Ok so my 300ex has a full fmf exhaust open air box k&n air filter and now im in the process of trying to jet it with a 400ex carb.im running a 150 main jet and a 38 pilot jet and my airscrew is 2 1/2 out.My problem is that its not very easy to start when its cold,it has a hesitation when i stab the throttle,it also does this weird kind of hiccup noise when it idles and it will sometimes kill it.It runs awesome just riding it around but the hesitation and the hiccup noise bothers me.Im thinking that i might need to bump my pilot jet up to a 40 but when i richen the air screw it makes everything worse.The carb is 100% clean i have been completely through it many times.please help!

 

First of all, why run a 400ex carb on a basically stock 300? even with full exhaust and intake,the stock 300 carb is more than adequate. Ive only seen the 400 carb used on big bore or hi comp racing motors requiring the extra fuel flow. It could just be a case of too much carb for the motor. My sons old quad was a 300ex, we ran no lid, Uni filter, full Supertrapp exhaust, and all i did was went up one size on the pilot and 2 on the main, and it ran great, and was plenty fast, it revved so nice, it constantly hit the rev limit,and I installed a +1000rpm cdi that let us take advantage of the extra hp and rpm the freeer flowing motor had. Hondas are notoriously cold blooded, every quad and motorcycle I haved owned that was Honda took a little extra care to get warmed up when cold.

 

actually i finally tried a 40 pilot jet and tweaked the air screw a little and its running amazing the 400ex carb made a huge difference in the performance on it.

 

i have a question for you about the 400ex carb setup. how did you even get it to fit into the air box tube? i did manage to get it to squeeze on the the intake boot. but its tight... it definatealy will not fit unless on the bigger opening end unless i sand it down a little... i bought a 300ex carb on ebay, and unfortunately got a 400ex carb instead. but if i gives more power than maybe it will be worth keeping and fiddling with. so you are running a 150 main, with a 40 pilot? i will be running stock exhaust, with k&N air filter with the lid on. hopefully you can shed some light on my questions. Thank you