just bought a used 300 ex 3 weeks ago ran great until this morning it wont take the gas after half throttle i thought maybe junk in carb. i took the air tube off the carb and revved it up till it cut out, gave it a touch of starting fluid to richen the mixture didnt help made it worse replaced the plug with a new one, swapped the cdi, ignition coil, reverse inhibitor box from another good running 300 ex i have. it seems to be running a little rich when it cuts out blows a touch of black smoke till you let up on the throttle a bit then runs good till you push a little harder any ideas would be greatly appriciated thanks

 

i just sold a 300ex check all the fuel lines make sure they arnt plugged

 

i am haveing that same problem with my 01 300ex, im not sure what the problem is either, im gunna have to have my local atv shop take a look at it.

 

sounds like maybe your main jet. Check out the carb

 

thanks hondarider300 i also talked to a guy at the local bike shop he said the same as you check main jet i pulled the carb off today and found the main jet had vibrated loose and fell in the bowl put it all back together runs like a champ. i like easy repairs like this thanks again for all the help. later