powervalve clearence

D

dockter71

New member
Joined
Oct 29, 2003
Messages
56
Location
Maple Grove, Mn.
Has anyone seen a Powervalve break the upper piston ring? When I rebuilt the motor I adjusted the valves using the test lead, so the valve is flush with roof of the exhaust port when full open. after 400 miles after rebuilt and 10,000+ miles on the servo motor, I started getting a Servo motor open or short fault code from the water temp light. Only when the sled was cold. Seemed to work fine when warmed up. Thats when I should have stopped and check things out. Another 100 miles later the sled wouldn't idle. I checked compression. #2 & #3 are gone. upon dissasembly the Upper piston ring was broke right on the center of the exhaust port. The rings rotated and doubled up completely wearing through the liner. There are wear marks on the piston and the powervalve that match perfectly, as if the valve was hanging into the cylinder. There are what seems to be small machined stops in the valve boss, but this doesn't seem to have stopped the valve from contacting the piston. Anyone ever seen this?
 
My MSRX was doing the exact same thing but i still gotta tear into it to see what is up.
 
Hey, Turk. This sled ran amazing before this happened. I spoke with you about my lift /cut mod a few mods ago I did on this sled. thanks for all the help, Sled ran amazing before this happened. This is it for this sled...If this doesn't work its gonna be hit with a little jewish lighting. I'm installing 2 new pistons, cylinders replated, low milage (3500 mile) servo motor, new valves, new cables. Readjust the valves per the sevice manual and thats it. This sled has been a major pain in my checkbook this last year. And thats it. I'm done.
 
Doc..there is no way a powervalve can hit a piston.Mine was running crappy due to a plugged pilot. You might want to check you crank run out or see if crank has gone out of phase. This will cause some unexplainable burn downs by messing with the timing.
 
Happened to me.

Now I hate to disagree with you Turk (with all your knowledge) but I had a problem last year with my P-valve coming loose and hitting a piston. The cable bored out the top of the valve and pulled out and the valve came into contact with the piston (on clutch side) and I had to take the cylinder off and grind out the p-valve with a dremel (from inside cylinder)as it got mashed up pretty bad by the piston and would not pull out from the port. The piston had alittle bit of a mark (just on top, above the ring) but I did not have to change piston, ring, or cylinder. So it could happen, and could cause some serious damage if it goes unoticed. Luckily I caught it in time and only had to replace the p-valve (all 3, actually)
 
I looked at the way my power valve sits in the slot & I can,t see how it could hit the piston but hey...anything could happen. Just real rare & freaky occurence?
 
There is no evidence of a lean condition whatsoever. The only damage in the cylinder is from the ring braking then rotating. The 2 halfs of the ring actually tried to overlap each other, partially succeeding for 1.5". And when they rotated the other end hung on a port really tearing into the liner.

I was a believer that the valve couldn't hit the piston either until I saw the piston and the mark on the pistons matched perfectly with the width of the valve, along with the wear on the valve end. I don't think they could ever slide so far as to being on top of the piston, but they can slide in far enough to snag a ring. All it takes is a few thousands. I know there is material at the bottom of the Valve opening that looks like stops for the valve, making me think there is no way it can hit the piston.

Important note: all this happened when Servo motor was on the Fritz, doing its 7 or 8 flashes in 5 seconds deal. Oh well Lets hope it all works. I apprciate all your help. thanks guys.

"One disappointed Snowmobiler"
Doc
 
I have to agree with turk, I adjust my valves by feel, and without a tester...Only thing I have used is a !2 volt batttery charger actually, and I have adjusted them i would say about 10-12 times swith no problems...her is one thing I noticed: when I actually adjust them I apply 12 v to the harness for just a 1 sec enough to actuate the motor...I then do my adjustment...I then Roll back the servo manually...I roll it back so the cables have 100 % freeplay....IMO there is no way they can contact the piston...even with a broken cable...because I already have the servo on turned so that the cable dont have any tension on it...

My point: when I have the exhaust flanges off, I usually rotate the engine and take a look inside for scoring, blow by, etc.. on the piston...I have never ever turned that clutch over and felt a valve hitting......

Just food for thought, you may want to try it.
 
dock, you say the markings on the piston and the valve were identical??



Maybee some debris entered the chamber and caused some scoring?????
 
I have had a cable pull thru on a valve and it did not contact the piston. I have heard of it hitting the piston if the guilotine breaks off and then it can actually end up on top of the piston. The only other way it can hit is if the stop is worn or the cyclinder where the powervale rides.
 
U had my motor rebuilt this year,ran great then one sta morning didn't seem right.Powervalve on pto side destroyed piston & cyl.I guess shit happens.........
 
I had the cable pull out of my exh valve, but it never hit the piston. I had the tops all welded up and re-slotted/holed them for the cables. All is working fine. When I made my motor into a 780 bb this season, I made a fixture to hold the exh valves in place while boring the cylinders. After all the final dimensions were figured out, we made the proper clearance on the valves while they were out of the motor. When checking everything before final assembly, the average slop between the valve and it's slot were enough to make me trim the valves a little more. When rocking the valve from side to side, it looked like it could be too close for comfort. After 1000 miles, I tore it apart to check everything and make a few changes, and all is well! On a side note, an 1.25" track and the big bore work very nice together!
 
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