My brother-in-law has this atv and it developed a ticking noise in the head. Slowly got worse until it wouldn't start. When I got it apart I noticed the intake valve had been hitting the piston. Took apart the head and noticed the bent intake valve. I'm not sure what caused this. Did the valve bend first then rotate and hit the piston? Did the piston hit the valve. There are 2 small valve reliefs in the piston (look factory). The piston, wrist pin and rod bearing all look ok, no play.

Any ideas? Does honda make an oversize gasket for the head? The mark on the piston was slight.

Regarding retiming the engine(cam), does anyone have the notes on how to do this? There is a single mark on the gear, I assume TDC on the piston the the mark goes to the intake or exhaust side, usually even with the cover. Thanks for any help.

Thanks,
Rob

 

What probably happened was that the valve clearance slowly decreased to the point where the valve was being held open and hit the piston. This can be caused by either the valve seat or the valve face wearing. I'm not sure how close the 300 valves are to the piston at TDC. Another possibility is that the wrong cam is in the engine. I'm told that the 250 cam will fit, and has slightly higher lift than the 300. People have tried to make them work, looking for more horses. I had someone try to sell me one a few months back, sounded like hell.

 

Thanks for the reply Chilly. I did forget to mention in my first post that there was plenty of valve clearence. That was the first thing I checked (pulled the 2 little valve covers). They actually had more than what the motor called for, probably .015-.025". I also checked the compression release, all ok there to(of course that's the exhaust valve).

I doubt if this was had been worked on before. He bought it used but it looked like a regular road hard/put up wet east texas deer lease ATV.

I'm still curious over the chicken or the egg thing.

Thanks Again for the reply,
Rob

 

If the 300 doesn't have a rev limiter, the valves can "float" at high rpm and hit the piston, that would be my next best guess.

 

Was it quite rattly? If it is extremely loose, sometimes the timing chain can jump causing such a thing to happen. Or possibly, the guide has some carbon build up and caused the intake valve to stick a little. If this is the case, your valve will float easily, even without excessive RPM. You should check the guide to make sure it isn't cracked (from the valve being bent) and definitely ream it with the proper reaming tool before you reinstall your new valve. I believe there are two benchmarks on the cam sprocket, and they should be flush with the top of the head when the piston is at TDC. It doesn't matter if the cam lobes face up or down for cam timing, but it's customary that they face down. This will make installing the valve cover easier as you will be on TDC compression and your valves will be closed.

 

Again thanks for the replies. The valve float is a distinct possibly. Grand kids and the like come over and abuse the atv then park it. If it breaks they try something else. Let's see they filled it with diesel before, sunk it in water, who knows.

I'll have to double check the upper cam gear. I think there was only one mark, and only 2 bolts holding it on the cam with no key. With a single cam I should be able to figure it out since the valves rock 360 degrees from the TDC compression stroke.

Thanks for the tips.

Rob