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Discussion Starter #1
Had to replace output shaft on a 2007 900 custom. So completely disassembled and reassembled motor. Replaces all gaskets and o-rings, also checked valve clearance and adjusted. Went to run it and on the rear cylinder it is making a pop sound when that cylinder fires and you can see the flame through the throttle (it flashes not a full flame coming out) as it runs at low RPM at high RPM it goes away. Also if you ride it, it does not have the power it should have at any speed. I double checked both timing marks (stator and gear on top). Used timing light on both cylinders and they are dead on. Checked cylinder pressure front is at 158psi rear is at 153psi (book says 142-228) so i have pressure i should this makes me thing it is not a valve issue (pressure would be way lower on rear) and makes me think it is a timing issue. But how? What am i missing or not looking at?
 

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Discussion Starter #6
spark seems to be ok on both, not sure i would call it "fat" but is a blueish orangish. Also looked in the boots for the plugs, in rear boot it has some corrosion, front one is clean. also switched plugs and tried again looked the same.
 

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Discussion Starter #10
Ok disconnected tried running it. Sputtered a little bit when I started but ran and had same sound/issue. Pulled plugs and they both actually looked more normal. front one did not look lean and rear was less black but was little wet on tip. Only ran it for a bit not even enough to warm up motor. Checked spark and looked the same as with the PC connected.
 

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Well, I am thinking of two options:

1. It still sounds like it is out of time or you have an intake valve that is hanging open/too tight. I am leaning towards this.
2. It also could possibly be a dribbling injector... very very much less likely. How you would check for that is do a fuel pressure/leak down test.
 

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As an add, listen VERY closely when the engine cranks over. Does the starter slow evenly for both cylinders as they bump over compression or does it not slow down as much for one as compared to the other. Not a definitive test, but sometimes it is detectable.
 

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Discussion Starter #15
This might sound dumb but, motor was apart for few months as I did repair could valve stem have simply dried out? Could I simply shoot some oil down the stem? Or are you thinking need new valves?
 
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