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mcat

Starting and TDC

5 posts in this topic

I have read several times here about these bikes being easier to start by putting the piston just past TDC, and this makes perfect sense to me. However, the procedure for finding TDC has me confused. Everyone says to kick till you feel resistance and you will be at TDC. Maybe I'm missing something, but the resistance should be felt at the bottom or near the bottom of the compression stroke, or about 90 degrees before TDC. Are folks just making this simpler to explain? I realize it makes no difference if its really at TDC or 90 before, you just kick till resistance, then pull decomp lever and go a bit past then give kick it hard and off you go (sometimes), but I have seen this mentioned many times and I'm wondering what I am not understanding about TDC.

Marty

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The intake valve doesn't close till a bit after bottom dead center, and then you have to turn it a bit more before resistance if felt. The auto decompresser keeps the exhaust valve cracked until about 1/4 turn from TDC. So you will not really feel it till just a bit before TDC.

The whole reason to get at TDC is the resistance is less until the next compression stroke. That allow you to get the motor spinning fastest with the least amount of effort. It the speed that gets it going.

I'm a big guy and I kick like a mule. I usually don't even bother with the whole TDC thing. I "locked" my old 81 XR500, by disconnecting the automatic decompresser and removing the manual lever. None of my friends could start the bike in that configuration. One of my smaller friends, who weighed about 140, could just stand on the kick start lever and it wouldn't even move. I could usually get it going on the first kick.

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I didn't realize the auto decomp left the valve open that long. I understand the reasoning for getting it just past TDC before you try to start it, just didn't understand why everyone was saying TDC was when resistance was felt, the auto decomp makes perfect sense now, thanks.

Marty

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Yep. When people say TDC in reference to starting what they really mean is just a little before TDC.

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If it's that much of a mystery, you could remove the inspection cover off of the side case, kick it until it stops and then kick or move crankshaft to see where the flywheel is relation to the compression stroke cycle. :thumbsup: I'll bet somebody's already done that here, too.

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