Hi all . What Im about to ask may seem a dumb question but Im relatively young and inexperienced so I hope that begs for clemency in the event . I have got used to staring my WM20 on full compression and have up until now assumed that the decompression valve is for stopping the engine and nothing else , as during the actual starting of the engine it seems to have no effect . I have other bikes which feature decompression facilities , for example a 1980's ex British army Armstrong MT500 , in the case of which the decompression lever serves the purpose of making starting easier , as it does with the larger capacity unit construction BSA singles of the sixties , for example . What made me wonder if there is perhaps something amiss with my WM20 is that the other day during a visit to a distant town I saw a guy start an M20 very easily , using the decompression lever , I noticed , with an almost effortless swing of his right leg , whilst actually sitting on the bike ( I am just about young and strong enough to manage this myself on full compression , as I say , although obviously I find it a lot easier to start the bike while standing next to it with it on the rear stand , as I can then of course put my full weight on the kickstart . I did'nt get the chance to talk to the owner of the bike as I was waiting at traffic lights in my car and he rode away out of sight . So , can anyone enlighten me , should my decompression lever reduce compression to make starting easier , in which case I must presumably investigate a fault , of was I correct in the first assumption , i.e is it there just to stop the engine and do I just need to keep doing the leg excersises ? Regards , Leif .

If it stops the engine then the valve lifter must be doing just that - lifting the valve off its seat and removing compression.

If you gently bring the engine up towards TDC on the compression stroke, it should stop. Lifting the valve should allow you to effortlessly bring the piston over compression a fraction. If you then release the lifter and give it a good swing, the kick start should be able to travel the full extent without undue force.

Hi Leif..The primary function of the decompression lever is to aid starting, by allowing you to easily position the piston just after compression before starting the bike.
The external parts of the lifter assembly are pretty self explanitory...
At the engine end the operating arm fits on to a fine spline on the operating spindle. This passes into the valve chest, where on the other end is a peg that is machined eccentrically to the spindle.
The exhaust tappet head has a flange around it and when the operating spindle is turned (by pulling in the valve lifter lever), the eccentric peg contacts the flange and lifts the tappet and exhaust valve..thus releasing the compressed gases from the cylinder.
Common possible problems that prevent this taking place are incorrect setting of the mechanism initially (it is quite fiddly),incorrect free length/adjustment of the cable, a damaged flange on the tappet head or a worn peg on the operating spindle. Plus any combination of the above.
If you remove the tappet cover you will clearly see the internal operating parts at the exhaust valve side...Ian

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

Is this a windup??

There's a crusty old guy near me who lifts the decompression lever, starts kicking the bike like a madman to get her going so the thing "free wheels", then drops the decompression lever.

It's hilarious to watch him go through this routine.

He usually has 3 inches of an old stogie in his mouth muttering to himself. He wears a dirty old moth eaten greatcoat, leather helmet with goggles and filthy pants.

What a character! But, the bike runs like a watch. I think he can get a slow tickover of 20-30 RPM!

email (option): britool51@hotmail.com

Hey, Robb, that's how I do it and the rest sounds like a reasonable description as well! Do you have a camera set up around here, or something? They didn't give the M20 a 300lb. flywheel for nothing. Give 'er a few good kicks and dump the valve, she starts right up because she's just been breathing a few good breaths.

email (option): jonny.rudge@verizon.net

When first introduced to M20’s as a teenager I used to start the engine from a seated position by bringing it up on compression, releasing the kickstart lever, then leaping athletically into the air to bring my whole weight down on the lever. That generally worked fine, but when it didn’t I simply repeated the process a dozen times until I’d bullied the engine into submission.

Not so now.

Leif, the M20 is a forgiving beast. Rik’s method is the official version of how to start it although the Robb/John method and other techniques such as above also work, so just use whatever suits your temperament and fitness level.

An analogy:
An old bull and a young bull are in a field next to a herd of cows. Young bull says “Let’s run over there, jump the fence and have our wicked way with a cow”. Old bull says “Let’s amble over there, push open the gate and have them all”.

