Weird top end cut-out 400ex...solutions?

[YZROOSTINYA]

Just did a 416 on my buddies quad. We are having a problem with it cutting out in the top end or under load.

The quad was broken in and ran fine. Halfway through a day of riding it felt as if it was loading up and when you started to rev it out it would cut out and bog.

The plug is super rich and fouls out.

The jetting is right.

Running a 155 main, 42 pilot, 4th clip on the needle. We even went back to a 148 main. Still same prob. Removed the airbox lid. swapped Carbs, CDI, Coil and still no luck.

It runs fine when we switch to a new plug but after 10 mins it fouls out.

It seems like a jetting issue but we have swapped carbs. cleaned them, changed jetting and still no luck.

VAlve lash is fine. compression is good. no smoke(other than carbon when revved) accel pumper works fine.

any ideas?

[1fst400]

Have you cleened the air filter? sometimes it can make a suprising difrence.

[YZROOSTINYA]

yes, airbox and carb boot all ok

[1fst400]

this is a strech, but mabie carbon build up is on the valve seats or somthing. when you checked the valve cleerences, did you have to change them?


Mabie an air leak on the carb boot.

Mabie posibly a faulty cdi?

[YZROOSTINYA]

the head was fully cleaned during the rebuild and relapped the valve seats.

Carb boot leak would make it lean, its super rich. the boot is ok even though.

top end has maybe 15 hours on it.

CDI and coil were swapped along with the carb. No Go:confused:

[1fst400]

Next thing to check is mabie flywheel key, the ignition timing may be off?????????????????

its possible for the key to sheer off.

[bwamos]

Check your petcock filter for debris?

It may just be clogged. Allowing just enough fuel to flow for up to 3/4 throttle.. but once you pass that point it can't keep up, and the engine gets starved for fuel?

[YZROOSTINYA]

no prob with that.

that would still cause a lean condition. the prob is it is too rich.

ran 100% then all of a sudden it was way rich and continues to be

[EXevan91]

does it have a prefilter if so take that off and drain that gas and put New gas in it, cuz someone could have put super rich premix in it or switched cans on him?

[greg_gorrell]

well it is definately your jetting man....if you tried a different carb and its still doing it then your jets are off. thats about the only thing that would cause it...what pipe and airfilter does he have on it?

[YZROOSTINYA]

it had a 148 stock( like every 400ex) and we went to a 155 main. ran fine for 2 days then all of a sudden it went wacky. went back to a 148 still wacky.

its not the jetting or the gas. there is something wrong.

[greg_gorrell]

well i know its a pain in that *** messing with them a lot but the jets are really the only thing i cant think of....im positve it has nothing to to with the engine itself but other than that i dont know what to tell you

[cals400ex]

you don't think it is the stator or the plug wire do you? i have no idea. i am just giving suggestions....

[1fst400]

Are you shure the enigne doesent run smokey? Somthing like the valve seals leaking?

[YZROOSTINYA]

the engine does not smoke at all. the only thing you get is a VERY slight blip of carbon(black) when you rip the throttle hard off idle in nuetral.

We cleaned all the ports and reseated the valves during the rebuild. Plus it ran fine till noon then it just started loading up and then it cuts our from being fouled.

When you put a new plug in it runs almost perfect and within ten minutes it fouls the plug and runs like ***..

We have been kicking our heads in because we have checked everything that 99% of the problems would fix.

We are gonna tear into later this. Hopefully we will find a solution.


Thanks for all the help. I am gonna go bang my head into a wall again.:p :D

[jhr400EX]

My 400 used to do that every once in a while. It would run perfect all day, then all of a sudden it would start fouling plugs and not running right. If you kept it running, the problem would go away and it would run fine for another week or so. I changed over to an Iridium plug and that seemed to help the problem a little. Never did figure out exactly what it was though. It just stopped doing it one day and never did it again.

[MIKE400EX]

Sounds silly, but put a good ground wire between the motor and frame and frame to battery (-) and see if the problem goes away.

[suck my pipe]

i dont think the jetting is right, all i had was a rev box filter and alip on and i needed like a 165 and a bigger needle to get it al right, it ran perfect no spots, bogs, or misses.

But you said its running ritch all ready? this dosent make sence too me. You want the plug to be a chocolate color, good luck.

[Caseys 300ex]

Have u tried any jett besides the 148 and 155? Cuz maybe thats the problem.

[Colby@C&DRacing]

You maybe burning up the plug and that is why it looks rich. To much heat from not enough fuel. A 148 main with the mods listed is pretty small. Did you powder coat the frame or anything?? I would also switch coils if you have high resistance in the coils primary or secondary it will run fine for a few minutes then as it gets warmer the resistance goes up it causes weak or intermintent spark which will foul the plug and make it look like jetting. Just something to try

[boricua]

^^^^ i agree with him, it sounds like you have an ignition problem with a possibility of a jetting problem as well. i would look into that before jetting. fouling out a plug with or without your mods.....and stock jetting....i would look at ignition first.....