I need help with my 400ex

[JJ32]

the problem i have is when the motor gets into the high RPMs it starts to bog. its like its hitting rev limiter but way before it should. I have k&n air filter and everything else is stock. I have cleaned and went through the carb and bought a new ignoition coil and CDI box and none of that fixed it. So i hve no clue what to do next. it would great if someone could help me out.

[tri5ron]

restriction in the exhaust/ muffler?
maybe collapsed innards in the muffler?

Old gas?
Clogged petcock screen?
Cracked or Leaking Carb Boot?

[jcs003]

jetting.

[quadbod]

Main jet:macho

[jcs003]

Originally posted by quadbod
Main jet:macho

i am not 100 percent positive for the high rpm jetting. i believe it is a combinatrion of the needle and main when WOT. but there are some guys on here that know the carb set-ups very well.

but i have heard the stock carb does this on alot of bikes in different elevations.

[tri5ron]

I guess the first thing I should have asked is...

Did this bogging condition appear immediately following the installation of your K&N ?
and did you also remove your airbox lid?

or did you already have the K&N on, and it was running fine?

As was already mentioned, It is most likely your jetting. (providing this problem just became evident, after installing the K&N),
and everything I previously mentioned is in good running order.

So give us a few more details about WHEN this problem began, and then we can better troubleshoot it for you.

[JJ32]

the bogging happened when i had the stock airfilter in it. so bought the k&n hoping that would be better for it. so what jet # would be good to try? if thts whats wrong

[tri5ron]

Since your symptoms were first showing up on a completly stock bike, you first need to confirm that EVERYTHING is in proper running order, and within proper adjustment specs.
In addition to the below maintaince suggestions, you might want to also check the valves for proper adjustment, and check your sparkplug for signs of fouling. The idea here, is to get it running properly in it's intended STOCK performance envelope, before you start changing OEM equipment, and therefor possibly complicating matters.


Originally posted by tri5ron
restriction in the exhaust/ muffler?
maybe collapsed innards in the muffler?

Old gas?
Clogged petcock screen?
Cracked or Leaking Carb Boot?

I had originally suspected that your condition had started before you installed the new filter, and I still stand by my original suggestions, before you approach the jetting issue.

how long have you owned this bike?
do you know the past maintaince history of it?
did you recently buy it?
what year is it?
what kind of conditions has it been ridden in?
What did a previous owner do to it?
how was it ridden?

In other words,...
What is the actual overall condition of the bike?,

and have you gone through all the manufacturers maintaince, and suggested troubleshooting steps first?

Bottom line is, A stock bike needs to be maintained properly FIRST.
Then you can start thinking about mod's and changes to the stock configuration, for enhanced performance.

The 400ex is easily the most modifiable quads out there, and can be made to run rather impressively.
but as with ANY engine, it must all be in proper working order, before you go changing things.

There are numerous threads here on this forum, that can usually answer just about any question you can ask,
and after doing a few searches, and alot of reading, you will undoubtedly learn more about your bike than you could have ever imagined.

hope this starts you off in the right direction, and is of some help to you.
and BTW-
Welcome to the forum

[jcs003]

after reading triron5's post, your issue could be excessive carbon build-up. this is an overlooked condition that would show up at high rpm. your conditions will be effected due to restriction. you may actually be getting too much fuel.

i know thiis happens in 2-valve head 4-strokes so is a possibility in 4-valve configurations. picking at straws here...

[JJ32]

i bought the bike used from a guy the bike sat for a year. it was ridin on street alot he said.and everything seems to be good on it i have checked the timing, the valves. so idk whats up with it? but thaks for all help so far this is a great site.

[JJ32]

how would i fix that or how could i tell thats the problem

[jcs003]

Originally posted by JJ32
how would i fix that or how could i tell thats the problem

i think a compression test would tell you if you had too much carbon build-up. im not sure how to remove it.

[JJ32]

ok thanks but i remembered we did that and the compression was good but i appericate the idea if anymore let me know

[tri5ron]

Since you say that it sat for a year,...
and the timing, and valves are in proper adjustment,...
I'd say it would be a good Idea to pull your petcock and screen filter to see if they are gummed up.
rinse out the fuel tank to get rid of any residues.

even if you had put in fresh gas, they could be gummed from the old gas, and not allowing the fuel to flow properly.
you could also have gummed up gas in the fuel line. I'd replace that just to eliminate the possibility of it being the problem.

let us know.

[JJ32]

i have already replsce the fuel line but i never thought about the petcock so ill check that out. thanks again guys

[derekhonda]

1 question. Did the problem get better or worse when you put on the K&N? My guess, is it got a little worse. If that is correct, it is starving for fuel, and you need to bump your main jet up 2 jet sizes, and dump 6 oz of seafoam in the gas and then I'd say you are good to go.

[JJ32]

i thought it was the jets so tryed to bigger and that didnt help. but the bogging isnt constant. its like its hittin rev limiter. Is there anyway there is rev limiter function i could check. Or would the alternator cause this if it was bad?

[derekhonda]

I'm still not convinced it isn't something in your carb. What does your plug look like? 400's don't have alternators but they do have a recharging system, this shouldn't be a factor in whatever troubles you are having right now.

400s are great cause they are virtually trouble free. And i recognize sometimes there are problems but you need to check the simple stuff before you go tearing into the engine. Tell me what your plug looks like, dark and wet? white and crusty?

Airfilter is clean, correct? Do you have an inline fuel filter and when is the last time it has been changed?

[JJ32]

the spark plug keeps folwing out.and i have no ckue about the fuel filter Its an 04 so if u could tell me wheres its at that would be great.

[derekhonda]

Ok, so its fouling out...too much fuel. Drop your main jet 2 spots from where it was a before you started tweaking it and do a plug chop (or just ride it for 10 minutes) and then see what it looks like and rides like. It does make sense that if you main jet is too big and when giving it too much fuel, it will bog on full throttle.

[JJ32]

ok so any idea what # jet it might be good to tart with

[derekhonda]

Man its been so long since i've been in my 400s carb, i think 138 or 142 is stock, i would try one of those.

[05trx300ex]

Stock is a 148K

[bigd's ex]

Might want to try taking the lid off the air box to let it get more air.
I'd start with a 152 main jet & go from there.

[derekhonda]

I guess the big question, is what size mj are you currently running?

[droppedmazda]

is the lid still on? if so pull it off and ride and see what it does. it sounds like its starving for air.

[JJ32]

The main jet that is in it is a 142. Just to see if needed more air we disconnected the hose from the air to carb and rev it up and it was a little better. So if you have any suggestion on what i need to do let me know.Thanks

[BEAVER.989]

Is that a DynoJet 142? If so, you're running awfully rich for a stock machine. That's about the same as a 155.

You definately need a smaller main, or you could remove the airbox lid and add a slip-on and be in the ballpark.

[JJ32]

so what #do think i would need

[BEAVER.989]

I'd try either a 138 DynoJet or 150 Keihin