Has anyone done a compression test on a sound M20 engine or knows what the reading should be? Ron

email (option): ronpier@talk21.com

Is that a numeric test - or simply how long you can stand onthe kickstart, before i goes over TDC

As if you were using a compression test gauge in the plughole. Ron

email (option): ronpier@talk21.com

Does it have any relevance?

Depend how briskly you can kick it over.
Hot/Cold
Open/closed throttle ?

Hi nigP,

You always test compression with a cold motor
And the throttle fully opened.

If you do not have an electric starter, then you kick it hard... (-:

Noam.

email (option): noam10 at gmail dot com

I would question the 'cold motor' part of your reply.

Standard practice with 'leak down testing' of big diesels is when engine is hot, otherwise the cold oil can form a better piston/liner seal ......

Part of analysis then is to place oil onto the piston to try and determine which is leaking piston(rings) or valves .......

But then again its years since i did one and i may have got it wrong ..

The reason I asked is that a guy has phoned me with an M20 non starter after having the mag rebuilt and a new concentric carb. I'm trying to talk him through it, and he tells me he's done a compression test and got a reading of 50 psi. I agree, if I had the bike in front of me, I would have a fair idea if it had reasonable compression or not, but just trying to find out what the reading should. Ron

email (option): ronpier@talk21.com

From my memory 50 psi seems low.

As it happens, today my old cylinder pressure gauge fell out of a cupboard on my head !

I'll stick it in the ES2 tomorrow, with it cold and see what i can get.

That sounds roughly about half of what it should be. If so, the engine would kick iver way too easily.

About 120 psi would be more like it I'd have thought...

However, there are variables in the test procedure to consider..Personally I think the 'standing on the kickstart' test is just as effective...

How you measure compression doesn't really matter, only that you have enough..and 'enough' in this case is to just take your weight on the kickstart...

Saying all that though, M20s will usually start even when worn...I'd check everything else (ignition spark, timing, carb set up, fuel supply, plug etc.) before getting to compression or lack of it...Ian

email (option): ian@wright52.plus.com

I've seen mention on other forums of swarf in new Concentrics. Is it a Premier, a standard Amal 600 or a Wassell ? What size bore is it ?

It seems a characteristic of the Mk1 concentric that any blockage of the pilot air drillings will cause a great deal of difficulty in starting and extremely poor running up to very wide openings.

I've only had them on twins and it's possible then to isloate what a single defective carb is doing but I can imagine that on a single, it wouldn't run at all.

The 276s seem more forgiving in this respect - probably because they spray fuel everywhere.

Sometimes it starts even if exhaust lifter is on (when cold), so compression isn't problem. In my experience low compression affects most idle running.

I agree with everything so far. But it's difficult dealing with someone at the other end of the country who's never owned such a bike before. I'm trying to talk him through the starting procedure, and now found out the bike has been stood for a number of years...So I've directed him to the sump drain plug for his next job Ron

email (option): ronpier@talk21.com

ian is right, right & right.

If 50 PSI is a real value it is way too low and there is no chance of him kick starting that bike. It can be tow started or clutched on a reasonable sized hill.

A newly ringed M20 should go 120 to 150 psi. throttle wide open.

Most people do not have the kit needed to make accurate compression readings.
So in most cases the numbers are meaningless.

Tell him to squirt some oil down the plug hole, kick the bike over 3 or 4 times then take another reading.
If it is higher than the first one his rings are not sealing, which on a fresh rebuild is not uncommon.

HE can try towing it around the block a few times throttle wide open in 2nd gear with no lead on the plug to get an initial seal on the rings.

However if this is his first job on a bike like this I would start by asking what grit it was honed to,
Chances he got it done at a car place who has honed to the standard 320 to 400 grit in which case the rings will never seal.

email (option): bsansw1@tpg.com.au

Timed on the wrong stroke is my bet if he's put the mag back and he's a newbie

email (option): ronpier@talk21.com

RON -

ES2 buried under jumk, so tested 3HW compression.

Engine cold and was running ~Xmas -
110 psi throttle closed & 125psi throttle open.

It required ~3 kicks to max the compression as my cylinder gauge has a pressure hold facility showing max pressure attained.

In my experience it depends on what causing the low compression;
If its the valves - then yes it does effect low speed.
If its the rings/piston - they will usually run very well at low speed (up to the point the plug oils up!) but top speed & high load is effected.