Hi everyone. 2005 Honda Rancher 400, completely stock. 234 total hours, 1060 total miles. The ATV starts up right away, idles fine, and runs great for about 5 minutes. After warming up, it starts running rough, backfiring and will stall. It gets harder to start and won't stay running, especially if you give it any throttle. If I let it cool down for a few minutes, it can usually get it started enough to make it back to the house, but it still runs rough.

- I've drained the tank - fuel looked good. Refilled with fresh fuel.
- I've checked the fuel cap and it is venting properly.
- I've gone through the carburetor and it looks good. Cleaned all the jets while I had it out.
- Removed the spark plug and it looks good. Couldn't check compression because I don't have a small enough adapter for my gauge, but turned engine over and it blows a good amount of air forcefully out of the spark plug hole.

Any ideas what it could be or what else I can check? Thanks in advance.

 

Are you sure the plug "looks good" the symptoms are those of an engine running rich and the plug would be black and sooty. This would indicate a stuck choke or blocked air filter. Alternative faults are valves too tight or electrical problems, but could be any ignition component "breaking down" when hot.

So check the air filter and the choke plunger for sticking, if it isn't that, fit a new plug, if that doesn't cure it adjust the valves. It may be worth trying a new coil, but going further could be best left to a dealer. For example, they can try a new CDI and put it back on the shelf if that isn't the problem.

 

Here's a pic of the plug.

 

I posted a picture of the spark plug, but 9 hours and I'm still waiting for a moderator to approve the photo. I'll try my best to describe the picture. The spark plug is dry, with kind of a brown tint to the tip. Definitely not wet or looking like it's running rich.

 

Did you check the diaphragm when you had the carb apart, as a holed diaphragm or loose needle can throw up odd faults? I would still check valve clearances and fit a new plug. Also check fuel flow from tank to carb. Beyond that and fault finding is going to be difficult.

 

I took your advice and brought the atv to the dealer. I'll post back when I find out what the cause was. Thanks.

 

Sounds like compression to me...runs great when everything is cold (tight), but wont run when it warms up (loose)...I don't think a machine warming up would have anything to do with fuel delivery (other than choke), but one never knows with these mysterious beasts!!!


Other possibility is electrical...but if it had a good strong spark after warm-up I'd rule that out as well...

 

Have you tried new spark plug? Sounds lack of fuel to me like due to clogging.

 

Voltage regulator can cause this as it warms up...check charging system