Good evening.

My BSA M20 is missing it's decompression lever . When I previously owned an M20 4 years ago, I used to cut the engine off by depressing the little thumb lever .

How can I turn off the engine on my new thumper without it ?

Kind regards,

Mark

email (option): towersmark84@yahoo.co.uk

Put your hand over the carburettor Bell mouth/turn off the fuel/stall the machine/ fit an exhaust lifter!

Hi Richard,

Thanks very much for your post !

I will certainly invest in exhaust lifer on that note


Cheers,

Mark

email (option): towersmark84@yahoo.co.uk

I dunno, Mark. Most of us put so much effort into keeping 'em running and you come on here asking how to stop the blasted thing !

Yes, Apart from one method of stopping the engine, if you have an engine with good compression the valve lifter is really a requirement for an easier life when starting it as well (which is its actual function)..Ian

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

Ian, I should think you need to give up the vegetarian diet and get some real food. At 5:1 compression and a cylinder of 4 inches diameter, the maximum backpressure, with the throttle wide open, would be about 25 lbs. total. And that's without consideration of any leverage, gear reductions, etc. Yes, an exhaust lifter makes things easier, as you can spin the crankshaft to 100 rpm or more before you drop the valve. But I would hardly say that it's a "requirement" to overcome a 25 lb force with your leg.

Mark: Did you leave the engine running while waiting for a response to your first posting about how to stop the engine? If not, then just keep using whatever method you used then.

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

put one finger on top the spark plug, and your thumb on the cylinder head to short out the spark.

email (option): deadsheds[at]yahoo[dot]com

['But I would hardly say that it's a "requirement" to overcome a 25 lb force with your leg.']

It's a requirement on my 720...Whatever the theory might be, with only 7-1 compression ratio I can't kick it through compression without the lifter..and I weigh 15 stone or thereabouts and enjoy red meat...

Even with a standard M20 if there are starting problems the lifter will make life easier...If a standard motor has good compression it will more or less bear your weight on the compression stroke after all...

With the engine on compression both valves are closed so I don't quite see how opening the throttle will affect the cylinder pressure....

I tend to think BSA fitted the 'decompression' lever for a perfectly good reason and wouldn't have bothered if they thought it unnecessary and that they could save a few bob by omitting it...Ian

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

Just turn the petrol of not many of em run then

Even with a standard M20 if there are starting problems the lifter will make life easier...If a standard motor has good compression it will more or less bear your weight on the compression stroke after all...

With the engine on compression both valves are closed so I don't quite see how opening the throttle will affect the cylinder pressure....

[/quote]

Most of us don't have a 720 with a 7:1 compression - at least not unless you start pushing them out like Mr. Ford. But even with my A10 I can't stand on the kickstarter for more than a second or two before it gives in. My reference to open throttle is meant to clarify the calculation: if the throttle is not wide open when you're kicking, you don't get full atmospheric pressure in the cylinder before the squeeze stroke, so the resistance to your kicking is less than the maximum: atmospheric pressure x compression ratio x area of the piston top x gear division/multiplication.

Let me know when those 720s become available to the general public.

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

I don't actually 'need' the valve lifter to start the 16H (although I can stand on the lever for some time before it starts to sink) but there is no doubt that the most efficient point for starting to kick is just over compression. The low gearing means that if the crank is to be spinning fast enough, it needs to commence past tdc.

Without a valve lifter, pushing the lever past tdc will usually cause the piston to go half way down again.

I can stand on my 850 Commando's kickstart all day. The cylinders are only 414cc but it takes a real swing to spin it over. Did the British ever fit decompressors to parallel twins ? Yamaha had one on the XS2, didn't they ?

I don't worry about timing the drop. I lift the valve, kick it over vigorously 2-3 times so the flywheel gets spinning fast and then drop the valve. That way by the time there is compression, even with the exhaust open, the cylinder has probably filled and exhausted several times.

Keeping in mind the delicate work of adjusting the exhaust lifter on a single, I'm guessing that it would be four times as time-consuming to do it on a 180 degree twin, unless the mechanism only lifts one valve, not both.

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

Just turn the petrol of not many of em run then[/quote]

Hi Rusty nuts, funny you mention that.
My Triumph 5t had a internal leaking tap. The bike kept going with the tap closed !

['I don't worry about timing the drop. I lift the valve, kick it over vigorously 2-3 times so the flywheel gets spinning fast and then drop the valve.']

I don't use this method of starting at all...I get the piston into position just past TDC using the decompressor and then give it a good hard swing to get the flywheels moving...That always works OK for me...

The 720s will never be common...An 'unlinered' cylinder is needed for the conversion along with an M21 flywheel assembly...Neither of those two are quite as readily available these days...At least not in the UK...Though they can be found with some searching...

Also the pistons are custom made for the job and aren't cheap, so they are only made when there is sufficient demand to cover most of the minimum order number...

A lot of people don't want to do that to their M20 anyway, or don't have the need for it...

I ride mine over reasonably long distances periodically and that makes the performance improvement worth it in terms of improved journey times...If the bike is trailered on longer events, which most are, it's probably not worth it...Ian

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

I agree, in my book trailering equals defeat. It's as though you are conceding ahead of time that your bike can't make it on its own. I've ridden to every meet I've gone in North America (except the very few that the wife decided to attend), including riding the old M20 to meets far into Canada (from New York City).

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

Best way of stopping an M20 is to park it infront of a moving tank. JT

I much prefer the war time method of using a brick wall or a hedge to stop the bike.

['Best way of stopping an M20 is to park it in front of a moving tank. JT']

That's more likely to stop the tank... ...Ian

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

Like it guys 😃

11 days and 15 hours until My M20 is delivered

I will try the stalling method until I can afford a decompression lever 👍

Cheers

Mark

email (option): towersmark84@yahoo.co.uk