96XP Burned Rear Piston with Factory Pro Pipe

[daboostcreep]

I finally got my ski running right, or so I thought...then burned the rear piston.

Here's the skinny:
96XP bored .50 over (found out after pulling head when cylinder lost compression)
Rebuilt carbs jetted to Factory Pipes recommendation 150 Mag, and 152.5 PTO with stock pilot.
Uni filter clamp on filters with billet adapters.

I set the carbs to Factory Pipe's specs.
The ski was hard to start, and would bog in midrange, then run good at higher RPM.
I run a mix of 100, and 91. Equates to 93 octane.

Here are my questions.

• With the cylinders being bored .50 over and not knowing it, did that cause an issue with jetting? Do you need bigger jets because of the bore?

• Should I have run, or run in the future the stock airbox?

• Should I remove and sell the factory pipe and go back to stock exhaust, or stock exhaust with a shaved head?

I want good reliable performance. This is a disaster, and nedless to say, I'm pissed and don't want to repeat this mistake again.

 

[Robj]

I don't think the oversize bore had anything to do with your failure. It was more likely caused bu the missing airbox and pipe. Factory jetting is set up for a stock pipe and air box. By changing the pipe and not using the airbox you should have gone with bigger jets.

 

[daboostcreep]

I don't think the oversize bore had anything to do with your failure. It was more likely caused bu the missing airbox and pipe. Factory jetting is set up for a stock pipe and air box. By changing the pipe and not using the airbox you should have gone with bigger jets.

I stated that I used jetting as instructed by Factory Pro Pipe.
Reading comprehension is your friend.

 

[Robj]

I stated that I used jetting as instructed by Factory Pro Pipe.
Reading comprehension is your friend.

I think you should reread my initial answer. You used the jetting for the pipe, but did you account for the removal of the airbox? Remove the airbox and you need to increase your jetting to account for the higher air flow and lower differential pressure.

 

[daboostcreep]

Factory pipe says those jetting directions are for stock motor with aftermarket flame arrestors.

 

[Dr Honda]

I responded to your email.... but I will also sound in here too.


Going with what you guys are talking about... did you have some type of air box on? (Factory or aftermarket) Normally "Factory Pipe" wants the OEM box in place.


What kind of 100 oct fuel did you use? On a 2 stroke... high oct fuels aren't always good. Since they burn slow... they have a tendency of overheating the piston crown at the exhaust port. Generally... regular 91/93 oct fuel is fine. Also... mixing a "Race" fuel isn't always the best thing, since they have things like Toluene in them. (it's hard on the crank seals)

With that said... I don't think that's where your problem lies. Since it was hard to start... and had a "Bog"... I'm going to guess that you have an air leak in the fuel system (Fuel valve, or filter) or high clearance in the rotary valve.

 

[Matt Braley]

I don't know if your Factory "Pro" pipe is a Spec 1 or Spec 2? If it is the Spec 2 then it is just for racing and may be impossible to make reliable on pump gas.

Also you probably have a rev limiter in there. Many limiters also have timing choices that are controlled by switch location. If your timing is off for this reason then that would surely do it too.

 

[daboostcreep]

I responded to your email.... but I will also sound in here too.

Going with what you guys are talking about... did you have some type of air box on? (Factory or aftermarket) Normally "Factory Pipe" wants the OEM box in place.

I ran Uni Filters with clamp on adapters, because the instructions on FPP's site says "All of our testing was performed on a stock engine with aftermarket flame arrestors & choke plates retained."

What kind of 100 oct fuel did you use? On a 2 stroke... high oct fuels aren't always good. Since they burn slow... they have a tendency of overheating the piston crown at the exhaust port. Generally... regular 91/93 oct fuel is fine. Also... mixing a "Race" fuel isn't always the best thing, since they have things like Toluene in them. (it's hard on the crank seals)

I was running Sunoco 100.

With that said... I don't think that's where your problem lies. Since it was hard to start... and had a "Bog"... I'm going to guess that you have an air leak in the fuel system (Fuel valve, or filter) or high clearance in the rotary valve.

Should I do a simple fuel system, straight from the fuel feed at the tank to the carbs, and return? Cap the reserve, and leave the vent alone? Thus bypassing the switch?

In your email you talked about ignition timing being wrong. I did install a MT rev limiter that day. I did however make sure 10 times that I did it right. I verified the # stamped in the engine case, and set the jumpers accordingly. Maybe thats the death box right there?

I've attached a picture of the piston, the damage facing the camera was the exhaust port.

 

[daboostcreep]

Comments, opinions, suggestions?

 

[Matt Braley]

Are you using the stock exhaust manifold? This will tell me if you have a spec 1 or 2. If it is a Spec 2 then there are different chambers available. Hopefully you have the larger longer one they call the "rec" version.

One very important thing I don't hear is a squish measurement. You can use a fatter cylinder base gasket to raise your squish and reduce your risk of this happening again.

 

[Dr Honda]

Yep... what he said. Dropping the compression a little will let it live longer.

 

[daboostcreep]

I'm running the stock exhaust manifold.

I don't have a squish measurement. Is it possible now that I have the cylinders off the motor?
Even then, I'm not sure I want to put a bandaid on the problem. If the exhaust is this big of a pain, I will go back to stock. I need the ski to work every time I drop it in the water. I drive 175 miles to the lake and camp for a few days at a time. A non running ski is a kill joy.

 

[Matt Braley]

It is a Spec 1 and it should not have done this to you. You have really thought this out in a smart way and I'm sorry it has been a bummer for you. Going to stock specs can certainly give you reliable performance but your mods are not so out there that you should be suffering from it. Hit up Spim on this site. He has a similar setup and puts mega riding hours on it.

 

[daboostcreep]

So I used a hone, and did the best I could to remove any and all imperfections.
The picture attached is what the hone didn't remove.
I can run my fingernail, or fingertip over it and there is absolutely nothing felt.
Would you run this cylinder?

 

[daboostcreep]

Anyone have input?

 

[CReynoldsMIZ]

just throwing this out there, but the rave valve had been shaved down to obtain proper clearance, correct? My 787 kind of looked like this then i realized i didnt shave the rave when i put my overbore piston in. i dont even remember which side the arrow points to, but i just was throwing something out.

 

[Dr Honda]

So I used a hone, and did the best I could to remove any and all imperfections.
The picture attached is what the hone didn't remove.
I can run my fingernail, or fingertip over it and there is absolutely nothing felt.
Would you run this cylinder?
View attachment 15100

I'd use that... assuming it's still round. Have you measured the bore yet? Personally... I can live with scratches... but if it's tapered, or out of round by more than a thou or so... then it needs cut.

What kind of hone did you use?? A ball hone will hide problems.

 

[Minnetonka4me]

That failure looks like it had nothing to do with the fuel system. Looks to have snagged a ring or something and didnt burn down right away...making the edges round off and look like detonation.

Raves need to be trimmed above .50 if I remember correctly.

If you cant deal with engine failure your in the wrong sport. This kind of chit happens all the time. I had the weirdest failure...a lower ring...chunk out a small piece of piston. The top of the piston and head were trashed, but theres zero damage to the port or the cylinder wall. Engine had about 20hrs on a full top to bottom rebuild.