burn hole in pto cylinder (rear)

[robgould16]

I have a 94 polaris SL 750 w/ 130 hours on it. Twice this summer I burned a hole in the rear cylinder, closest to drive shift. The hole is in the top of piston.

Here is what happend. At 110 hours I burned the hole in the piston the first time. I replaced it with 0.5mm overbore piston, rings and had the cylinder rebored. I checked the fuel mixture on the rear carb and confirmed it was set a 7/8 of a turn. Suspecting a lean mixture, I then increased it one 1/4 turn counter clockwise just to ensure the mixture was correct. For the breakin, I added 12oz of oil to the first tank of fuel.

The first 17 hours after the repair I rode it very easy. Half throttle mostly, never above 4500 rpm. Periodically I check and found the plug to be either greyish on the electrode and insulator or sooty black but NOT WHITE. After 10 hours, the breakin was done and I started raciing the machine @ WOT where it would stall once and a while. I check the rear plug and it was black/sooty. I increased the carb mixture another 1/8 turn and it never stall again. Approx 2 hours later, again while @ WOT, the piston burned a hole.

I pulled the engine from the craft, pressuriezed it to 9psi and checked all crankcase and crankshaft seals with soapy water. No leaks were found. Any advice? Please respond to RGOULD@TELEWEB.NET

[pcalonge]

I'm not really familier w/ the Fuji motors, as my old 96 750 never broke, so take this for what its worth-
A grey spark plug is never good- would indicate detonation to me- sparkplugs should be medium brown, about the color of coffee w/ a little cream, not light tan. Black is usually OK, means rich which will just foul plugs and cramp your performance, not destroy pistons.
When your engine stalled at WOT, I suspect the #3 piston was sticking in the bore from overheating- the dome and piston crown were probably exchanging aluminum as well, and had to be badly pitted well before you holed the piston.
There are lots of things that can cause overheating. You didn't say if you replaced the dome on the bad cylinder when you replaced the piston- if it was badly pitted and had pieces of sacrificial aluminum from the old piston, it should be replaced, as this will cause overheating. Did you clean or rebuild the carb?- could be blockage in the jets or the oil passages, a bad popoff spring, etc.-this could cause a lean condition that would not show up at low throttle settings. Also check the fuel line to this carb- it usually is the #1 piston that has the problems in this engine, hence the number of aftermarket pump fixes available, but you might have an internally rotted line that is not flowing enough gas.
Lastly, check out the cooling system- there could well be some blockage in the waterbar, compacted sand or mud, which could be restricting flow to the cylinder and water jacket.
Good luck!

[DavidF]

A twisted crank can cause the PTO piston to hole. You should check for proper indexing of the crank.