compression

Mckorkins · Post by **Mckorkins** » Sat May 26, 2012 9:14 pm

Ok guys, another hare to chase: if I were to do a compression check on my 1956 Model 18 , what would be considered a good reading in psi given that I would be kicking it over? And I don't mean kicking the entire bike over!

saltbox alf · Post by **saltbox alf** » Sat May 26, 2012 11:28 pm

anywhere between 110 and 130 ilb's per squinch is usually good enough. a materess is handy for kicking it over

rex.webb · Post by **rex.webb** » Sun May 27, 2012 9:40 am

I like to ROLL OVER on a MATRESS ??
RIDE with CARE and LIVE LONG to ENJOY your Bikes and Scooters

Mckorkins · Post by **Mckorkins** » Sun May 27, 2012 9:15 pm

Wow! 130lb seems awfully high compression reading for an old plodding thumper like mine. Are you sure you are talking about a Model 18 and not a G50 or a 7R?

saltbox alf · Post by **saltbox alf** » Sun May 27, 2012 10:24 pm

A single will run with 90 lbs sq in but it will not give full power and need more fuel to start.
I would try a repitition test 5 or six kicks and see if the compression rises at each kick. or test your compression once then pour a little oil in the cylinder and see what improvement you get!
For racing engines you could be getting 170 to 190 odd lbs. generally under a ton it needs looking at.
Rex I am rubbish at spelling, perhaps I ment misstress ho ho

rex.webb · Post by **rex.webb** » Sun May 27, 2012 11:08 pm

Rex I am rubbish at spelling, perhaps I ment misstress ho ho
And MORE Ho H O !!
This SANTA IS all ways looking for some
Ho Ho H o ing to enjoy .
As i say Ride with CARE and LIVE LONG .

aobp11 · Post by **aobp11** » Mon May 28, 2012 10:29 am

It looks less if you convert 120 psi to 8.3 bar. It happened that someone reported this result in Dutch magazine (May issue), along with a long list of non-fault circumstances (like revs and throttle opening) that can dramatically influence such readings.
I would add that a high reading isn't good either, this might indicate a thick coal layer inside.
Albert

iandusud · Post by **iandusud** » Mon May 28, 2012 6:25 pm

I presume a comp test shoud be done with the throtlle wide open.

Ian

1608 · Post by **1608** » Mon May 28, 2012 7:18 pm

A comp test should be carried out on a hot engine and with the throttle held fully open. Not sure about the single, but my twin gave a reading of 115 psi on both cylinders. Not wishing to teach anyone how to suck eggs, but if by pouring approx 1x spoon of oil into the plug hole increases the psi considerably then suspect cylinder/ring wear. If there is negligable improvement but still poor comp then suspect valve wear.

Mckorkins · Post by **Mckorkins** » Mon May 28, 2012 8:52 pm

Thinking about it, I tested my Morris Minor's A series engine cold and the cylinders pushed about 165psi cranking it over on the starter motor, so it will be interesting to see what old Maisie will produce. On reflection 9-10 bar does seem achievable, as the motor has only done about 300 miles since it was rebuilt some 10 years ago. No, I'm not kidding - it's done that little mileage in a decade!

Can't do anything to it at the moment because I'm in France getting fatter by the day: food vg...Edited by - Mckorkins on 28 May 2012 7:57:37 PM