ChrisJ

Well , thanks all of you for your input , and actually no , it was'nt a wind-up . What is patently obvious to some may not be so to others . I have often given help and advice to people less clued up than I , (in the world of modern diesel engines , mainly) and I am also a guy who will gladly admit to ignorance and inexperience in order to benefit from the advice of the wise . It sounds as if my decompression valve is faulty to some extent , as it will stop the engine , but only just (takes forever and a day, rather like an old lister generator) , and try as I might I cannot persuade it to give any sign of releasing compression during attempts to start the engine , regardless of the method employed . I would much rather remedy this than continue with the bull in a china shop approach to starting the bike , as this in time would put added strain on the kickstart mechanism . Best regards to all .

Hi Leif..I agree...it's a learning curve for all of us and some are just a bit further up the curve than others. When I used to train apprentices it was always stressed that you should never think any less of them because they didn't have as much knowledge as you did..Ian

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

maybe a donkey engine would do the job

email (option): roger.back@node6.com

Lief, I've said it before. The M20 valve lifter is not the best design in the world. I think you need to go back to the basics of re-adjusting the mechanism at the bottom of barrel. The trouble is with these is:- Over adjust and it will keep the exhaust valve of the seat, under adjust and it just won't work....like yours. Ron

email (option): ronpier@talk21.com

Our aim with starting technique ought to be to preserve our own legs but also to show as much mechanical sympathy as possible. Trying to avoid jumping on the kickstart can't be a bad thing.

W.C. Haycraft's pre-war 'Book of the Norton' gives the starting technique as follows :- (He doesn't list any alternative)

"To start the engine, stand astride the machine, put the piston on compression, raise the exhaust valve lifter, depress the kick-starter with a vigorous kic, and drop the lifter when the crank is about three-quarters down."

...so it obviously was considered a legitimate manner in the old days.

Personally, I don't find the 16H easy to start astride the machine. The gearbox is too far back and the wide part of the saddle is trying to push my leg outwards as the kick-start lowers.

Me too. I always find it easier by standing along side the bike while it is on the stand. Ron

email (option): ronpier@talk21.com

A number of years ago I was attempting to start my M20 and it fired but at what part of the stroke I can't remember because I was hopping around in agony as the she fired the kick starter back up at my leg!
Now,I know my technique is pitiful, but has this happened to others or have I set something wrong on the bike that bit me so bad.

If the timing is correct and you start the bike with the lever at full advance, there is a tendency for the engine to kick back. Best to retard the ignition by about half the lever travel before attempting to start up. Ron

email (option): ronpier@talk21.com

I always start the bike standing over it, setting the piston just past TDC using the valve lifter and then giving it a good firm kick. I keep my foot on the kicksart, relaxing my leg muscles just as it reaches the bottom of the stroke so if it does kick back there is no resistance to the movement.
The key is to kick hard and quickly so that the inertia of the flywheels keeps them going in the right direction..
It's best to start the bike with as much advance as possible, so if it kicks back I will retard it progressively, just enough to stop it kicking...You soon get familiar with where exactly the ignition should be set on your own bike..
I find M20s start best if they are not over flooded and the throttle is left on the stop until the engine fires and is then used to 'catch it'..Ian

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

I've never had one kick back when using the "spin the flywheel as fast as possible and then drop the valve" method. I think the inertia of the flywheel would be too great to reverse course. In any event, that method permits you to take your foot off the kickstarter before closing the valve, so even if it kicked you would not feel it. Two or three robust kicks with the valve up is all you need.

email (option): jonny.rudge@verizon.net

I always straddle my bikes to start them.Use full advance on all of them except my B33 as it has a nasty kick back.Best not to start a rigid M20 on rear stand as you can fracture the mounts where stand bolts to frame.My WM20 always starts 1-2 kicks hot or cold unless I tickle the carb to much..Dave

I start my 850 Commando astride and the 16H generally starts well but there is something odd about my geometry or that of the 16H because the position of the saddle and that wide rear section on the standard Terry's makes me feel as if I'm pushing backwards rather than downwards