Carb troubles

Hello all....I've got a problem that I hope someone can answer before I wind up taking my bike to the shop... Ok, I've got a 1993 300EX with a Carb from a 99 300EX on it. Now, when I give it a slow push on the thumb throttle, everything works good. But when I stab the throttle as you would if you were to try to get the holeshot it stutters off the bottom. I tried adjusting the screw on the bottom front of the carb (air screw I believe) and nothing really happened but the idle changed. And of course I had to adjust the idle after that. But no noticable change anywhere when I adjusted that screw(minus the idle speed). Any clues as to where to start? I also noticed that it runs better with the choke on the first setting. It didn't stutter as much. All thoughts would be great....help the rookie out here, huh??
Ted :devil

[NVR2L8]

If the engine runs "better" with the choke partially on, that's a pretty good indication that you might be lean on your jetting. With the choke on you're slightly decreasing the amount of air the engine is getting, thereby richening up the mixture. I don't know what kind of mods you have done to your 300EX though, or what mods the owner of the 99 300EX carb might have done (and jetted the carb for his engine), but you need to do the same. First find the elevation where you normally ride, tell us what mods you have done to your 300EX, and disassemble the 99 300EX carb and determine what size pilot and main jet is installed, as well as on what groove the needle is positioned. It's pretty safe to say you'll probably want to bump up the pilot jet a size or two (I think the stock pilot is a 38), and I believe the stock main is a 122, so also pick up a handful of larger main jets.

FYI, when adjusting the fuel screw, first seat the screw by turning it in until it "lightly" (and I mean "LIGHTLY") seats, and then out 2 1/2 turns. After starting the engine and letting it warm up, turn the screw outwards until you notice a decrease in idle speed, and then inward until you notice a decrease in idle speed--once you determine this range, find the spot in the middle where you get the best response when slightly wicking the throttle from an idle. BTW, you might want to wait to do this until after you increase the size of the pilot jet...you can easily run a size or two larger for a little better throttle response and cold starting.

Anyway, tear into the carb and tell us what you've done to the engine, intake, and exhuast systems.

Thanx's for the input so far.... As far as I know, the motor was supposed to of had a cam in it.....but I don't think so cause it doesn't really beat any Warriors, but anyway...the rest of the motor is stock with the exceptions of a few holes in the air box lid. I am going to have to tear the carb apart to see what the jets are. I still haven't gotten to that yet. I did change the plug today and noticed that the plug was almost white on the electrode. I'm not possitive but doesn't that mean that it's running lean? I opened up the air screw to 3 full turns from the seated position. Tomorrow I will run it and see if that changes the color on the plug. Until I get the carb apart.....Cya L8TR:devil

[big-guy]

does the 300ex have an accelerator pump? that's the first thing I'd check. sounds like it's not squirting enough gas when you mosh on the throttle

Ok, I've been searching thru all these old posts about hesitations and carb troubles and the thing that they all have in common is the needle jet. They all say to lower the clip to get rid of the off idle hesitation.Hopefully I might have just found the answer to all my problems. It all makes sense...but does it work??? We'll see I guess.

[gojk]

I have 400ex with the head pipes ground a yoshi pipe and the choke removed and a dyno jet kit. Once I installed the jet kit I would hit the gas and if I didn't kind of flutter the gas first it would just bog down and die. SO I raised the clip up one notch and my problems went away. There is an awesome article on jetting in the April Dirt Wheels mag. I answered all of my questions. They say if it is just a slight stumble then revs it is lean and to lower the clip one position if it stumbles hard then raise it to make it leaner. Hope this helps. Get the mag, it explains it a lot better.:p :cool: :)

I just ran into the same problem on my buddy's 400. We called Markhams and he said we were to rich. To much gas to little air. So we lowered the jets and dropped the needle one notch below stock and it cured the problem. You might have to fool with the air/fuel screw inside the carb as well.

Well, I lowered the clip on the needle, thus raising the needle. It still has a slight hesitation to it and I'm noticing that it shoots out a flame about 16 inches as it backfires when deaccelerating under no - and under load. I did notice that it is smoking a little now too. It's black smoke so I'm thinking it seems to be rich now. Also seems to have a little more power as well. I still need to check the plug to see what color it is though. Any thoughts on the backfiring ??? What causes it and how to stop it??? Thanx's a mill!!!:devil
Ted