I have no idea if it's been sunk. Nothing bent on it and in good shape but oh does the knock bother me. The Rubicon had a lot of rough farm use from what the guy who sold it to me. Hard to say what they may have done to it. I find most sellers of these ATV have a dark secret they do not tell you.

I'll mess with the cam tensioner as its running and see if I can hear any slop in the chain. I guess you can turn a screw inset to back off the tensioner or apply the reverse pressure manually. A stop is necessary to keep it there.

 

I agree with you. Usualy when the oil heats up and thins out something like a rod knock gets louder, not the other was around.

I guess if the skirt was knocked off of the piston it would get a little quiter because of piston growth in the bore, but????

Let us know what you find out.

 

I'll let you know. The guy I bought it from knew of this issue I'm sure and unloaded it. He actually drove it warmed up and for a reason I'm sure. Sort looking forward to putting this unknown to rest.

 

Sounds to me like piston slap especially if it goes away when warmed. When warmed the piston and rings expand slightly and thus fit more snug within the cylinder. If this is the case I would not worry about it too much as it will not damage anything. My question is what could have caused the premature ware on the piston rings and or cylinder, maybe not letting it warm up enough before laying into the throttle, or too long between oil changes. I know you got the ATV second hand so it is hard to tell what the other person did to it. However, if it bothers you and you are determined to tear into it I would bet a new set of piston rings is all you need. Maybe a compression test would be a good test to rule out/in my piston ring theory. Obviously this is my opinion and I am not a mechanic. Keep us posted.

 

Thanks! Gives me an objective. I wonder if the piston slap would score the cylinders? If so I'll have to have that bored out as well. I just hate the sound of it with that knock. It would not surprise me the original owner did both in not allowing it to warm up and lack of oil changes.

I've also been thinking maybe a bearing but then again that sound would be constant would it not. I'm pulling the engine apart Wed night. I'll let ya know what I find.

Thanks,
Dennis

 

It is possible the cylinder is scored and may need to be bored and re-honed, but if caught early enough you maybe in luck. However, if and how much boring is needed you may still be able to get away with the original piston and just install larger rings. I agree that if it was a bearing the knock would be continuous.

 

If you don't have the proper micrometers, take the cylinder and piston to a machine shop.

Measuring them against specs for out of round, taper, and diameter service limits is a must-do.

May as well measure the cam, and be sure to inspect the decompression thing closely.

If it has ever been hydrolocked by water or fuel, it is possible it could have slightly tweaked the rod.

Anyway, a tear down should tell the story.

 

Yes excellent suggestion. For the work I want to finish this right.
I'll let you know what I find.

 

Tore down the Rubicon engine last night and found the problem is not what I thought it was, being piston slap theory. In fact the piston, rings, cylinder bearings are in great shape by the look of it. Whats messed up is the decompression mechanism on the cam. The piece has a warn slot guide and does not properly move as it should and binds. The knock sound is on the upper end of the engine everything is tight so it almost has to be this decompresser on the cam. I would have like to replaced the cam chain but it's way to big of a job to tear the front half of the engine / clutch out of it so I left it as is.

Ordering parts / gaskets today.

Thanks everyone

 

I kind of thought that might be the case.