hello, i have an 01 scrammy which likes to melt pistons down,3 to be exact.on the last 1 i had a bike shop rebuild it and it lasted 3 tanks of gas and then a hole in the piston.it has a hotseat pipe ,vforce reeds,12-1 hs head,dg silencer,42mm tmx carb,0.40 overbore wiseco[the one with the hole in it],and a hs clutch kit.the jetting is as follows a 420 main, needle 2nd position from top,pilot not sure of but it is stock for the carb.if the clip was wrong would it have caused this?secondly the first piston was a stock polaris after i did all the upgrades,itmelted down at the ehaust port,the second was a wsm it did the same, and last was the wiseco it has a dime size hole in it now towards the center of the piston.the bottom end has just been rebuilt new rod ,crank seals and oil injection has been eliminated.32-1 premixed.the main jet is what was recommended by hotseat any suggestions why this bike keeps doing this?im at 2600 to 3000 ft. and cant seem to get it right.also has a kn clampon filter.this bike hasnt even had the brakes replaced so it doesnt have much riding time on it.thanks to all who replies.also it seems like when it quit running it was mid to full throttle.also air screw 2.5 turns out and a br8es plug.

 

Sounds like it leaned out on the midrange.drop the needle clip to the one bellow the center.Do plug checks often after ridding at differant speeds.

 

thanx for the reply i was wondering how to check the plugs for jetting correctly?do i do a 1/4 throttle run,mid throttle run then full and check my plug in between runs and how far of a run/thanx again.

 

Do a search on here for plug reading there was a good post awhile back on it

 

Typically, a hole in the piston is from detonation or "knock" as the old timers call it. The plug is firing before the Piston gets high enough (Not close enough to TDC). This means that for some reason your timing is off. Several things can affect timing. 1) is the engine timing itself - when the coil receives the signal to fire the sparkplug - which I believe is not adjustable (Correct me if I am wrong). 2) Compression ratio - an increase in compression ratio can increase (advance) ignition timing. 3) Spark plug specs & Fuel Octane can affect timing as well.

If you were seizing (Sides of the piston skirt and the cylinder wall scratched and or gauged), then you might have a jetting or oil issue - but that might play a role as well.

Normally, an engine will run quite hot from timing that is running too advanced - is your engine temp light ever coming on? The higher comp piston and/or higher compression head is likely to be a contributing factor. If you are indeed runing a NGK BR8ES. You may want to go one plug colder (Auto parts stores can usually cross reference this). Colder plugs also help to reduce Pre-ignition (Piston firing before the plug fires) by allowing the insulator tip to remain at a slightly lower temperature than the next hotter plug.

Upgrading your octane is another necessity when upping compression and/or performance. My pipe manufacturer recommends no less than 93 octane for proper performance. That recommendation is just when adding a measly pipe! You have much more modification than that! Higher octane is less likely to explode (detonate or knock) before it is fired by the plug. Some porting jobs and higher compression ratios require Race gas (104+ octane). Check with your head & piston manufacturer for their fuel recommendations in regard to octane.

If you cannot get it sorted out, find a good used stock head & Normal (Not increased compression, but proper bore size) piston and swap those in. I'm willing to bet that your issues would simply vanish and you would instantly become a happy scrammy owner.

 

Once you figure out how to do a throttle chop plug read (Kinda hard to do properly on a belted polaris), here is a basic key on identifying the plug's condition: http://www.theultralightplace.com/sparkplugs.htm

 

thanx for the replies.i was thinking about the timing being off also.what i forgot to mention was that i had to replace the flywheel cause the shop that did the motor said it had chipped magnets.so i bot i believe to be the same number 1 off ebay ill have to check it to make sure.also i hooked the fan up direct after i rebuild it to make sure it was workin ok.and i never saw the overheat light come on ever but it may not work anymore.do u guys think this happened from to high comp. or maybe from the jetting not being correct?thanx again to all.