hey i was just wondering if running something too lean would burn a hole in the piston. what does a smaller main jet run leaner with gas or air? this is why i am asking because i just re jetted my 250r with smaller jets, and it change how the bike ran all together. it had stock jetting too. it is stock except for a rps silencers and lo air cleaner lid. it was at a 145 main and i went to a 142. i got the 142 off a different 250r that had that pipe on it. any help is appreciated.

 

Lean means HOT, and yes that can melt your piston!!!

The way to jet, is start out where you know you are to rich. Then, jet down until you are just a little lean. Then, back it off a little to be absolutely sure you will never run lean. Be careful, because performance can seem better when you are lean, and that can fool you...

 

reconranger is right on the mark, my son's 440 Polaris sled was jetted for grass drags when I bought it, waaay too lean for winter riding, didn't find that out until the first ride, ran fantastic, ripped the heck out of my 500 in quick little runs across the field,then took it for a ride, got less than a mile down the trail, and it melted one piston, and scored the jug on the other side, 700 bucks in parts later, I learned to check out new purchases BEFORE riding, butthead I bought it from never told me he jetted it for summer grass runs !!! So to sum it up, lean will make a 2 stroke scream....... for a little while, then melt !

 

do you think that is the right jet though?

 

whats the air screw adjustment

 

i got the jet from another atc 250r same year and all, but it had no motor. so it has me thinking, did the guy take it out to rebuild it casually or to fix it since he screwed it up

 

<div class="FTQUOTE"><begin quote>Originally posted by: stageIVbanshee

do you think that is the right jet though?</end quote></div>

We can't just tell you a jet to run. You need to get a set of them, and start with a fat one and work downward....it can be tedious....and it's fuel injection for me from now on!

Might find some good advice here: http://www.dansmc.com/MC_repaircourse.htm

 

One way to check jetting is to do a plug chop. Find a straight shot area. Run through all gears and when you are 6th gear at wide-open throttle, pull in the clutch lever and shut the motor off. Come to a stop. Pull out the spark plug. What you are looking for is a mocha brown color. If its grey/white-ish in color, too lean. If it is real black, too rich. My plugs usually have a very small amount of black. That's b/c I would rather buy plugs than pistons. And I'm running 110 octane.
Hope this helps. It's not an exact science, but it will get you close. Also, start with stock jetting and go frome there. If you pull the plug after it sits idling, it will usually be black b/c of the oil in the gas not being able to burn off completely at idle.

 

its like a gray with a tint of brown on it. i mean it ran so bad with the jetting in it so now it seems like a different machine. whats the air screw adjustment? would that cause it to run a little lean. i have it at two turns out.

 

Air screw changes SLIGHTLY with aftermarket filters & pipes. Sounds to me like you are very, very close. 2 turns out sounds about right. Did you check out the link in the reply above. Lots of good pics for spark plug colors...