bump starting my 426

8 replies to this topic
  • NorCal426

Posted March 29, 2005 - 05:07 PM


HEy, i have an '01 426 and i need some help on tihs. it is quite easy to bump start, all i do is put in first gear with clutch and decomp in, then release the clutch and soonm after release the decomp, it starts right up, but i read somewhere that killing it with the decomp is very bad for it. is this true? and if so isnt what im doing to bump start it essentially the same thing? so would bump starting my bike like this a bad idea? let me know what you think, any input is greatly appreciated, thanks

  • buck_y_lee

Posted March 29, 2005 - 06:34 PM


I guess there is the possibility of the valve and piston becoming intimate while the engine is under load, but I would think it unlikely. That said, I don't and won't do it.

Maybe someone with some real knowledge could tell us how close the valve and piston can get at TDC with an open exhaust valve. I don't understand the mechanism that opens the valve either - I wonder if that can be damaged?

  • tricksta400

Posted March 30, 2005 - 05:43 PM


hey buddy, i got first hand experience in this field, i did that and it put a pretty big chinck in my piston and then caused the valve to freeze and it was a whole mess, so just take the time to kick the darn thing and use the kill switch, thats what its for, it'll save you alot of money in the long run!!

  • Fastest1

Posted March 31, 2005 - 01:58 PM


Tricksta, did you damage it starting or killing it? It shouldnt have collided, especially during start up when things arent up to speed. I have and known many others who have started their bikes this way with no problems. I would be hesitant to kill it that way though. If you go with the 450 exhaust cam, it will bumpstart with no problems.

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  • grayracer513

Posted March 31, 2005 - 02:45 PM


Your bump starting technique is fine, and you run no particular risk of damage by using it. The difference between starting and killing the engine with the release is simply a question of speed. The point at which the exhaust valve and piston are closest is around TDC at the end of the exhaust stroke/beginning of the intake stroke. If you watch the operation with the cam cover off, you will see that the valve is farther open at this point than the compression release is capable of raising it, so clearance is not an issue. It would not have been built this way if it was.

So, what's the problem with using the release on a running engine? Visualize the assembly and think about what's going on. If you are just idling, the engine's probably turning 1500 rpm at least. So as you bear down on the valve lifter with the release shaft, you are increasing the clearance between that lifter and the cam, and when the cam closes the valve, it smacks it back down against the release shaft rather ungracefully. That's the biggest problem, in fact, because the impact of the shaft striking against the lifter off center imparts a rocking load on the skirts of the lifter. This load will naturally increase with engine speed.

Compared to even a low idle, the age-old (older even than me) technique of rolling the engine off with compression release in produces much lower speeds and loads on the cam follower (300 rpm or less).

The only thing I'd say is that second gear works better, and pulling the bike backwards in gear against compression gives you a two-revolution head start.

  • PumpkinHumper

Posted March 31, 2005 - 06:03 PM


Ive bumpstarted my bike while rolling down hills like what you describe. No problems at all.

  • KiwiRacer

Posted March 31, 2005 - 06:22 PM


I've heard of several guys bending there valve bump starting the bikes with decomp,So I'd say NOT to do that. Now I don't know anything about killing it with the decomp.... WHY :naughty: do that.... You've got a kill switch. :naughty:

  • grayracer513

Posted April 01, 2005 - 09:35 AM


You can reduce the danger (if any) of valve/piston contact or any other damage even further by increasing the amount of free travel at the decomp lever to 50% of the pull of the lever. That will shorten the amount the compression release is capable of lifting the valve.

  • Ga426owner

Posted April 01, 2005 - 09:58 AM


Mine would only roll start in 3rd or 4th - too much compression in 1st and 2nd.....never tried the decompression lever -

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