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Discussion Starter #1
Hi folks just got me a ktm 85sx 2007. Just to get into it again. I’ve been having a few problems with low compression. First of all the coolant was spewing out the overflow on the rad so I replaced the head gasket and while I was messing I did a compression test (throttle wide open) kicked it over like my life depended on it but I’m only getting 75psi which is half of what it should be from what I’ve read. Any ideas what else it could be? thanks in advance.
 

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I'd be wary of compression testers, first of all is the gauge accurate, secondly where is the non return valve and how much extra volume does the tube running from the head to the gauge add to the combustion chamber volume. (This is why the location of the non return valve is important). With a small engine the stuff that I've mentioned is going to have a bigger effect than say on a 300.

Rather than being concerned by the PSI reading you would be better off looking at the state of the bore, the piston and ring(s). If they are all good then I would not worry about the PSI reading. Also what does the engine feel like when you push down on the kick starter by hand, if you can feel the gas just leaking away because the kick starter just moves under steady pressure then stuff is worn out, if the kick starter can be held against the compression without moving quickly then the top end can not be too bad.
 

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Discussion Starter #3
Makes sense with the extra pressure in the line the release valve is at the gauge. The bike seemed to run just fine till the coolant starting shooting out but changing the head gasket has stopped that, I’m getting white smoke but I think that’s because coolant got into the exhaust system so I’ll see if that clears up.
 

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The release valve is one of the valves but it is probably not the one way valve, the one way valve will be 'in line' at one of the connection joints.
 

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Discussion Starter #5
It has a quick release when the gauge connects to it and the screw adapter at the plug connection. The bike starts first or second kick maybe I’m over thinking it after using the comp tester.
 

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If it runs it runs, when the rings have finally burnt out then it will kick over super easy and not start.
 

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Personally I would not let the rings wear to the point where the engine won't start, if you do they will be so knackered that they may end up breaking and then you are into loads of money.
 
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