400EX trouble shooting engie problem.

[mthomas]

My 400EX is sick. Here's the history...I purchased the EX with a PC T-4 pipe on it. Stock airbox with the lid on, no holes and a Uni air filter. The jetting is currently #38 pilot and 150 main jet. It has always ran great, put the plug gets pretty black. More recently, it started fouling plugs. It got really bad a few weeks ago when it warmed up outside - even with a new plug it would cut out and not run too good. I decided to re-jet, so I took the carb apart and cleaned everything, checked the float level, etc. and then I installed the stock 148 main jet. Now I can barely get the think to start! So, I fiddled with the pilot screw until I could get it to start. It now starts, but if I give it any throttle, it dies. If I choke it, it dies (to get it to start, I have to leave the choke 100% off). As soon as I start it, I can give it throttle real fast and it will run real bad, with white smoke coming out of the exhaust and eventually it dies. If I change the pilot screw setting, it does not increase the idle like it is supposed to. I check the valves and they are properly adjusted. I seem to have good compression. What the heck is wrong with this thing? Any help would be greatly appreciated!:(

[TC426EX]

Fouling plugs means its way too rich probably, but the jets youre running dont sound like they should be that rich for the setup you have. Also, white smoke usually means oil burning, so you sure its not black smoke? Black smoke would indicate richness and go along with your fouled plug. If its indeed burning oil then you either have a ring problem or a valve seal problem. I have to think about this for a minute to see if I can come up with anything else. Try running it with the airbox lid off and see if it runs any better...

[Fortman_400]

oil burning is blue,, not white
white is coolant burning or water condensation boiling in the exaust.

[mthomas]

The smoke is definitely white. I will check the header and drain the tank. If I take the airbox lid off, It runs the same.

[buster369]

Just an idea but the rings could be shot from not gettin enough oil to them from something clogging the oil flow. it happened with my bike and then i changed the oil filter and oil and ran fine after that. Just a thought tho

[plkmonster2]

White smoke can also be from extremely excessive gas. On my 200, if you leave the choke on for 2 minutes or so, turn it off, and rev the motor, it will hesitate, blow out white smoke (That really, really stinks, but not like oil) and then it will run perfectly. Check to make sure that your pilot jet didn't fall out. Also, it seems liek your needle could also be set too rich. Check all that.

[mthomas]

I ran a cold (it's hard to warm it up, when it won't run)compression test today and I only got 90PSI. The service manual says it should be between 100 and 128 PSI. I left the pressure on the gauge and after several minutes, it had not leaked off any pressure. It looks like I have a worn top end...

[TC426EX]

Time to go to the next bore

[mthomas]

I put some fresh gas in it today and got it too start. It idled for a while and then died. If I give it any throttle, it dies. After I played this game for a while, I did a compression test. This time, I held the throttle wide open and cranked it over. I got approx. 145PSI. So, it looks like the top end is in good shape. Any other ideas on what is making this thing run so crappy?

[JOEX]

Originally posted by mthomas
I put some fresh gas in it today and got it too start. It idled for a while and then died. If I give it any throttle, it dies. After I played this game for a while, I did a compression test. This time, I held the throttle wide open and cranked it over. I got approx. 145PSI. So, it looks like the top end is in good shape. Any other ideas on what is making this thing run so crappy?
Did you drain out all the old gas from the tank and carb?

[mthomas]

I took the carb off of my sons 2003 400EX (all stock) and tried it on mine - it ran perfectly! Obviously, during my last two carb inspections, I have missed something. For the third time, I have the entire carb apart. All of the passages are clear, I have the stock jets in it, the air valve seems to be working properly, the slide and needle are in good condition ans move up and down as expected, etc. I can't seem to find any differences between his carb and mine. Any ideas on why his works and mine doesn't? If the O-rings in the air valve or pilot adjustment screw leak, would it cause the issues I am having? Thanks for your help.

[mthomas]

Yes, I tried new fuel before I tried the other carb. It did not help and I ran the old gas in my lawn mower without any issues.

[maxoutput]

ck the clip on the needle..make sure its in the middle grove...

