Ok guys, I;m stumped on this one.
I have a 86 250R w/ a rad valve, k&n on the engine other than that the engine is basicly stock with about 30 hours on it.
The bike has ran great all winter never fouling a plug (BR9ES gaped at .35)at 5,000 ft sikolone mix at 24:1. now all of a sudden as it warms up it won't go 30 min without fouling the plug, the plug color at the base is a nice light black to almost brown. I have gone from a 145 main jet to a 148 and it has helped the plug last a little longer but 40 min later the plug is done. Like I said the plug looks good when u pull it out, put in a fresh one and the bike runs great, I have gone up once on the main should I go up again?
most info i have read says you will have to go up 2-3 jet sizes when it gets warm out but i want to make sure this is corect and if anyone else has had this happen?

Thanks for your reply.

 

I'm no 2-stroke expert but I believe as the air gets warmer - and hence less dense - you should actually lean your mixture out to compensate. I would expect that now that the weather's getting warmer you're running too rich and should jet down instead of up.

 

I agree with dhines. Go leaner. But a little at a time.

 

Isn't colder air more dense?

 

Colder air is more dense. YOU are actually putting less air in the engine and the same amount of fuel which causes the engine to run rich thus you foul plugs. If you are at 5000', I'm sure the air is even less dense than at sea level(I think). I don't know if sikolone oil is synthetic or not but most synthetics are 32:1. Mix your oil at whatever ratio is recommended by the maker of the oil. Changing gas/oil ratios also causes your engine to run richer or leaner. You could also try using a hotter plug(BR8ES) or taking off the air box lid if it isn't already.
I have an 86 engine bored .050 over, K&N filter, no airbox lid, RAD valve, thinner head gasket, 152 main jet, needle clip on 2nd position(from top), use AMSOIL 50:1, and ride a little above sea level.

ERIC

 

Isn't air more dense at sea level?

 

Yeah, air is more "dense" at sea level; also, air expands with temperature increase; warm air (allowed to expand) is less dense than cold air. (PVT, or something.)

Thus, theoretically anyway, you can use a mixture more fuel-rich at sea level on a cold day than you can at Denver on a 100-degree day, because there are more oxygen molecules in each intake gulp of denser air.

However, I think the subtle air density changes in slight variations in altitude and temperature may be too small to cause the extreme changes in fuel-richness of the combustion mixture, seriously fouling plugs.

Is your choke mechanism fully open? I'd check that, and maybe going to a step or so hotter plug (I think NGK numbers decrease with hotter plug rating, e.g., a BR7EA is hotter than a BR8EA), before I'd go radical on the carburetor jetting. Monkeying with the externally-adjustable mixture screw might help; worth a try; then, maybe you need to lower the needle, if nothing else helps and the needle can be dropped.

Beyond that, with proper fuel-oil mix, you certainly have a puzzle.

Good luck.

Tree Farmer

 

Isn't air thinner at higher altitudes? It's harder to breath when on a mountain? Does that mean there is less oxygen or less dense air. Maybe I had that backwards.
ERIC

 

The answer to the air density question, this is one we learned in scuba class.

At sea level the air is dense. At higher altitudes the air becomes less dense.

That is why it is harder to breath in the really high mountains.

 

Thanks for the ideas, and yes the air is thiner here than at sea level.
I had read somewhere that during the summer when it was hotter you had to jet up so the fuel will help cool the piston?
again thanks for the advice, if anyone eles has any ideas come on speak up...