Siezed 951 PTO piston

[JimZ]

Well, if first you don't succeed try, try again. Well at least as long as the money or my sanity holds out.

99xpl, rebuild carbs, fly F/A, rejet kit (high 170 - 1 1/2 turns, low 68.5 - 1 turn, I bought this as a kit, thought the lows were strange since stock is 75), R&D 49CC head

Tuned lows for max RMP at idle, started tuning highs, was getting about 9000RPM. Didn't notice any drop off at high speeds. Started high speed straight runs for plug coloring.

I couldn't get any color on the Mag plug, was thinking that I might be rich enough to wash all carbon out. Think I've heard about that happening.

PTO plug was giving dark color on plug, though it was looking good.

Started WOT turns, engine died. I was below half a tank so I filled it up thinking it was fuel starvation. Died again on WOT turns.

Pulled head and PTO piston had signs of detonation on crown and light scoring on walls. Mag piston was OK. Something that caught my eye was the O rings on the head. They had become thin in spots and had starting to melt(?) to form ridges where the head met the block.

I was wondering if this jetting sounded OK, particularly the lows. Could the Orings have been defective and allowed water into combustion chamber. Scoring was all around piston. I'll have to see if I can post some pictures if it would help.

Also wondering about RAVE line off of PTO side of block, how can I check to see that there is no air leak here.

Very light scoring on walls, possible get by with a light hone and a stock size piston?

Thanks for the help.

Jim

[mattgross]

jetting is fine, probably rich accually, but those high speed turn can kill your mixture. now i have to ask, but i thought you said 9000rpm, am i mistaken becasue i would have to say thats your problem right there.

[JimZ]

Matt
No, I was daydreaming when I typed in 9k. What I meant was 6900. 9k would be a screamer wouldn't it. More along the lines of a motocycle with a big V-twin.

I have had some feed back on the pilots with people saying that I should be looking at 85 to 90s.
Thanks
Jim

[Bill O'Neal]

Jim,
There is no such jets as 68.5's. You need 85's.

170's at 1.5 turns out ? Unless you have twin pipes, it should be running like crap from being too rich.

The orings that you described are typical of a motor that detonates. It is actually lifting the head with each stroke, hammering the orings out of the motor.

My guess is that you have some sort of major air leak, or the carbs are not functioning properly, maybe dirty, or put together wrong.

49cc head is ok, but what is your squish clearances? You need at least .065 or more for pump 92 octane gas with 49 cc domes.

If either of the pulse lines from the block on the pto side of the motor were leaking air, the pto side piston would detonate badly. It is not uncommon to have an air leak in the line from the block to the rave valve solenoid. I'd check that line first, as your description of the piston is exactly what would happen if it were leaking. I have repaired similiar skis with that line either completely off or split and sucking air into the cases. The leak is usually right close to the fitting on the block, between the check valve and the fitting.

[JimZ]

Thanks for the reply Bill

Looks like I read the jet wrong, getting older and need to buy one of those pair of magnifing glasses for the small stuff.

Since I though it read 68.5, maybe it really was 85. Might there be a character before the jet size that I mistook for the letter 6.

Jim

[Technical Support]

You probably say the square like shape of the Mikuni symbol.