I bought this 300ex a few days ago and have been having carb trouble. i got it and the guy said it was dirty. so i took it apart and cleaned it. It has a cobra silencer but not header and said he had rejeted it. It is blowing black smoke when we get it running but it has only been running for like 30 seconds. We took off the the tube from the air box to the carb and it idled but when we gave it gas it bogged out , we have a new spark plug in it.It has a 122 jet in it. when it was running we slipped the tube from the airbox to the carb back on and it ran for like 5 seconds more and backfire and flames would come out.I think it is running rich because the spark plug turned black any suggestions.
thanks

 

if u got the jet kit box with it, read the instructions and see what it says about the fuel mixture screw, i think that you turn it in all the way and then come back out 2.5 turns let me know if it works. the fuel mixture screw is on the bottom of the carb with a flat head screw. that sounds like the problem if not let me know

 

Is the choke open or closed?The symptoms you describe are those of lean (bogging and backfire)., except for the dark plug. I know when the lid is off, and the filter is off on mine...its very lean, and runs like crap. If you put the filter and lid back on...how does it act?
Hows the compression look? Is it posible the valves are adjusted too tight?
How old is the gas in the tank?

 

It has great compression it will run when the airbox is off it will idol but when i keep it at idol and put the airbox with filter back on it dies out and wont run. its slow jet is 40 and the others are stock, it blows black smoke to. choke is off it will start with choke then i take it off i have tried it off and on, brand new gas, i have not checked the valves yet but i do not have the book so can you tell me what the clearnce should be.The airbox lid is off
thanks

 

If it runs easily with the lid off, its richer than normal.
Hows the mixture screw setting? You may have to go in further than the 2.5 turns out.

The valve specs are .004 " on both intake and exhaust.