Need help with cam positioning


26 replies to this topic
  • grayracer513

Posted August 03, 2011 - 01:27 PM

#21

That was pretty much explained to the OP in post 2 by Gray.

I've never been certain that he understood it entirely, either.

  • upthemaiden

Posted August 03, 2011 - 07:56 PM

#22

I didn't understand why you were saying it at first, but I think I see why it affects things now. If someone with a kickstart bike can answer these questions I think it'll help me understand how it affects a bike more.

When you kickstart a properly functioning bike, how far does the kickstart lever go? Will it go all the way down until it hits the footpeg, or will it stop on it's own above that? (assuming you already followed the steps of pushing the lever until it stops from compression, pulling the decomp lever, moving the kickstart lever slightly further, and only then kicking it)

and...

If you took the same properly functioning bike, disconnected the electrical to the spark plug and tried to kick it over, would it feel the same, or would it be more likely to stop earlier because it wouldn't have the ignition to push the piston on the downstroke, to help make the compression stroke easier?

I guess I assumed that spark or no spark, kicking the bike would still feel exactly the same, just with different end results of either the bike starting or not. I never noticed a difference when I'd forget to turn the power switch on, which was keeping the spark plug from igniting. I'll pull the plug as soon as I get home from work tomorrow and make sure it has spark.

Edited by upthemaiden, August 04, 2011 - 03:20 AM.


  • upthemaiden

Posted August 04, 2011 - 03:17 PM

#23

Just an update, my bike is getting spark, if that means anything to you guys

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  • banda

Posted August 05, 2011 - 09:27 AM

#24

Spark or not, the feel at the kick start lever is not affected.

However, the kick start lever does not turn the crank though 2 full revolutions in a single stroke, so whether or not you reach peak compression in one stab at the kick starter depends on where the crank was when you kicked.

The normal starting procedure begins with working the kick start lever slowly as many strokes as it takes to reach peak compression. Might be two strokes, might be a quarter of a stroke. Then you return the lever to the top, pull the compression release, and push the piston gently past TDC with the kick start lever. Then return the lever to the top again, let go the compression release, and kick it hard to start it.

  • grayracer513

Posted August 05, 2011 - 11:34 AM

#25

The normal starting procedure begins with working the kick start lever slowly as many strokes as it takes to reach peak compression. Might be two strokes, might be a quarter of a stroke. Then you return the lever to the top, pull the compression release, and push the piston gently past TDC with the kick start lever. Then return the lever to the top again, let go the compression release, and kick it hard to start it.

The portion in bold is wrong. The correct procedure as outlined in the video I posted by Doug Dubach is to find compression by pushing the lever until solid resistance is felt, as you said. That is start of the compression stroke. From that point, the release lever is pulled and the starter advanced about one more inch. The point of this is actually to AVOID going past TDC. Instead, the piston is moved a little closer to TDC, not past it, and not past the point before TDC where the spark occurs. That way, the starting kick drives the piston the rest of the way up through the reduced compression stroke, past the ignition point, and fires it.

TDC is top dead center, where the piston reaches its highest point in the bore. If you go past it, you have gone past the point where the spark occurs, and past all of the compression stroke, and there's no chance at all that the engine will start.

You can feel where TDC is if you force the starter through compression; as you push the piston up against the pressure, there is a short little spot where the resistance disappears and the lever jumps forward a bit until the resistance comes back. That little easy spot is TDC, and the resistance after it is the result of trying to force the piston down against a vacuum through the power stroke.

As you pointed out, the engine only goes a little farther than one full revolution with each kick, so if you go just past TDC, you can't kick it far enough to get to the next compression stroke, never mind through it. If you go far enough past the first compression stroke, you can drive the piston down the intake stroke and into the following compression stroke with the momentum of the crank behind you, and that was the way that old school thumpers with 50 pound crankshafts and much lower compression used to get started. It doesn't work with these new ones.

  • banda

Posted August 05, 2011 - 08:34 PM

#26

Thanks for spotting my error and clarifying. You have a gift for precise and meaningful explanation.

  • grayracer513

Posted August 06, 2011 - 06:31 AM

#27

That error is not only yours. It's extremely common these days to hear "TDC" used to describe the point where you feel the resistance from the compression stroke. But actual top dead center is something entirely different, and the use of the term in that way causes significant confusion.





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