i have an 07 400ex. im running white bros slip-on with 6 plates, UNI filter, airbox lid mod, 195 main and 42 pilot.
once it gets started it runs great, doesnt overheat. might be a shade lean but no excessive backfiring and such.
ever since i added exhaust and tuned it with the jets, it has been a pain to start. it could be 40* or 90* outside, doesnt make much difference. i had removed the choke but i replaced it to see if it would help, didnt help much either. what i essentially have to do every time i start it, unless the engine is warm from being run in the past few hours, is bump the starter and try to keep it running at idle. a slightly higher idle speed helps but if it is too high then it wont run without stalling(while its cold). it usually stalls after 2-3 seconds of running for about a minute of doing that constantly. even once it will hold idle if i give it any more than 1/8 throttle it stalls. it will eventually hold idle and then warm up and once it is warm it starts with a flick of the switch. the only other problem when it runs is it sputters when i am riding it constantly barely above idle speed/barely giving it throttle, like cruising down a road in a high gear at low rpms, barely above idle. would never cause it to stall or anything like that but it acts like it isnt getting enough fuel. aside from that it runs flawlessly, can tach it out at 80mph with no problem. what is the deal with the starting issue? i know there are the adjustable screws inside the carburetor, i havent messed with those but i may have accidentally changed something while cleaning the carb, could that cause this? thanks.

after doing some reading, alot of people say to pump the throttle 3 times and then it starts right up, would this fix the issue? i dont live where i ride so i cant simply walk out and try it right now.

 

hi, by adding those mods especially the jets, you should adjust the air mixture screw by adding mslightly more air in may fix your issue, iv even switched to a hotter plug 1 size up. it made a world of a difference for me..

 

so what plug do you run? i have the ngk iridium, whichever model was recommended for the 400ex.

how much should i adjust the mixture screw?

thanks for the help

 

i use ngk dr7es the bike is desined for dr8es.. you should stick with the same brand, also for the air mixture screw try by turning back quarter turn at a time, reassemble and see how it runs.. it may take a few times to get it right but it will be worth it,, id recommend getting the hotter plug 1st and try it.. then adjust the screw


my quad has dyno jet stage 3 and runs flawlessly

 

ok i will, thanks. what all mods do you have and whats your needles at?

 

hey man np we all just wanna go out n ride in the mud lol... wow a list...

msr handle bars
full fmf exhaust with header
dg nerf bars
stage 3 dyno jet
stage 2 hot cam
wieseco pitston and rings
stage 1 cdi
white bros clutch
rk racing x-ring chain
renthal sprokets
twist grip throttle

so for my language but its loud as ****, my dad hates it lol but when he rides it he dosn't comlain lol...it tops out around 96kphr, i painted the frame last year and didnt end up likeing the color so im doing another rebuild the winter/spring and changing colors and fixing other lil things....where bout's in the world are you in? lol

 

Here is what I would do. Start by adjusting the valves, they can be hard to start when the valves are out of spec. Then make sure the carb is clean. If you are lean she will pop on decel. But you really want to read the plug. Get the ex out in 3rd gear 3/4-full throttle then turn it off without backing off of the gas. Pull the plug and see what it looks like, white/grey = lean, black = rich, brown is where you want to be. After you read the plug you will know how to adjust the fuel/air screw on the carb. The f/a screw should be about 2.5 turns out from seated. If thats where the f/a screw is now back it out a 1/4 turn and try it out. Trial and error.
All the 400ex are a pia to start. With the 42 just bump the gas 3 times, then start. Probably going to be as good as it gets.

 

96 kmh is about stock gearing, throw a bigger front sprocket on and u can easily get some more speed out of it. i ride more wide open trails so i ride in 5th all the time and its always fun to get up to the high 70s on the road lol.

ive never even tried pumping the throttle 3 times like so many people say, i guess i will try that next time i go riding.

and my white bros is insanely loud, it sounds like an airboat when its riding away and you can hear it for about a quarter mile, all of this with the spark arrestor and only 6 plates, sure does sound good though.

 

sweet dude lol,, my bud has a 400ex and always has probs starting, and he has no mods...just something with that bike...good luck with fixing your quad...let me know how it goes..im always interested to see what happends...

 

My WAG and that is all it is:

#1. Adjust the valves.
#2. Drop the clip two slots
#3. Back the idle mixture screw out 1/2 a turn.

Go ride it and see how it starts and feels.