Down here in Devon it's raining, which is not unusual. So I just started leafing through an instruction manual for my 1965 B40F (well, it lists various C15 and B40 models on the cover, but I'm not absolutely certain it's the correct manual for my year.) Anyway, I noticed in the opening pages, under the heading "To Stop Engine" it says "Close the throttle and turn the ignition switch to the OFF position." Fair enough. It then goes on to say "An exhaust valve lifter is provided on Trials and Scrambles models which can be used for stopping the engine." A couple of things puzzled me about that. Firstly, mine is not a trials or scrambles model, but it has a valve lifter. Is that not normal on a 1965 B40F? Secondly, I thought the lifters were for use prior to starting the bike, to overcome compression and get the piston just past TDC before releasing the lever and swinging the kickstart? So I was interested to see they can be used to stop it. Is that common practice?
In any case, when I operate my lifter lever I'm not sure it makes any difference noticeable when kivking over. Maybe there's something wrong with it, but I'm not too concerned as I don't feel I need it.
Any comments on any of these points please? This is mostly just out of interest really. Phil
Hi,
I think a lot of trials bikes had decompressors to aid slow down hill sections on a closed throttle when if on full compression the back wheel would lock and stall the engine - 2 stokes especially. Any trials riders out there?
When I had a B40 I never used the decompressor as with only 6.5:1 compression and a squarish bore and stroke it was not needed.
Dave.
I think the valve lifter is completely unnecessary on the B40 models. My SS90 has higher comnpression than the standard model but I still don't need to use it.
I start this bike with the same techique used for most smaller engines, bring up to just before compression, let the kickstart lever back up on the ratchet and then kick through.
On the other hand my M21s couldn't be started without one as I do need to lift the valve to get just past TDC before initiating a long and confident kick.
Hope the sun shines soon Phil!
Cheers,
Bernie
According to the B40 owners manual, the valve lifter is there to be used for starting the engine. Essentially, the technique was to rotate the engine until you feel compression and to then use the lifter to release the pressure before kicking the engine over. For most owners this is not really necessary but was probably thought necessary by BSA to cater for riders with a slighter build. The valve lifter was fitted to the C15 competition models more as a way of being able to quickly stop the engine without taking your hands off the handlebars.
For larger single cylinder engines (eg B50), the need for a valve lifter is a bit more necessary since it's important to correctly position the piston just after TDC on the compression stroke before trying to kick it into life. If you don't then the engine momentum you can impart through the kickstart may not be insufficient to carry it past the next compression stroke and get it to fire up.
There are a number of other uses for the decompressor lever on the larger single cylinder engines. As said, it is sometimes used in scrambles to avoid too much engine braking on loose downhills sections. It can also be used to allow a flooded engine to be cleared which can happen during the use of a cold starting device - even my kickstart Honda XR250 is fitted with one for this purpose. I also use the valve lifter to stop the engine on my large capacity CCM single with electric start - the problem is that the engine will normally stop after it hits compression at TDC which will then jolt it backwards leading to unnecessary wear to the Sprag clutch. I also do it this way on my other large kickstart single as it provides a much gently way of stopping this large engine.
The valve lifter or decompressor was or is used to help with starting.
It allows a good swing on the kickstart to use the momentum of the flywheel to spin the engine.
It also saves excess loading on the kickstart mechanism which is not the strongest of assemblies.
When I did an overhaul on a fairly new 4-stroke KTM enduro bike, I found that it had an automatic decompressor in the cam lobe. So decompressors are still regarded as useful. On Vincents, some people advice against using it for stopping the engine. Though I have not yet experienced any problem on mine.
Most more modern OHC singles are fitted with auto-compressors. The one on the Honda XR series of trail bikes is very sophisticated because it not only helps with starting by reducing the compression but also makes it impossible for the engine to kick-back - basically, the exhaust valve opens if the engine starts to rotate backwards. If only my B50 was fitted with such a device it wouldn't then be quite so frightening to start sometimes!
In terms of stopping the engine using the decompressor, I think it may be a question as to whether the mechanism was designed for this purpose or not. The BSA unit singles decompressor is ok in this regard but maybe the one on Vincents was not?