I have a 99 400sc with a ported cylinder, headwork (pump gas) and pipe by RCR. Carb is bored. Currently running a 350 main. I seized the piston the other day and just got to tearing it down. The piston has a half-moon shaped notch out of the exhaust side and the top seems to be a little dimpled. I currently run 92 gas (highest we can get in this area)with klotz on a 32:1 ratio.
I have a suspicion that the engine is pinging and this is causing my problem. Does anyone agree w/ me?
Also.. I was wondering if anyone has changed timing/ignition on this type of performance mod. It just seems to be a logical upgrade on a modded quad.

Thanks for all the help.. any information would be extremely appreciated.

 

Sounds like it is/was running really lean which will cause excessive cylinder pressure,tempeture,and detination{ping}. those things will destroy a piston in short order just as you mentioned.
How does the plug look? I bet is that it is white and the insulator is possibly melted.

I dunno how you would go about retarding the timing but a richer fuel set-up would probley cure your problem in the future.

Andy G

 

Pinging usually burns a hole in the centre of the piston. You just have it way too lean I would say. Remember it is not only the jetting but you could have an air leak somwhere else!

 

With the work that you have done you should be running a least a 270 main 30 pilot.

 

As I stated in my original post I am running a 350 main jet, shouldn't that be enough?? Any thoughts about using higher octane gas for this setup? At what compression level should you have to jump up above pump gas?

 

It sound like it was a "stock" Polaris piston, right? Have you asked Rick about what the fuel requirements are for the head pressure you are running? In my opinion a 350 main is way to fat. Alsmost sounds like a slide or needle problem where it's running lean in the 1/8 - 7/8 throttle position. Usually you can feel a stumble when it's not correct coming off the bottom and in transition to the main.

 

Jack--

No the piston was a wiseco. This is actually the second time the bike has done this, both on wiseco's. I talked to rick after 1st incident and he said that the 330 main he gave me when he did the work should be big enough, but due to the extreme cold we had this winter i went ahead and went up 2 more sizes to be safe, but it didn't pay off. I have only dropped the needle clip one postion from stock, which is what he suggested. Both times it has happened, the bike was at <1/2 throttle and fully warmed up. This is the range where i can seem to hear pinging. If i increase or decrease throttle, it goes away. What do you recommend doing in this case?

 

Sounds like you may have to much compression for 92 octane. I had a head shave for 92 octane and had the same problem.

 

I was at HPD a couple days ago an was talking about my friend motor an he said my friend was not running enough octain in the motor he has a was proble pinging an top end an would eroad the piston the way you are discribing. An he told me to run an br9ev NGK plug in my raceported TB so maybe you need to raise the octane an use a br9 plug
Just a thought my friend went throught 2 piston before we found out about the plug an octain he needed his is an HPD race motor not the full mod

 

Did you make sure of the correct piston clearances? It would seem to me that the lean condition would be in the mid-range and not on top end. That's where I've personally seem more meltdowns. If it siezed, there was not enough fuel or you have an air leak. I've not heard of a problem with octane ratings with normal head modifications.

I siezed one up three times in a row once running on 110 so I knew it was not the gas. It turned out the be a strange carb issue. What carb are you running?