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Discussion Starter #1
I got an 09 Brute 750 that I did a full rebuild on a couple of months ago and have only ridden it lightly for 20 hours or so. It smoked when I first started it when I rebuilt it (normal) and it kinda went away but after a while, it started getting worse and I was getting crankcase blow-by, I could tell by the oil in the airbox. It also seemed to lack power. Anyway, after a while, I figured I should investigate. I found the front cylinder to have low compression. (My memory is failing me but somewhere in the 20-30 psi range I think) . I put a little oil in the spark plug hole and it went way up to the 80ish psi range. I figured I installed a ring wrong or something so I removed the head and cylinder only to find everything looks perfect. There is very very minor scoring on the piston but that's it, nothing I can feel just visual marks. I bought new cylinders and new Wisco pistons and rings when I rebuilt it. Any idea what's wrong with it?
 

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Discussion Starter #2
I'll add that the top rings were offset by about 120 degrees with the very top one facing towards the back of the quad.
 

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Which way is the arrows pointing towards for the front and rear pistons ? They both should be pointing towards the front of the machine.
 

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Goin through the same issue right now with one of my cylinders. It was 78 psi before I overheated it in September. Now it has 40 psi.
The following things have to be checked:
Camshaft compression release weights and pin.
Valve seats and valve guides.
Head gasket.
Cylinder head warp.
Cylinder cracks.
Piston/ring damage.
Cylinder/piston clearance.

The last two are screwed up on mine.
 

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Discussion Starter #6
Wouldnt the oil test elimintae all those but the piston/ring damage? Or is it not as fool proof a test as I'm thinking?
 

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Wouldnt the oil test elimintae all those but the piston/ring damage? Or is it not as fool proof a test as I'm thinking?
I've put oil in thru the spark plug hole on brand new pistons/rings and badly worn ones. In each case, the compression went up. Oil is not compressible so it will always give a false read, in my opinion. A leak down tester is much better for finding the problem.
 

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Did you hone the cylinder? It sounds like the rings aren't seating.
 

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Might be a bad break in after the rebuild. There are so many different opinions on how to break in new cylinders------it's frustrating. I myself believe in the "drive the crap outta it for 20 or 30 minutes or so" to fully seat the rings but there are others who say take it easy to prevent damage. Break in is different if you have a fully rebuilt engine, but for just cylinders and rings, I'd go with the drive it like you stole it break in. The problem is, if the rings didn't seat, you have to rip everything apart again and rehone the cylinders as they become glazed and will not break in no matter what you do.
 

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Discussion Starter #10
Might be a bad break in after the rebuild. There are so many different opinions on how to break in new cylinders------it's frustrating. I myself believe in the "drive the crap outta it for 20 or 30 minutes or so" to fully seat the rings but there are others who say take it easy to prevent damage. Break in is different if you have a fully rebuilt engine, but for just cylinders and rings, I'd go with the drive it like you stole it break in. The problem is, if the rings didn't seat, you have to rip everything apart again and rehone the cylinders as they become glazed and will not break in no matter what you do.
It was a fully rebuilt engine but the lower end should be broken in now. I rode it pretty hard right away but I did have a lot of trouble getting the timing right so it got chugged over and ran on one cylinder a lot before I got it running right. Do you think that could be enough to glaze it over?
Did you hone the cylinder? It sounds like the rings aren't seating.
Brand new from RevZ cycles. Should have come honed right? It had good crosshatch.
 

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Discussion Starter #11
Also, could a bad head gasket lose that much compression? I didn't get the heads machined flat so it's possible I have a warp and the gasket blew.
 

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Bad head gasket would smoke when running and put pressure on the coolant system. A leak down test will show if its the rings, head gasket or valves.
 
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