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10/12/2013
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Roselle, IL
US
someone just mentioned compression release
there is something on the exhaust camshaft that appears to be a compression release
Does this engine have a compression release?
How does it work?
Can I do a compression test (by kickstart) on this engine if it has a compression release?
I'm getting 60 PSI compression reading (after top end rebuild) when kicking this engine. Is that normal? I thought its supposed to be >150 PSI ???
how does engine with compression release start if there is low compression? makes no sense 
It does have an auto decompression. It works on centrifugal force with a weight and spring on the exhaust cam sprocket. Once the bike starts and the rpm comes up, the weight will be forced out and the shaft/pin rotate with it. If you wanted to do a compression test, you could disassemble the decompression mechanism. A leak down test is the accurate way to test modern 4 strokes though
Ok sure. Makes sense except for 1 thing........
If the exhaust valve is slightly open during start - how is there enough compression to ignite the fuel mixture and start the bike?
I thought you need a compressed air/fuel mixture for the spark plug to ignite it ???
It allows enough compression to start obviously. It only pops 1 exhaust valve open slighty for a very brief amount of time and at a specific time during the cycle
Sure I get it.
Seems incredibly simple yet effective.
Devil is in the details I suppose.
It must open only 1 valve for an exact amount of lift and for an exact cam duration.
A little too much opening or a little too much time and there won't be enough compression to start.
Not enough opening or not enough time and it will require too much force manually start kickstart it.
Hmmm........Never really thought this through until now......Strange how that works.
Also explains why I'm only getting 60 PSI on compression tester when I give it a few kicks........
Post a reply to: 2014 RMZ 250 Compression Release & Compression Test - HELP