OK, I know the basics of jetting, but I'm not really experienced at it. I got my 1990 L250R back in November. It was running rich and I would foul about 2-3 plugs per tank of gas (mostly in trails, so I was between idle and 3/4 throttle the majority of the time. Very rarely did I get to full throttle).

About two months ago I was riding down a snowy road in sixth gear pretty much full throttle. The bike suddenly cut off (no bang or any other noise) and when I checked it out, I saw that I had blown a hole in the piston.

Does this mean I need a bigger main jet (to richen 3/4 to full throttle) and I should raise the clip on the needle (to lean out off-idle to 3/4 throttle)?

My top end is being bored out to .40 over. I have a full DG pipe and boyesen rad valve.

Where can I get a good jet kit for cheap? I want to go with a K&N filter with prefilter cover setup because I won't be riding in any mud or water.

Any opinions/ideas on how I should jet it?

Thanks

 

Someone correct me if I'm wrong,but isn't a hole in the piston a sign of "detonation" not poor jetting? Wouldn't the motor sieze before the hole was made if it was lean?And surely rich jetting wouldn't make a hole.Is your compression stock? [img]i/expressions/face-icon-small-confused.gif[/img]

 

Ok,First what kind of carb is on it??Probably a Mikuni,call your local dealer and find out what the stock main jet is for that carb,check the one in the carb see if they're the same if not get the stock one for riding in the snow,if you have the stock size already then the engine is sucking air somewhere like maybe a cracked intake boot,the base gasket could be leaking,crank seal's could be leaking.An easy way to check this is with a can of W-D 40,start the motor let it idle,spray the boot,if the engine idle's faster it's sucking air replace the boot,do the same for the base gasket,if it doe's it there check the torque of the base bolt's,if torque is ok then the gasket is bad,replace it.Now for the crank seal's,they usually go bad when the crank bearing's are bad,to check them,take off the stator cover,crab the flywheel and try to move it in and out,up and down,side to side.If there is any movement then the crank bearing and seal's are bad. oh yeah if it doe's idle faster when spraying the boot also check the bolt's on the intake boot,don't over tighten them,they strip out easy.

 

Thanks, ZillaDaddy409. It has the stock carb on it and I will check all the things you mentioned.

LT80, if it is detonation, does that mean I should start running race gas or at least a mixture of 93 octane and race gas?

 

I've had this same thing happen to me. Winter riding can also bring another problem to the surface. Cold temps can freeze any moisture that may be present in the fuel system. Right at the entrance of the main jet, there is a drop in pressure that can cause any moisture present to freeze under the right conditions.

Even if you are jetted correctly for the colder air temps (Air density changes with temperature which usually facilitates a jetting change), a frozen/plugged main jet will lean it to the point of failure. Consider yourself lucky that it melted in the center of the piston, and didn't get the cylinder walls too.

Really watch your fuel quality..