Help me troubleshoot

[mjmisek]

Here's the story along with what I've done so far to try and fix the issue:

My wife's 400ex is "cutting out" at half to full throttle. It only does this when the motor is under a load and it seems to get worse if I richen the main jet. Right now I have a 140 in it, which is DEFINITELY lean for the application (440 stroker motor), but I don't think that carburation is the key since it began cutting out when I had a 39mm FCR carb on it. My first attempt to fix was to put the stock carb back on. This did not fix the problem.

So I am thinking that the issue is electrical. I followed my service manual and I have effectively troubleshot the entire "ignition" section. .. but not at high rpm under a load. .. I had a friend's 400 in the garage for some time (same year . .. '99). . and I swapped out the spark plug (with new), CDI, coil, AC regulator, and stator (including the pulse generator). To make sure it wasn't my killswitch grounding out, disconnected the ground from it.

In an attempt to make sure it wasn't a valvetrain issue, I pulled the valve cover off and inspected the cam timing. It was as dead on as possible (seems like the 400exs are always somewhere in between where you can adjust by the teeth) Anyway, I put it back together and set the valves. ... still coughs, hesitates and cuts out at mid to high rpm under a load. .

If I persist and keep trying to make it operate in that range, it will die out and need to sit for 10-20 mins before it runs again.

I can ride it for hours if I never punch the gas and it won't have an issue.

-Just hoping someone on here can give me some insight beyond what I've tried to do. My next step is going to be replacing the wiring harness alltogether. . . thoughts??. ..

BTW, please don't mention taking it to a dealer as I'd sooner part the quad out than let any dealer within a 50 mile radius touch it.

Thanks!!!

[mjmisek]

ttt

[zrpilot]

please post everything you have tried including all the mods you've done.

I have an FCR running a 175 main on a 416EX with Wiseco 11:1, HRC cam, HMF full exhaust, XC porting and I'm thinking about trying a 180.... Your's sound lean.

[mjmisek]

when I was running the FCR carb, I had a 195 main. . . and it seemed about right for a while. . . then this issue started.

Like I said, I know it's horrendously lean right now, but as soon as I richen it up any, the problem worsens. .. this leads me to believe that it's electrical. . . but only seems to happen under a load at higher rpms. . .

[mjmisek]

mods. . .

440 stroker kit
11:1 piston
stage 2 hotcam
stock header /
WB E series slip on (I know a full system would be better, but honestly, this quad is for my wife and I don't care to get any more performance out of it)


Currently has stock carb

[400exracer511]

I'm no mechanic but I'm going to throw this out there anyway.... maybe it is sucking air? I would check to make sure everything is sealed up good from the air filter to the motor.

[zrpilot]

Then in my opinion, you gotta look at spark.... how old is the plug? How is the spark? Crisp and blue?

[400exrider707]

Jetting is one of the problems at this point, but it does sound like you have more problems than that. Check and make sure your intake is nice and tight and that there are no cracks in the intake tube. Spray something flammable around the intake and see if it revs up. That would indicate a leak. I doubt that it is electrical, though you cant rule it out.

[mjmisek]

Originally posted by 400exrider707
Jetting is one of the problems at this point, but it does sound like you have more problems than that. Check and make sure your intake is nice and tight and that there are no cracks in the intake tube. Spray something flammable around the intake and see if it revs up. That would indicate a leak. I doubt that it is electrical, though you cant rule it out.

Yes, that's a good trick, and I already tried it. (Sorry I forgot to mention it in my description).

-To answer another's question, I have tried a couple new spark plugs, checked the voltage at the coil and swapped the coil to no avail.

As far as the jetting goes, I know its on the lean side right now. .. but I've been through the gammit on this and think I have eliminated jetting issues since the issue did not change at all when I switched carburators.

[zrpilot]

Originally posted by zrpilot
Then in my opinion, you gotta look at spark.... how old is the plug? How is the spark? Crisp and blue?

I'm sorry for this post, after I read your original you had already covered all this and more. ME BAD

How about something more mechanical. Have you done a compression test? Could a funny valve problem be creating some sort of funky reversion?

This is a tough one for sure. Good luck.