 2017, 07:01

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"?
Stan Thomas
Club Member
 
Posts: 134
Joined: 01 Jun 2004, 18:34
Location: Stafford

Re: Compression test question.

Postby The Shipley Squirrel » 12 Sep 2017, 16:02

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
The Shipley Squirrel
Club Member
 
Posts: 46
Joined: 19 Jun 2013, 23:51

Re: Compression test question.

Postby BRIAN MARSHALL » 12 Sep 2017, 16:21

If it runs well I would leave it alone. There is an old saying, "If it ain't broke, don't fix it"....
BRIAN MARSHALL
Club Member
 
Posts: 1812
Joined: 31 May 2004, 21:58
Location: East Midlands, U.K.

Re: Compression test question.

Postby Tony Kitching » 22 Sep 2017, 06:50

There's an even older saying, "if it ain't broke, keep fixing it until it is!
Tony Kitching
Tony Kitching
Club Member
 
Posts: 27
Joined: 17 Oct 2004, 15:26

Re: Compression test question.

Postby gammakeith » 22 Sep 2017, 16:55

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
gammakeith
Club Member
 
Posts: 168
Joined: 18 Sep 2012, 16:43
Location: Nottingham, UK

Re: Compression test question.

Postby BRIAN MARSHALL » 22 Sep 2017, 17:15

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 !
BRIAN MARSHALL
Club Member
 
Posts: 1812
Joined: 31 May 2004, 21:58
Location: East Midlands, U.K.

Re: Compression test question.

Postby gammakeith » 22 Sep 2017, 17:34

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

Keith
gammakeith
Club Member
 
Posts: 168
Joined: 18 Sep 2012, 16:43
Location: Nottingham, UK

Re: Compression test question.

Postby gammakeith » 23 Sep 2017, 11:59

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
gammakeith
Club Member
 
Posts: 168
Joined: 18 Sep 2012, 16:43
Location: Nottingham, UK

Re: Compression test question.

Postby BRIAN MARSHALL » 23 Sep 2017, 14:38

If you want to borrow my gauge for comparison let me know...
BRIAN MARSHALL
Club Member
 
Posts: 1812
Joined: 31 May 2004, 21:58
Location: East Midlands, U.K.

Re: Compression test question.

Postby gammakeith » 23 Sep 2017, 15:31

Thanks! That sounds like it is worth a try.

Keith
gammakeith
Club Member
 
Posts: 168
Joined: 18 Sep 2012, 16:43
Location: Nottingham, UK

Re: Compression test question.

Postby Bob Mather » 23 Sep 2017, 23:47

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.
Bob Mather
Club Member
 
Posts: 96
Joined: 20 Jun 2004, 01:29
Location: Adelaide Australia

Next

Return to General Scott topics

Who is online

Users browsing this forum: Google [Bot] and 3 guests