I have a 1985 atc 250r, with cr 250 reeds, a 38mm carb, a full fmf gold series exhaust, and it is running a k&n airfilter with no airbox, it has had two pistons blown, so therefore has been bored out twice, enough to smooth out the inside. My question is what octane gas should i run in my trike and how will it effect if if i run too low of an octane

 

What was the cause of you 2 piston failures? Too low of octain will cause detonation (basically melting of the piston) if you are running a high compression piston. BUT you can also get piston failure due to air leaks on your 2 stroke, and running the engine too lean. I would do some carefull inspection on your engine to see exactly what is going on. Too low of octaine is bad; too high is just a waste of money.

 

that is the problem, i do not know what the cause of the first two problems were. The best i can figure is that it is leaning out, but i dont know how, My carb was jetted by esr, so what else could cause it to lean. Im just trying to figure out what is making it happen before it happens a third time

 

I hope some of these 250r guys on here will chime in soon! I am on my first 2 stroke right now (yz250). What did the first 2 pistons look like? were they melted on one side, or was that from a previous owner?

 

the first one had a crater blown in the top, and the second one blew from the top all the way through the piston, i too am new to two strokes and dont know how to tell what is causing this problem, the thing ran great when it was stock, but the carberator leaked so i just decided to fix it up, ever since then it blows everytime i get it running

 

wow .sounds like you have a problem [img]i/expressions/face-icon-small-happy.gif[/img] there could be several things causing your meltdowns,,, crank seal sucking air from the ignition side, air leak somewhere in the intake system, float level to low and starving for fuel on topend, maybe jetted to lean .. just cause esr jetted your carb doesn't mean its right,,when they jetted it did they do any plug chops? detonation burns holes in pistons,,,,,, oh by the way ThrashAndCrashUntilItsTrash there is no such thing as a high compression piston for a 250r, compression is set by dome volume and headgasket thickness,,,

 

i am not sure if they did plug chops, isnt that something i could do myself? and how do i do it and what am i looking for when i look at the plug, and what would make my float level too low, and how do you fix that

 

4th gear WOT for a good 1/4 mile and hit the kill switch leaving the throttle wide open. do not let it come down on RPM's at all. just kill it. pull the plug and check for whiteness and dryness of the plug. white signals lean. brown/tan signals ok. this method is still not 100%.

if you're going through top ends like this, your jetting isnt your problem IMO. it was thought in your last thread about this that you might have a bad crank seal.

 

and 91+ should be fine. i have more mods than you and I run 91 all day long. without compression mods, you're usually good. just dont run 87 or 89. sure you could get away with it, but its cheap insurance to get the premium.

 

thanx for the help, i dont think its jetting either, as soon as i get it back im gonna get a leak down test to see if there is a leak, does anyone know if they leak down test tells where the leak is, or how to find out where it is if it doesnt