Went riding with 2 friends with Z-400's and they both came home on a rope.

Bike #1 : Was bought near halloween' 02. It is riden by a girl that rides pretty conservativly and has only been riden 4-5 times. Bike will run about 10 minutes. Then it will begin to backfire out the exhaust and quit. Doesn't matter if your going 6 or 60 mph. You can attempt to start it but nuthing will happen, engine turnes over fine, Just wont run. Has spark and the plug looks ok....Not fouled. Also has gas in the carb, checked by loosening the drain bolt. Replaced plug anyway and it still dies. Drained the gas tank and refilled with gas from my can, Same deal. After it sits 20-30 minutes it will start again and run another 10 minutes. Air filter cleaned and oiled prior to ride...It's still clean.

Bike #2: Bought in the middle of November ' 02. Has been riden by a pretty agressive rider, Riden every weekend. Owner is **** about maintenance and keeps the filter clean and the oil changed. Bike "Shoots ducks" (pop, pop, rattatat, pop) at all engine speeds. Changed plug, The old plug was very black, Choke was not left on. No difference. Drained carb and gas tank and used the rest of my gas in my can. Has an aftermarket gas filter "inline". Got so bad that we towed both bikes back to camp and came home.

My bike ran fine all day on the same gas as the two 400's as did a 400EX so I think we can rule out water in the gas or bad gas. Any ideas?

 

The first thing I would do is take some gas out and look closely at it, matter of fact, I would pull the carbs off and look in the bowl. It sounds like to me you got something in your gas, like sugar.

 

Two of my friends bought kfx400s. The one wouldn't start. We pulled the gas line and there was no flow. I loosened the gas cap and it started to flow. He told me he had some problems starting it before...this was like the first real ride we were going to do. The quad can't get enough gas flow.

I would pull the gas cap and see if the vent can be enlarged. If the other end of the vent hose is connected to anything I would let it hang free.

The other quad had no problems.

 

I'm not thinking it's somethin with the gas. The bikes are from 2 different towns about 30 miles apart, 2 different gas cans ect. We dumped the gas we drained from them into my truck and drove home just fine. They still acted up with gas from my can and my bike ran fine on the same batch. Keep the ideas comming tho.

 

Well when you find out, please keep us posted.

 

The vent makes sense on bike #1. Forming a vacuum and not allowing gas to flow.......Hmmmmmm!

 

Aren't both bikes still on warranty?? Take them back and make it the dealers problem!! Could bike #2 be the reverse rev limiter getting stuck on?? This use to happen with Warriors back when, and the symptoms sound similar (bike cuts out and runs like it is way rich).

 

The fuel valve is activated by vacuum in the ON position so it won't flow unless motor is running, In the prime position it should flow even when motor is stopped. Check for a plugged or kinked hose etc. Especially if the carb has been off to rejet it. Also I believe the choke cable could be out of adjustment causing to choke to not fully stay open, happened to a friends Z400 after they rejetted it.

Let us know what fixes it.

KFXBob

 

They are still under warranty. The truth is that I (we) don't trust the dealer where they bought the bikes. Guess they could take them to another dealer, but It doesn't seem right since they didn't buy the bikes there.

As far as jetting and such the bikes have never been touched.

We did try the fuel valve in the various positions and nuthing changed. I'll let ya'll know what we find out.

 

I agree with KFXbob (no not just cuz i have a KFX also LOL) check out the gas valve. when it starts to crap out turn the valve to the prime position and see if it runs again.
Just because you saw gas flow from the carb drain doesn't mean there was enough to run the engine. The bowl has to be at least half full for the engine to run properly and half the bowl looks like alot of fuel when draining from the drain hose. Both quads - same problem most likely.

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I posted the same time as you and didn't see your reply.
Pull the gas hose off the carb (from the tank) and see if fuel flows in the "prime" position.

The dealer probably won't ride the thing for 10 mins to diagnose the problem, I would try the easy stuf before taking it there. Personnally I don't trust the dealers.