300ex jetting help

[Curtis300ex]

Im running a hmf full system exhaust and a k&n filter with no lid, i bought the moose racing stage 2 jet kit, put in the suggested main jet and i put the clip on the needle to the fourth notch from the bottom like the kit suggested. we also turned out the mixture screw to 3 turns out.

The bike idles and starts up fine, but about mid rpms it really falls on its face, bogs down and has no power, but does really great in the top rpms. i still have the stock pilot jet in, should i got up a few sizes with that? im not sure what will fix the mid rpm bog. it also pops a bit on deceleration, i also took out the spark plug to check that and it was black around the threads but on the tip where the spark comes out it was white.

Im 17 and this is my first time jetting, im fairly mechanically inclined and my dad is helping me also, i just figured i would ask you guys cause we both have no clue when it comes to jetting, so any suggestions would be great, thanks guys.

[Quadkid440]

Popping on decel is too rich. Could also make a mid-range bog. Try turning the screw in a bit or going down one size on main jet.

Could also drop the needle a notch or too to lean it out a little bit.




Could also turn to fuel screw in all the way and go 1/4 turn at a time until it idles fine. From what I've learned anything past 2 1/2 turns out is bad. But 300ex carbs are new to me. Try leaning it out a bit first. Plugs chops are good too.

What color is the spark plug? If its all black and chitty its too rich. if its like whitish yellow its too lean. you want a soft brown color on your plug. Check the plug and get back to me. We can get this figured out.

[Curtis300ex]

Well the plug is black, actually very black around the base but where the spark comes out its pretty white, but that might be because i have the amr monster coil, and the nology spark plug wire which both together provide a really hot spark. As far as the mixture screw, 2 1/4 out is stock, and 3 turns out is apparently not working for me so ill try like 2 and a half or 2 3/4 out and see how that goes, and for the needle, that was actualy the first the i was gonna try, bringing it down one notch.

[Tommy Warren]

a white tip on the plug usually means its way too lean I would definitly richen it up. its better to be too rich than too lean. also a pilot jet would help your mid range a bit. I think your pilot sends fuel up until 50% throttle and then the main takes over from 50% to WOT (wide open throttle) something like that.

[Curtis300ex]

Well i finally got it jetted properly(i think) it runs smooth throughout the whole rpm range but it still pops a TINY bit on decel but im sure thats normal as it is an open style pipe. I took off the spark plug, and its a darkish gray color, and i matched it with a spark plug chart my dad has and it seems to be where it should be, im not too worried about it. Thanks for the help guys!

[Quadkid440]

Aftermarket exhaust will always give a nice little grumble on decel.

Glad to hear you got it! What exactly did you do?

[Curtis300ex]

well my final "tuning" is a 128 main jet, third notch from the bottom on the needle and 3 turns out on the mixture screw.

[Quadkid440]

That seems about right. Jetting is simple once you get the hang of it. Carb swapping is a little more difficult. Thats the kind of fire I was baptized in! LOL

[Curtis300ex]

Yeah, at first it was a bit overwhelming, but i think i got the hang of it now, im just not looking forward to next winter when i do some engine mods and put a 400ex carb in it, however i am looking foreward to the outcome!

[Redhotrod]

Speaking of swapping carbs, I am in the middle of trying to jet my 300ex with a 400ex carb,
I just bought the bike, 1997 300ex, it had a dynojet jet kit in the original 300 carb, but it fouled the plug out at WOT. So I figured to keep up with my buddies with 400+ccs I would get a bigger carb at least for mine, My bike does have a full White bros exhaust, and I am running with my air box lid off with a pair of pantyhose overtop of the air box with a K&N filter with outerwear and the motor has been bored out recently, I just don't exactly know how much.
My problem is I cannot get the jetting right. with either carb. I kinda understand the concept, but I guess I cannot exactly tell the difference of a lean or rich condition.
with the 400 carb, I have the stock pilot(38) air screw 2.25 out and the needle in the 3rd groove down, it idles great, plug looks brand new with it idleing for 5 minutes,
coming up to 3/4 throttle it has a little popping on the top end:confused: tried moving the clip up and down but the best I seem to get is at the 4th groove down
I have a handfull of main jets, running with the stock 400ex 148 main jet, it was fouling plugs, not sure if its cuz of not knowing if my needle is wrong or not. I have been playing around with the bike on a stand and in neutral revving it up to check jetting, will this work? or must the bike be under load?
So with the plugs fouling out I got a bunch of jets, the smallest I have is a keihin 135 and have been trying to get the plug right with that, I can't imagine needing a smaller main for the setup I have. Even with the 135 in I still have the same popping with the plug turning brown/black
another weird thing is I will even pull the rear air boot off of the carb to help lean it out more and I notice at 1/2 throttle the carb is spitting gas back towards the air filter:confused:

anyone have any suggestions? I would love to get it running right, heading to the hills of WV in a couple weeks