The mystery of the lost oil and compression, where did they go?!?!

OPmini

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Hello all! I recently noticed that my predator 420 lost some compression and started burning oil (due to dust getting in the engine, where I ride in the desert the dirt is as fine if not finer than baby powder and I previously didn't have an oiled air filter), so I changed all the rings out for new OEM ones and put it back together. It worked fine for a few hours but then I noticed extremely low compression when cold starting the engine and some smoke when accelerating, so I pulled the dipstick and sure enough the oil smells like exhaust (I use Pennzoil platinum synthetic 10w-30). The head gasket looks good to me. I also checked the valve seals they look fine to me (also shouldn't there be valve seals on both valves???). In one of the pictures below I have my finger pointing at the cylinder wall, where I'm pointing I can feel a very small lip is that normal? (The lip isn't anything extreme I can't see it but can feel it, just barely) I also did the 'oil through sparkplug hole' test and there was no compression gain pointing to valve seals being the issue, as well as mostly seeing smoke when accelerating or idling. But that would not explain the lower compression or exhaust smell in the oil. Does anyone have ANY ideas on what is wrong? Thanks in advance! :2guns:





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JTSpeedDemon

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Uhhhh did you get it bored and honed oversized? Did you home it at all? If not those rings aren’t seated, and of course it’ll lose compression and burn oil! The cylinder doesn’t necessarily look scored from what I can tell, at least not badly, but it looks glazed at the least.
 

OPmini

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Uhhhh did you get it bored and honed oversized? Did you home it at all? If not those rings aren’t seated, and of course it’ll lose compression and burn oil! The cylinder doesn’t necessarily look scored from what I can tell, at least not badly, but it looks glazed at the least.
nope. so I need to hone the cylinder?
 

JTSpeedDemon

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You might be able to get away with it, but I'd suggest getting new rings again to make sure they seat right.
 

JTSpeedDemon

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I honestly don't know, I haven't had to or attempted to hone an engine myself yet, but I do know they need to be honed when new rings are put in.
 

Kent

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I am no expert so have limited knowledge on the engines but I did change out the rings on a Subaru/Robin last year. The engine was pretty old with much use. I just got a new set of rings and cleaned up the cyl wall by hand with some 220 grit. That did not really hone the wall, just cleaned the glazing up. Runs good now.

Might be all you need to do. If the valves still need attention after trying this it's much easier to pull the head and fix them and won't have to go all the way into the case. Good luck.........
 

65ShelbyClone

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Yeah, you always need to hone the cylinder when installing new rings. A 180 or 240 grit is probably what I would get if I was only going to have a single hone in my tool set. You might look around to see what ring manufacturers recommend for a surface finish and compare that what grits the hone manufacturer recommends to achieve it.

Have a look at the valves as well. If the engine has been eating enough dust to wear the cylinder and rings like that, then it's also likely that the seats are also wearing.
 

Denny

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That ridge at the top of the cylinder is no good either. Ridges cause ring lands in the pistons to wear, rings to chip or wear funny (seen both happen). Buy a ridge reamer and get rid of the ridge then home the cylinder.
 
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