Machine is very hard to start (even on starting fluid) which lead me to believe this was an ignition problem. Once the machine is started, it will idle perfectly until you give it any throttle at all then it will bog and backfire everytime. So I have replaced the CDI, the ignition coil, and the spark plug. Same problem still exists. Tore apart the carburetor to discover the diaphragm had a tear in it so I replaced the carburetor, same problem still exists. Digging deeper I have tested the stator (well within spec), the rectifier/regulator (was a bit out of spec so was replaced). I have confirmed the timing is still perfect and adjusted the valves and I'm still experiencing the original issue of hard to start, but idles perfect and backfires upon any throttle. Any help I can get as to what I am missing will be greatly appreciated!

 

I know this sound weird but I have had alot of atv have this problem with clogged air snorkel try running with air box lid off if runs better check for blockage in snorkel get an try an hollar back you did say you replaced spark plug did you use a new ngk Dpr8ea9?

 

Just one point, you replaced the CDI, Honda made at least four different CDIs for 300s, the first three use the same shaped wiring plugs but are electrically incompatible with one another. I once had a 300 with the spluttering when revved up problem and found someone had fitted the wrong year of CDI. Now you have fixed the carb problem, try refitting the old CDI. Also do a compression test, low compression makes bikes hard to start.

 

I will try this tonight after work and report back, will also change spark plug to the one recommended to make sure

 

Problem solved! You were correct, i re-installed the original CDI and she is running like new. I wasn't aware of there being that many CDI's for that machine so thank you very much for that information. It's crazy how it mimicked the original problem of the carb diaphragm to a T.

 

Honda originally colour coded them, white, red, or green plugs and sockets. However so many now have aftermarket CDIs, which usually have off white sockets, that it is hard to tell which model CDI is fitted.