250-500cc Need help whit jetting (WR 250 Mikuni carb)

My bike keeps fouling plugs every about every 3 hour

I am riding at sea level and it is Swedish summer so the temperature is about +20C. I am mostly running thigh singel tracks and I never get to use the 4 gear.

When I am at the MX track the bike foul plugs as often. So I guess it is the thigh singel track that keeps fouling my plugs. So what settings in the carburettor should I use?

I have a 390 main 30 low speed and the needle in the 3 clip and the air screw 1.5 out. Everything according to the manual.

So now I don't know what to do next?

Oh and a another ting. I spoke to a husky mechanic, he said that I should use 3% premix. I have tried to use 2.5% premix one time, the bike felt much happier whit that. Do I dare to use 2.5% premix whit out blowing the engine?

Modelyear?/Årsmodell?

do you know what slide is fitted? #4 or #5?

2006 and it has a new piston and rings.

#4 slide and I guess the orginal needel. I can confirme that later.

The manual says a #4 but I'm pretty sure it's a #3 slide, that's what's was in mine. I have a friend with the same year and he has a 35 pilot, 400 main with no problems. I have a #4 slide and had a 30 pilot and 3rd clip on the needle with no problems either.
Right now I am running a JD jet kit with the blue needle 2nd from the top 390 main and my bike runs great at Sea level and up to 3000', I do start noticing a drop in bottom end power the higher I go. By the way I run stock gearing as well

The plug cap on these bikes sux. I replaced it with a resister plug from an xr100.

Okej i think I found the problem.

The float level is wrong. I think...

I read at TT that if you tilt the carb 45 degrees the distances between the carb body and the top of the floater should be 20 mm (+ - 2 mm ) Can anybody confirm ore deny this.

If I measure mine I have 14 mm between the carb body and the floater.

I don't think it's the float level, i have had fuel leaking out of the overflow tube for some reason when it is not running and my bike still starts and didn't fowl a plug. I have had my float set too low and it would run out of fuel at WOT

14mm sounds about right. 18mm is the measurement for the older style tmx carb. If the needle closes when the seams on the float are parallel with the base of the carb, you should be close.

I have been busy whit a midde season big clean whit the bike, but I am going to take it fore a spin next week.

Mmmkay so I wont touch the floater level...

I am going to drop the needle a notch and turn out the airscrew 2 turns and use 2.5% in premix next time I ride.

Ore do anybody have a better idea?

PS I am gong to buy a new plugg cap next time I am at the bikestore.

An update:

I tried the things I said I would.

I have only ridden on the mx track so I cant evaluate how the bike behaves in tight single tracks yet. But the plug has no oil residues on it, and that is a god thing

By the way I have a 6DJ8-60 needle and a 3.0 slide. So what kind of needle do I have, does it make the mixture richer ore poorer?