Why did my bike run like this?

[FMX_novice]

1995 CR 125
Ok, in practice at 11:30 it was 100% humidity and 85 degrees and my bike ran perfectly. I warmed it up for about 45 seconds before taking off. The bike ran perfectly! At 1:30 for my race, i didnt change any settings and i warmed the bike up for 2 minutes at the starting line. It was 95 degrees and 90% humidity then, and by the time i was around the 2nd turn the bike started to bogg and die everytime i let off the gas, the revs would go from approxamitly 10,000 to dead in 1 1/2 seconds, the motor would literally stop when i let off the gas and pull the clutch, then i would let out the clutch and gas it and the bike started. I reached down, pulled the choke (thinking it was lean) and rode the rest of the race with the same problems and the choke on, but it was just a little easier to control the stalling.
I pulled the BR9EG plug and it was black with a little bit of oily residue. I switched it with a BR8EG and turned my air screw in 1/2 turn how my friend on a Yamaha had it. The bike ran perfectly the 2nd moto without the choke. Why?

 

[Hucker]

I have similar probs when I ride on really hot days. Usually the bike is fine for the first couple of laps, but then seems to richen up alot. The hotter the air is the less dense it is, so therefore it can't hold as much O2 as it could when it is cool outside. This probably led to your rich condition. Might have had something stuck in the carb to, you wouldn't think 10 degree's would make it run so badly....

 

[Yamamoto]

My Guess

Running with the choke on is bypassing the pilot jet. Not really By Passing it but giving fuel to the engine that should be supplied by the Pilot jet.

I think a peice of dirt is the problem.(in the pilot jet)

Your plug would look black running with the choke on because the choke will give the engine more fuel then the pilot would.

I am really suprised that you finished a moto with the choke on so you are running lean.

At that temp if all was well when you Choke it , it should DIE.

 

[FMX_novice]

I checked the fuel filter in the petcock and it is a little clogged, the fuel moves through pretty well tho. Well see, looks like im runnning fine now tho.