400ex coughing/missing/sputtering

[Warrioreater400ex]

My 400ex used to run just fine with a wb slip on and k&n, well the pipe broke, so while waiting for a new one, i polished some stuff and ground the welds down on my headpipe. well now sometimes after i jump or hammer on it, it just dies and wont start back for a few tries. also it misses on the top 3/4 throttle, so i thought it was just my main jet, however, i went to get a new one and all the threads on the honda jets were a different size??? so i couldnt get a bigger jet. but it doesnt act just like jetting because when it starts coughing when i hammer on it, if i just hold the throttle steady, say at 3/4 or so, it jumps forward, then coughs/misses, then jumps forward, coughs misses etc... i took the carb and cleaned it and look at the floats and it looks ok... can someone please help me!!!

[FoxRacing81]

Umm...Try these things.

Clean air filter

Buy new spark plug, start up quad, rev to almost for throttle for about 1 min then shut off the quad quickly and take the plug out and check the color. If it is completely white, your too lean. If it's completely black, your too rich. If its brown, its just right.

Mine would sometimes spudder when I would just get on the throttle turns out the filter was way to dirty causing not alot of air to get through...which would be just like creating a rich state.

-Justin

[TCracin440ex]

sounds to me like your main jet done fell out....or...you have trash in your gas tank...take the tank off empty it out....take the petcock out and clean it out too...might solve some problems

[Warrioreater400ex]

ok so if my jetting is wrong, ive already tried to fix it and none of the honda dealerships jets will fit!? their threads are too big for some odd reason? and my jet is like a 146??? and they said stock was 148? supposedly the guy i bought it from in 2001 had it jetted.. but idk why its less than stock and an odd jet?

[DeerNuts]

Im assuming you have a Keihn carb, which is stock. the stock main jet is 148. Dont know why someone would ever go down a size:confused:

you need the keihn jet. I ordered one recently, and when I went to screw it in the threads didnt line up. well, here it was a Mikuni jet, so obviously it didnt work.

It should say the brand on the jet, but now on the 150 I just installed I dont think it said the brand, but it fits perfectly.

[DeerNuts]

Originally posted by Warrioreater400ex
and my jet is like a 146???

Pull the jet out and check. the size will be stamped on it.

[plkmonster2]

sounds to me liek you have the dynojet kit. You have to take out the adapter that the jet screws into. Is the jet hex, or is it slotted?

[Warrioreater400ex]

it is stamped on it, no brand, but the numer is 46, but it has a slot on it and the number is beside the slot, so the 1 just got slotted because it is obviously a 146, it has 2 hex shaped nuts that tighten down, then at the top of it the jet screws into them. it is round with a slot in the top of it. My pilot jet has the keihin "K" stamped on it, so if it is a dynojet... what do i do to get a jet that works?

[plkmonster2]

You take out that jet, and there is an adapter above that. Take that out also. it is 6mm I believe. then replace those 2 parts with the 1 keihen jet from your dealer.

[cals400ex]

yeah, it sounds like you are running dynojets. are the dynojets round for the 400ex? the adaptor may get confusing for him because the stock mainjet actually screws into what others call an adaptor. it is actually called a needle jet holder. however, with the dynojets, you have an adaptor that screw into the needle jet holder. the dynojet mainjets are definately shorter than keihin jets but i don't know if they are round or hex shaped.

[Warrioreater400ex]

thanks guys, i knew i could count on yall, im pretty sure its just the jetting thats all messed up. ill let yall know when i get the jet it.