i have a full fmf...? ehaust , i bought it off of ebay and the seller didnt tell me what series the pipe was, so i have a k@n airfilter with outerwears dust cover, closed airbox, and i was told by a mechanic (adams cycle) that it didnt need to be jetted. i put the pipe on and it idols fine but runs like crap. i half to let it warm 5-10 min befor riding in 95 degree weather!. once its warm if i get on it too hard it will cut out for a few sec but then catch back up. for some reason i cannot hit the rev limit, it will just loose all power at the same spot the rev limit should be. can anybody help please ????
thanks

 

are you using the stock main jet after those mods??? have you checked the spark plug???
sounds like you might be running to lean... allowing the motor to be free flowing like that is manditory rejetting!!!

 

i told adam that it needs jetted. and he said that it didnt and i foul a plug every time i start it. i take it out, get a new one ,heat it up and throw it in. it works but im afraid to start it anymore beacuse it overheats and i already fryed the top end 2 times and i cant find jets or a jet kit for cheap. i got a main jet for me ol 84 atc 250r and it was only 3 buks. but hes telling me its gonna cost 40 buks 4 one little jet. i just told him bulls****t and never went back. but i just dont run it anymore but i love to ride it. it sucks

 

I don't know a whole lot about the 300ex but, call the nearest hondata dealer.. im sure the have the jets you need. Just tell them the carb name and size and they will match you up... I did this for the past 3 atvs I've owned and they had them for around 4 bucks...unless hondata makes the 300ex jets out of gold

 

Maybe Adams meant $40 to rejet it FOR you, but just to buy jets is cheap. You can buy a handful of them for $40 !

 

if you put a FMF and a K&N on, and DIDN'T rejet, you will cause heat damage to the engine from running it too lean....hopefully that hasn't already occurred

Go to the nearest Honda dealer, and get the next two richer main jets over stock. Install the biggest one first.

Ride it, keeping in mind that a rev limiter causes the engine to shut off/misfire when you reach that rpm limit

 

Lower the clip on the needle jet 1 notch it should be the 4th from the top ( stock is 3rd grove from the top ) and go up 2 sizes on your main jet stock is 122 you can go 126 or 128 ? I am not sure what sizes are available for a 300ex .
When does your bike start to cut out ? If it is cutting out between 1/4 to 3/4 throttle its the needle jet . If its bogging on topend its the main jet .
Changing the main jet wont change how the bike revs between 1/4 and 3/4 throttle
The main jet is used at 3/4 to wide open throttle .

 

Any aftermarket exhaust.......start with 2 sizes bigger main jet for safety......lower e-clip on needle jet and try. Then work from there depending on elevation that it is going to be run at. Always look at the top of the piston to determind proper jetting.....the plug won't tell you crap on a two stroke.
Use a benda' light and look at the top of the piston......you should see a light colored center with diffening marks on the intake and exhaust ports.
Don't listen to anybody that says look at spark plug.......you won't last long.

 

umm........300ex is a 4 stroke......and plug read work just fine if you know how to read them.

 

LOL, I thought I was missing something!