When you kick a Commando motor over to start it, both valves in one cylinder are closed. How can cam timings or profile change the pressure at TDC ? Different cams only enable a bigger gulp of mixture when the motor is running. I suppose if you stood there kicking the motor over rapidly at about 4000 RPM, the cam might make a difference.
Ashley, every Triumph 650 I ever owned was kick start and had race cams. I have even kick-started a 1000cc Vincent which was on 13 to 1 compression. It is the same procedure - push the piston up slowly up slowly until it is well up the bore, then give it a strong shove. If you kick a four-stroke motor over with the piston travelling from BDC, it will often bite you. The piston only needs to go past TDC to get a spark. The rate at which the piston travels up the bore does not usually change the strength of the spark, even with a magneto.
My electric start for my Seeley 850 needs to be used with 3rd gear engaged. It is powered by an angle grinder and It works, but my motor is only on about 9 to 1 compression. I am to old to bump start it, these days.
I have had old motorcycles since I was 15. When I was 29, I began road racing. Until then, every bike I had was kick start. If the bike will not start by kicking it, a bump start will not usually make it fire up. If you have compression, mixture and spark - it has to start, unless the ignition timing has slipped. Then you will hear it backfire through the exhaust.
The hot cams I have looked at close on inlet later than the stock cam.
This means that at kickover and at low to mid rpm the hot cams trap less mixture in the cylinder than the stock cam does, so the effective cr is lower with the stock cam. That is why the stock cam makes more power at low to mid rpm and the hot cam makes more at high rpm.
Once the ideal rpm is reached with the hot cam (comes "on cam"), the late closing effect of the cam actually helps to pack a little more mixture in than the stock cam does.
That late closing does ease kickover some, but the effect might be fairly small.
For example, it is enough on my 1360 that I can just kick it thru even with 11 to 1 cr. I doubt that I could manage this with stock cams.
A friend in Norway built a 1200 cc Egli with 9 to 1 cr and less aggressive cams than my 1360. On paper it should be much easier to kickover than my 1360. The cr is a lot lower and the engine is quite a bit smaller. The only thing not in his favour is the less aggressive cam, which is still not as early in closing as the stocker.
He is a sturdy fellow, a mountain biker, but has never been able to kick start the bike nor has anyone in the local vintage MC club. They have all tried and several ride kickstart Norton Commandos that they kick over easily.
He eventuallu fitted an estart type that normally works well on 1000 cc Vins, but the drive just kept shearing off.
End result the bike is bump start only and it requires a big hill at that!
So I think the cam type does make a difference to kickstart effort required, however since there is a huge range of cams out there, it's difficult to say just how much difference it makes.
Glen
