Hello all,
I rencently bought a 2000 Honda 400ex knowing that it had been sitting outside for awhile however, it did start but won't stay running. I have changed the battery and put a k&n air filter on it , drained out old gas and put in new, also changed the spark plug. any ideas on what NEXT? It won't stay RUNNING.

 

How long does it run for? 2 seconds...one minute? Choke on or off?

 

it will stay running 10 sec a few times it stayed for a couple of min. when the choke lever is up is it on or off?

 

yeah I had a similar problem with mine...it sounds like a carb problem...maybe clogged jet...might want to check from there
Hold on.
when your choke lever is in the UP position, your choke is ON. lever DOWN, and its OFF.
with that said, if you crank it and it stays running for ten seconds... have you tried moving that lever down before it dies? cant be that easy though, right?

I'd start with the carb.

 

Sounds like a classic case of a clogged slow jet. Happens when the quad sits for a long time and the fuel dries up and leave a scum behind in the carb. Clean the carb, and pay particular attention to the slow jet.

I bet it backfires on deceleration as well (???).

 

It could also be a too low idle speed. If you blip the throttle to keep it running...will it keep running? It might take a half minute or so, of slightly increased rpms , to warm it enough to keep running.

 

well I cleaned out the carb. found all the jets to be cleam from anything (carb clearner blew through all holes) emptied tank and replace it with new gas still same problem it starts then stalls. any ideas????

 

Were the rubber tubes on the carb clean thru? If they are plugged, they will stop the gas from flowing, because they are vents.
Did you remove (unscrew the jets from the carb body), to clean them out? If you didnt remove them, chances are they didnt get cleaned. The pilot jet has an extremly small center hole, that must be clean. The main jet wont cause your symptoms, so just redo the pilot jet. If you cant see light coming thru the center of the jet, just replace it with a new one, and if you buy a new one, go for the number 40. Thats one size bigger than stock, and really helps to start the quad when cold.
Also look at the gas tank vent, and make sure its venting.

 

i will check the rubber tubes, the main gas lin is clear. also the gas is flowing due to gas coming out of the overflow when i try and start it. The pilot jet is completely clear of any obstructions not only can i see light through it carb cleaner sprays through ever hole. ( and yes the jet was completely removed) The gas tank vent? if you refering to the hose on the gas cap it is also clear. please!! anything we might be missing?

 

Check for a leak in the air intake boot??? Sounds like you are lean.

Remove the air filter and hold your hand over the opening. Crank the engine a little, until you feel a vacuum. The vacuum should stay a while after you stop cranking.