Compression test M 50

Can't advise on compression, John, but I can turn my 1960 Model 50 over by hand. I'm happy with performance too.

Previously John Oldridge wrote:

I have just fitted a reconditioned head to my 1960 M50 in the hope to improve the compression from 30psi ! It has improved slightly -my tester now reads 50psi after about 5/6 brisk kicks (Throttle open!)

âThe head has had new seats,valves, springs and guides, and was done by Mike P, and is a very good job. The bore wear is well within limits (I think) , but I am surprised how relatively easy it is to kick the bike over compression.( I have removed the valve lifter as not required) When I had a 16MS 350 AJS about50 years ago! I'm sure Icould stand on the kick start for a number of seconds.

âIt could be that when I was 18 I was lighter than I am now...... but not sure!............... Performance is ok....... 65 mph ish on level with not too much effort.

Could anyone advise what compression I should expect.....

Regards John O

John; Compression reading of 30 or 50lb psi sounds a little low to me.

Not familiar with your model but as a simple guide, air pressure nominally 14psi.

2:1 ratio would be 28lb psi, 3.5:1 would be 4lb 9psi

I suspect your machine should be in the order of 6 or 8:1, meaning pressures around 100lb psi are expected. Did you oil the bores before your tests?

Next I would check your gauge; simple if it has a Schrader attachment, pop it on a tyre or go to a air line with a gauge on it.

If not try it in a know cylinder, (another bike or petrol engine car).

Then go back to the motor, checkyour valve clearances and head bolt torque.

When you stand on the kickstart can you hear a hissing of air or does it just slump down?

I guess if there are no mechanical maladiesgoing on; try startit?

It may well be ok...

Nice to know how you get on

Cheers

Jon.

If the compression ratio is 7:1 (Bacon says 7.3) and all is perfect, a slow pressure (constant temperature) test will give 7 times atmosphere 'absolute'or 6 atmosphere 'gauge' pressure as George describes.

But if it is a rapid ('adiabatic') expansion, its temperature will go up and the final pressure will be higher. If it were all perfect, my calcs reckon it could reach 200psi. But nothing ever is...

But my guess is either the rings/bores are a bit worn, or it was too dry to seal (might be higher with a drop of oil or when it's still warm and wet). My Dommie went to about 150psi last time I tested (if I remember correctly). That's over 10 times atmosphere although the compression ratio isn't 10:1.

P.(V^1.4) is constant as I understand it.

Previously George Phillips wrote:

Just a point on your figures, Jon. When you put the gauge on the "uncopmressed" cylinder it will not read 14.7. It will read zero as it's measuring gauge pressure. So you have to deduct 14.7 from your final figure to get it back to gauge pressure. So your "around 100psi" figure will be "around 85psi. BTW I got the CR from Bacon's Norton book. Cheers, George

Not sure about that george.... Are my tyre pressures 24 or 10 Lb The gauge could be calibrated to absolute or ambient George;

Pg= Pa- Pambient. .

Not withstanding i think the figure of 85 is still a lot more than the 30-50 that was seen.

PS: what was the CR for the Model 50?

Cheers

Jon

Yes.always oil the bore.

Performance is good compared to my bike. 45/50 is a happy cruise but I am 6'3" and 18 stone!

mine runs better on 20/50 oil than straight 30/40 and oil leakage is minimal.

I have seen 75 plus on the speedo once but it has never managed anything like that since.

Hi John. Your compression pressure is way too low...Should be at least 100 PSI. It could be your tester though, as some are designed with the non return valve sited a long way from the spark plug hole and have a long hose which adds air capacity/volume to the compression space and lowers the effective compression ratio. The valve needs to be at the very end of the testing hose (spark plug end) ideally.

By the way. All the above calculations have forgotten that the inlet valve closes at around 75 deg After Bottom Dead Centre which shortens the effective compression stroke and lowers the slow speed pumping pressure. However, what offsets this is that the tests should be made with a warm engine which raises the measured pressure as the compressed gas is expanded by the heat as well as the piston rings sealing better due to the oil coating the bore and the rings expanded and minimising their gaps.

I would check your tester on a known good engine of similar capacity or a 4 cylinder 1.4 litre car engine for example.

I would say it is very likely your piston rings have been trapped in their grooves and you can see or test for this by pouring in a dessert spoon of thickish engine oil into the plug hole and redo the compression test noting that several fast kicks are required to get the final reading. If the pressure increases nearer to 90 PSI say, then it seems the rings are not sealing well for reasons just mentioned....Les

PS...A glazed bore can also be the cause. This can occur when you run the engine in too softly with too high a quality oil. The microscopic bore honing marks get filled in with burnt on oil deposits so can't hold any fluid oil and thus the rings fail to seal properly.

John; a little oil in the spark plug hole will help seal the rings for the test. It is difficult to get an accurate reading as the subscribers above have said;.a piston works on a wet cylinder else it would wear rather quickly.

I feel your 30 - 50 reading is low but the reasons above, when addressed may well give you a better estimation. As previous if there are no mechanical issues then start her up. It should be a sprightly performer with the work you listed,.and after a few runs take the reading again an see where youare for reference purposes.

Keep yourgauge, faulty or not its your baseline reading, assuming performanceiswhat's to be expected, note the reading and monitor it over the season. If something is amiss it will change.

Hi John,

regarding the lightness of resistance on the kickstart - i have a 1960 ES2 which is similar in terms of going well but being easy to kick.

We did a check on the cam timing a little while back, it was woefully wrong, and discovered that the cams have quietening ramps. These cams seem to make the kicking softer while maintaining performance.

I've no idea whether quietening ramps were original or whether your model would have them.

I agree with Jonathon so start it up and take it for a ride. When the engine oil has warmed, ride the bike hard for as long as you can or wish, by using lots of throttle and revving it quite highly. This will really heat the piston up and loosen any stuck rings* that I expect is the problem and get them sealing again and thus restore compression. testing soon after this treatment will almost certainly show a compression improvement....unless the rings have broken .........Les

* Not absolutely guaranteed as stuck rings can be trapped under a smear of piston metal caused by a previous slight seizure (not always noticed when it occurred).....Les