I'm working on an '03 400 for a friend of mine that will not rev up over 20 mph. It only has 980 miles on it. It was running fine one day, then the next morning it would not rev up past 20. It just pops, sputters, and back-fires once it gets there or anything past half throttle, but up to there it will run perfect. It also revs just fine up to the rev limiter when in neutral. I have drained the fuel, checked the carb for anything in any passages, jets, and bowl. Everything was clean and the jets are stock. The diaphram on the slide is fine with no tears or holes and the needle is fine. The choke is working properly and not hanging up. I have checked the valves and they are fine. The spark plug is black, which is a sign of over-fueling, or not enough air flow. The air filter is in great shape, and the exhaust seems to not be obstructed. I turned the headlights on while driving it incase it was overcharging and nothing changed. I also pressed the override button to see if it would make a difference and it didn't. I swapped the CDI out with the one on my '04 400 and still no change. The quad runs fine right up until about 20 mph or 1/2 throttle. I've checked and tried everything I know to do. I'm open to any and all suggestions.

Thanks in advance,
C.J.

 

Coil or spark plug wire could cause this also clogged Fuel tank vent.

 

Mine does that when the choke sticks. You seemed to have ruled that out though. Can you swap the carbs from the '03 and '04? I don't know if they are the same. If you can do it, then that may give you an answer. Just something else to try. Keep us posted.

 

I forgot to list that I had blown through the fuel tank vent to make sure that it wasn't clogged. I also clipped about a 1/4 inch off the end of the plug wire to make sure it wasn't corroded or anything. I guess I'll try swapping the coil and then swapping the carbs. I'll keep you posted on what happens. Thanks guys.

 

check the carb boot?

 

Carb boot is good. Thanks man.

 

belt driven? check belt slippage

 

The belt seems fine. The motor is just poping, sputtering, and backfiring and wont rev up past 20 mph or half throttle.

 

is the plug carbon fouled? (kind of black dirty/gritty looking) or the exhaust smoke a darker color?

 

The plug is really carboned up, but i don't know how long he rode it trying to get it to clean out or something. If you screw in a new plug, it looks great after a short ride when you don't make it cough, but when you give it over half throttle or so it gets really black.