[Speedy81304]

could be your accelerator pump is bad. I seen the same problem at work and i turned out to be the pump. it could be stuck open and free flowing gas into the engine. what you should do is pull every thing out of the carb and soak, the carb, main jet, slow jet, in carb cleaner for about 2 hrs. to mack sure ever bit of junk is removed. then using an sir compressor and a blow gun. blow air though every opening in the carb even the air bleeds in the throat of the carb. this will make sure that every thing is clear. check every o-ring and the plunger for the accel pump. to see if there all good. then put back to gether. start off with the slow speed fuel mix screw out two turns then adjust out by 1/2 turns till it revs up and then once it starts to fall off ( rpms start to drop) go back 1/2 turns on the screw...

[mthomas]

I went though the entire carb (third time). I checked the air cut off valve (good), I soaked everything and blew out all of the orifices with compressed air. I set the carb up EXACTLY (stock) like my son's 2002 400EX - I even made sure the needle jet was screwed in the same measurement (this could vary, depending on how far you choose to screw it in before you lock down the nut). Keep in mind that I installed that '02 carb on my '00 and it ran perfectly! I did take the accel. pump apart, but I could not see anything wrong. I checked the float valve and all was well. I checked the needle (stock - middle pos.), the slide, the throttle stop, etc. I put the carb on my bike today and guess what...same damn problem! After several stabs at the the starter, it did finally start and idle, but if you give it any throttle, it dies. Also, it will not start with the choke on. I tried the '02 carb again and she purred like a kitten! Any ideas? I am just about ready to set my pride aside and take this darn thing to the Honda dealership.

[plkmonster2]

Does your accelerator pump work when you pump the throttle? in front of the slide, there is a small brass piece that will shoot fuel into the cylinder. If it doesn't shoot fuel, your diaphram for the accelerator pump is shot. It could also be a very small blemish or piece of dirt in one of the carb's venturi's.

[plkmonster2]

Also, make sure that the slide is still attached to the lifter arm. And check the synchronization of the butterfly and slide valves. The service manual specifies all this.

[Speedy81304]

well with out looking at you r atv. It hard to tell wahst going on now. if i was you take it to a very well known hando dealer in you area. I would say if you lived in the poconos of PA. to take it to R&D powersports. Since i work there. LOL well good luck its mostly something your over looking. but dont hurt to find out what it is.

[mthomas]

I checked out the accelerator pump and it shoots a strong stream of fuel when I pump it. Any other ideas?

same thing happened to my quad and my oil ring was completly disolved, check that imediatley.

[mthomas]

Are you telling me that the oil ring on the piston was deteriorated because of this condition or are you referring to another oil ring? Keep in mind that my bike runs great with my son's 2002 400EX carb.

[dirtmomma]

Look for cracks in the carb, I kno wthis sounds crazy but I had a 400 carb on my 300 & it ran fine FOREVER then all of a sudden BAM it was doing the EXACT thing you have wrote, we jetted, jetted,adjusted the air/fuel,adjusted the needle EVERYTHING then as I'm putting the carb back on something caught my eye, where the top cover screws go in, well one was not the right one or something cuz it got screwed in too far & BAM the whole top of the carb around the bottom(if that makes sense LOL) was cracked so it was sucking air, just a small crack can put a damper on things, so we put the stock carb back on & DAYUMMMM it fired right up & hasn't ran this good for a LONG time :D:D Good luck

Originally posted by mthomas
Are you telling me that the oil ring on the piston was deteriorated because of this condition or are you referring to another oil ring? Keep in mind that my bike runs great with my son's 2002 400EX carb. thats what was wron with mine,it started smoking and when we tore it down that was the problem. I guess thats not your problem though, if a different carb makes it run fine. Just buy a new one on ebay

[mthomas]

Update...I swapped one component at a time from the known good carb to mine. Float & valve, pilot & main jets, air cut off valve, and accel. pump. No change! I can get it to start and barely idle. If I give it some gas, it will run like crap and then eventually die (with white smoke). The plug is wet and if I use the choke IT WILL NOT START. It is definitely getting too much fuel. I took the air filter off, to see if I get get it more air and it did not make any difference. I tried adjusting the pilot circuit (less fuel) and did not see any change. HELP!

[dirtmomma]

ARE YOU SURE YOU DON'T HAVE ANY AIR LEAKS IN THE CARB OR OTHERWISE????????????????????????????????

[bwamos]

Silly question... but you have checked the float level right?

It coulda been thrown off when you were cleaning it. If it's too deep it will make it run really really rich.

It sounds rich. Not lean. He said it would only start if the choke was 100% off. A crack would make it lean. And then would only start if 100% on.