Port timing?? - Pocket Bike Forum - Mini Bikes
 
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post #1 of 5 (permalink) Old 10-28-2004 Thread Starter
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Question Port timing??

Can someone explain to me what this is and how it works?
Thanks

Modded FF and HF Cags
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post #2 of 5 (permalink) Old 10-28-2004
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Since a 2 stroke has no camshaft the time that each transfer and exhaust port are open is the port timing. The piston passes past the ports and the length of the ports and their location relative to each other is the port timing. Just think of it like a camshaft, it opens the intake valves and exhasut valves for so many degrees of the crank turn, well when you have no camshaft and the ports are controlled by the piston passing over them it is pretty much the same thing. Raising or lowering the ports as well as the length of the ports determine at which point of the cycle they are open, obviously changing that changes the timing big time and you could somewhat compared that to installing different camshafts that have longer or shorter duration to make the valves open a longer or shorter time. Of course on the 2 stroke it can also have a large affect on the compression since moving the exhaust port much higher can mean a shorter compression stroke.
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post #3 of 5 (permalink) Old 10-28-2004
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I couldn't have explained that better myself. Here's a question for you. I was working on a friends big bore head the other day and I noticed that it had a notch machined into the top of the exhaust port flush with the cylinder wall. It's about 1/16" in diameter and about 1/4" deep. The only thing I could see being it's purpose would be a compression relief, extending the exhaust timing allowing the piston to bleed off some slight compression before before reaching TDC. What the hell would you want to do that for? The other thing I thought of is that it may have been a guidline for porting the exhaust, except he has the BB head ported from the dealer for exhaust. Anyone know what I'm talkig about, have an explaination or suggestion, like fill it with epoxy?
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post #4 of 5 (permalink) Old 10-28-2004
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That is a compression release, it is there since the plastic pull starter pawls suck and break even with the compression that is there. It is small enough to where it does not affect the power after 2000 rpm, it just relieves it a little so the pull starts don't strip out.
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post #5 of 5 (permalink) Old 10-28-2004
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Oh yeah I forgot about that daamn pullstart. That makes perfect sense.
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