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Compression test PSI on M20....whats good? #278792
10/13/09 10:27 pm
10/13/09 10:27 pm
Joined: Oct 2007
Posts: 1,539
Auckland NZ
Ignoramus Offline OP
BritBike Forum member
Ignoramus  Offline OP
BritBike Forum member
Joined: Oct 2007
Posts: 1,539
Auckland NZ
Doing comp test on side valve is a PITA (with a straight shafted gauge) but it suddenly occured to me that if i make an adaptor out of a old spark plug and a bit of rubber hose to my gauge it will save draining the tank ect twice so i can test and compare cold/hot, also i wont be struggling to hold the gauge while kicking....anyhow:

I have no idea what a good PSI would be on a 4.9/1 side valve.

All i know is my A65 is 9/1 and gives 180 psi (hot)....i know you cant realy just say "well the m20 is around half that compression ratio" due to all the technicalities of how gasses behave under pressure ect
but would you expect around 90 PSI ???

It feels like there is plenty compression on cold kick start,perhaps slightly less when hot. But I recon its seems ok for a side valve


Any ideas
Thanks guys

PS:
motor isnt burning any oil and runs well BUT is leaking excessively out of breather valve. I have replaced cam oil seals, removed silly rubber extension tube in timing cover ex previous owner and shagged about making a new flapper valve disc....I am trying to eliminate "Blow by " as a cause

PPS:
Will do a test for crankcase pressure/vacum with a manometer another day but one thing at a time for now.

Last edited by Ignoramus; 10/13/09 10:53 pm.

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Re: Compression test PSI on M20....whats good? [Re: Ignoramus] #279008
10/14/09 11:00 pm
10/14/09 11:00 pm
Joined: Jul 2009
Posts: 3,549
Vic. Australia
P
Pete R - R.I.P. Offline
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Pete R - R.I.P.  Offline
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Joined: Jul 2009
Posts: 3,549
Vic. Australia
Between 83 and 100 psi. would be good on a warm engine.Above 100 psi. would be better than excellent.
Below about 72 psi,its about time for a valve-grind or ring-job.

A plug adaptor,with a brake-hose or grease-gun flexible hose makes life easier.To get accurate readings,you need a valve at the plug adaptor.If it is a spring-loaded valve,it must be a very,very soft spring.If you can blow through the valve with your mouth,it will lower your readings by less than 2psi.

If you only have a valve at the comp-gauge,about a foot away from the plug,expect lower readings because of the increased volume during compression.

Re: Compression test PSI on M20....whats good? [Re: Pete R - R.I.P.] #279044
10/15/09 1:45 am
10/15/09 1:45 am
Joined: Oct 2007
Posts: 1,539
Auckland NZ
Ignoramus Offline OP
BritBike Forum member
Ignoramus  Offline OP
BritBike Forum member
Joined: Oct 2007
Posts: 1,539
Auckland NZ
Cheers Pete
so my guess of around 90psi looks ok then?....

Ive got a standard car type compression gauge ("Vane") with a built in valve, all i was going to do was take a reading, undo the flexable hose (probably just with a "jubalee/hose (sp) clip", release the pressure, put hose back on then test again.

Think i see what you mean about hose lengh so i was just going to use as sort a peice as possible, probably only a couple of inches, I will test it with the plug adaptor on the A65 which I tested at 180 only a few mounths ago and see if it reads lower with the adaptor, then add the decrepencey to the measurement i get on the M20, .

Its a bit of a preformance sure, but not as big a one as pulling off and emptying the tank over a hot motor. (its got the balance pipe between the tank halves)


Thanks again

Any thoughts on reasons for excessive oil out of crank breather?


"There's the way it ought to be and there's the way it is" (Sgt Barnes)

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