Not the usual "kick starting a bike witha decomp lever" thread..

2 replies to this topic
  • cjard

Posted September 25, 2009 - 09:21 AM


I have a question about the theory behind starting a 4 stroke engine using a decompression system..

Is it the case that, when following the starting procedure as in the videos/manual (kick till compression, decompress, move 15 degrees, return to top, kick through):

Is the piston positioned before ignition, and kicking through ignites a partly-charged cylinder which is enough power to get the engine to complete an exhaust cycle and another (proper) power cycle

Or is the piston after ignition, and it is our kick that is putting enough momentum into the flywheel etc to get a full exhaust/intake/ignition cycle out of it getting us into proper running?

(At the risk that that isnt understandable, i guess the question could be simplified as: Following the starting procedure from TDC, is it our kicking leg that provides the power for the ensuing exhaust-intake-compression-ignition, or is it a weak explosion from a partly charged cylinder that powers it?)

I got to thinking that decomp cams must work in a different way to decomp lever, from what I've read; the cams hold the decomp open all the time until the engine is spinning at speed. Doesnt this mean that a decomp lever adjusted to decompress by the same amount as a decomp cam would mean that the starting procedure becomes: squeeze the decomp lever, kick the kickstarter, when engine fires, release decomp lever

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  • matt4x4

Posted September 25, 2009 - 10:07 AM


With manual decompression lever:
Your kick is enough to cycle it through the exhaust cycle and into the compression cycle, cause it to come up to full compression and fire - thus starting the bike. (So after ignition) - try it - go beyond 15 degrees - say 30 and it'll still start - believe me - I'm not that accurate with mine every time I start it

With auto decompression: Same thing except the auto decomp pin will stop working above approx 1300 rpm (centrifical force). So if your kick is lifeless and slow it's harder to start the bike, but if your kick is forceful and fast it's easy.

Also - the pin will tap the valve every time it comes around at just a hair before TDC, releasing the compression, BUT - it's only opening it at the right time for a brief moment.
Holding the lever on a manual system is completely different since it acts like a stuck open valve now and you will be getting close to 0 compression all the time because you are no longer allowing a buildup and then bleeding it off.

  • cjard

Posted September 27, 2009 - 04:46 AM


Many thanks to one and all who answered my queries on this; I got the bike started yesterday and it's now such an easy start, that a girl can do it..

I think the first time I was trying, being one of those too eager "It's 1am and I just finished putting everything together, let's see if it goes" attempts that probably flooded the engine.. Come back the next day with a bit more energy and having let the bike dry out over night, and it fired up a treat (though from cold I do have to blip the throttle the tiniest amount just after I kick it, otherwise it fires a couple of times and then dies)

Thanks again


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