[bohnjerry]

Thanks people for making forums like these what they are.
New member.. probably a common problem, but...

I took my son and his rancher to an old sand pit.. there's wonderful trails and hills and whatnot.. He barely was able to get back down from the hills after 5 minutes with a terrible knock with the engine not wanting to run. I thought the worst.. like "did you run it out of oil??".. Oil seemed fine, coolant was fine.. but now.. suddenly, it runs rough and clanks something fierce! We parked it and planned on having to do a rebuild.
Last night we pulled into the garage bay and pulled the head. I grabbed a hold of the piston and connecting rod, and everything is beautiful.. the piston and rings look great, the wrist pin and connecting rod has no play (except the slightest expected side to side rock. The piston, rings, pushrod all look almost like I opened up a brand new engine.
Before this, we did a valve adjustment just to be sure we didn't have any play there. It didn't need much to get back to exact specs, and certainly not enough out to be "out of spec". The valve seats and how the valves are seated look nice, there is no marks of valve contact with the piston, so I can't figure what else this racket is coming from. I didn't expect the valve adjustment to take care of this, because it's just too loud to be valve noise. It sounds to me like an engine knock.. but I've not worked with powersports engines before. Tractor, heavy equip, and automotive is my experience. It does sound tinny.. like it's being reverberated out the exhaust..
This machine was bought new by my father in law to tour logging roads, etc. while camping.. It's not been used to tow, and not beaten, overheated, swamped, jumped, rolled, crashed, redlined, etc. Since there isn't contact with the piston and valves, I would assume I don't have a major timing chain issue..
What next? I'm out of ideas....

 

[shadetree]

and what about the water pump ??. did you pull the piston and check the wrist pin in the top of rod for play ?.

 

[bohnjerry]

and what about the water pump ??. did you pull the piston and check the wrist pin in the top of rod for play ?.

Thanks for the response!

When you say "pull the piston" do you mean pull the head, expose the piston and check the wrist pin and connecting rod for play? I did that (and thought I communicated that).. but no.. I didn't check the water pump.. Can it make that sort of racket? I've usually seen water pump leak all over the place or the impeller becomes so ineffective that you overheat. Can it really be that out of whack and do neither of those things?

 

[bohnjerry]

Success!!
We assumed we might have piston slap.. since we have marks on the base of the piston skirt. The piston head to connecting rod swivel was questionably tight and felt a bit bound up. We took a leap of faith that we only had to rebuild the top end. We ordered a top end rebuild kit and went from there.
We took the cylinder jug and head to be bored, had the valves lapped and replaced the valve seals.. The mechanic saw right away (because we brought the old piston and wrist pin) that the piston was loose fitting in the cylinder. When he went to bore it, he found the cylinder was out of round, and he found that out at the top of the jug.. not the end towards the block. Since we didn't have any noticeable scoring in the jug, I don't think out piston slap could have pounded the top of the jug out of round. We had to press the wrist pin out.. and upon examination.. it looked like the original casting ridges were still inside of the connecting rod, rather than having the oil channel machined out.. Perhaps someone missed the machining on this?? BTW this is all original equipment that has never been disassembled. This machine has been in the family since new. I ended taking this ridge out and creating an oil channel myself, which allowed me to insert the new wrist pin by hand. So the historical theory of how I got here I imagine is this: The casting ridge in the connecting rod caused a lack of oil lubrication and generated heat until the wrist pin bound on the connecting rod.. the out of round cylinder caused concentration of heat unevenly on the piston, and got the piston hot enough near one of the wrist pin connections causing it to bind up on one of the wrist pin ends.. Bam.. piston slap.
The machine now performs better than it ever has.. a lot more power and smoother running and accelerating. We noticed that a delay in throttle response has disappeared that we simply thought was normal. I've been wincing ever since this repair every time my son take the machine out... waiting for the other shoe to drop... waiting for him to walk home and let me know it threw the rod or something.. waiting for another symptom or malady to pop up that we didn't account for... but so far, so good! Thanks all!