I have a 1999 LFT500F 4x4. When the engine is first started it runs fine for about 4 seconds, then it starts running rough and will backfire at full throttle. It will idle fine, but the problem starts when I try to give it gas, the thing just "spits and sputters". The spark plug is very black and I have changed plugs 4 times and each time the engine will "blacken" the new plug in just a few seconds. I have had the tank cleaned by a local repair shop and they said they cleaned the carb but I'm not sure. The motor will run rough when I give it gas, but if I give it a small amount of throttle and hold it for several seconds, the motor will suddenly stop running rough and rough perfectly. The motor will not do this every time I start it, but it is pretty frequent. I have noticed that when the motor is running rough under throttle, I can hit the starter button and for just a second the motor will perk up and stop spitting and sputtering, but immediately starts running rough again. The engine will do this whether the motor is hot or cold. Any suggestions are greatly appreciated. [img]i/expressions/face-icon-small-confused.gif[/img]

 

Rhallo, I'm just a backyard mechanic, not very good either. But.....here are some obvious suggestions. It sounds like you are running rich... too much fuel or not enough air.

These are the simple things I'd check

1) Choke, is your choke on? Choke lever should be next to your left hand grip. Towards the front of the quad is off, towards the back is on.

2) Fuel valve, on your fuel line, there is a fuel lever. The choices are ON, reserve, or prime. Make sure yours is turned on to the "ON" position.

3) Air filter, Is your air filter clean? Is anything blocking your air flow? If so, this will cause you to run rich.

4) Idle screw. There is a phillips head screw on the right side of the motor. It connects to a black plastic tube that runs to the carburator. This adjusts your idle. Turn on the quad on and slowly turn this screw clockwise or counter-clockwise. Listen to the engine for signs of it running smoother as you do it.

If these don't work post back, you could have other issues........Timing, exaust blockage, carb problem, valves out of adjustment. However, it sounds like you are describing a situation of too much fuel or too little air going to the combustion chamber. Good Luck!

 

Oh....one more thing..NEVER hit your start button when you are already running. Bad, bad, bad. [img]i/expressions/face-icon-small-wink.gif[/img]

 

Thanks, bsb for the reply. I should have stated that the air filter is new and the when I had the carb and tank cleaned the shop said they checked the choke also. I initially thought the choke was sticking.
I have never messed with the fuel valve so I guess it's okay. The repair shop told they did have a bike like mine come in with a messed up fuel valve, but I don't remember what symptoms they said that bike had. I do remember, though that they said a new valve was expensive, but everything for the bike is anyway.
I am familiar with the idle adjustment screw, and have tinkered with it before. I have the idle set at about the same speed as the factory (tuning by ear, of course). This probably doesn't have anything to do with the problem, but ever since I bought the bike new, the exhaust pipe will get glowing red from the head until about 8 inches past. The glowing exhaust pipe has been going for 5 years now, but the rough running problem has just surfaced in the past few months. I also had the repair shop check the valves and they said the valve adjustment was okay. Needless to say, the local shop has tried to fix my problem three times with no luck. I was trying to resolve the problem before taking the bike to the dealer and spending who knows how much they will charge. I never thought about the blocked exhaust before.
I guess I could take the muffler off and try, huh? I looked and found two vent tubes coming off the top of the carb. I took them off and blew air through them with no problems. I'm beginning to think I have an electrical problem (something with the coil?). Shop said it could be the on/off switch, but I don't think so. I thought about the engine timing, but I would think the engine would run rough 100% of the time with a timing problem.
Thanks again for the input.

 

Since it started running rough, has the exhaust still been glowing? Doesn't a glowing exhaust indicate a lean condition? I'm just guessing but if you were running lean for 5 years and you are now running rich and out of whack, you may have done some internal damage. Hope I'm wrong. Keep playing with it. Perhaps it is something stupid.

Would be nice if someone else would chime in here and help this guy.