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I purchased a basket case..It dropped a valve on rear cylinder but engine was running. I replaced the heads and valves and I set the timing and adjusted the valve clearance. Afterwards, I did a compression test and on the front of which I did not touch and the compression was 180 lbs. The rear was 90 Lbs. Would it be that the rear cam is 180 degree off or what is the problem. I need assistance and my email is [email protected]. Please some help me on this. The cams are aftermarket so I am not alarmed at the 180 lbs on the front but on the back I am concerned. The top of the piston was chewed up a little and I did not replace it. Please get back to me.. Si.
 

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The front is high, even with the compression release disabled.
Are you sure the cylinder wall on the rear cylinder wasn't scored/gouged?
The cam being out 180 degrees won't reduce the compression.
 

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The front is high, even with the compression release disabled.
Are you sure the cylinder wall on the rear cylinder wasn't scored/gouged?
The cam being out 180 degrees won't reduce the compression.
Thank you for getting back to me! I had checked the cylinder wall and they are smooth. The top of the piston was really chewed up but it did not look like it was cracked. Of course, from what I had discussed with the person who I had purchased it form he had been running it on one cylinder, (his children). I think I will squirt some oil down the spark plug hole and then run a compression test and if the psi increases, then I know there is by pass and I will just go ahead and replace the cylinder and piston on the rear. I think this is my best course of action. Your thoughts?
 

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The front is high, even with the compression release disabled.
Are you sure the cylinder wall on the rear cylinder wasn't scored/gouged?
The cam being out 180 degrees won't reduce the compression.
Thank you for getting back to me! I had checked the cylinder wall and they are smooth. The top of the piston was really chewed up but it did not look like it was cracked. Of course, from what I had discussed with the person who I had purchased it form he had been running it on one cylinder, (his children). I think I will squirt some oil down the spark plug hole and then run a compression test and if the psi increases, then I know there is by pass and I will just go ahead and replace the cylinder and piston on the rear. I think this is my best course of action. Your thoughts?
A cam with the compression release still hooked up will show 50-80psi.
 

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Regardless of cams I have never seen stock pistons give 180psi compression. And just FYI, when an engine is stopped by something in the combustion chamber...like a valve, you may only see a few dents on the piston but, the act will evacuate the oil film between the rod bearing and crank damaging the bearing insert and making an impact mark on the crank which between the two will lead to it's failure sometime in the near future. Enough of an impact will even peen some of the bearing material away and can cause the piston to be a couple of thousands shy from reaching the top of the cylinder...which will cause a drastic drop in compression. It may also distort and changed the balance of the piston and may effect the rod itself. If those people were riding it with a valve head bouncing around in there or continuously hitting the piston, if it were mine, it would be in for a complete rebuild...just my opinion. :) Hope its something else and and easy fix.
 
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