Why Your ATV is Running Rich

When an engine is “running rich”, it means there's a problem with its combustion process that's making it get too much fuel and not enough air. Running rich (or vice-versa, "running lean") can be caused by any number of fuel and air system issues.

Our resident ATV expert John Talley goes live on our YouTube channel every Friday at 3pm Eastern, and in this session, he answers a couple of questions from our viewers about a Honda ATV and a Polaris ATV running rich. Here's what John had to say about a Honda TRX350 and a Polaris Sportsman 850 running rich.

Honda ATV Running Rich

I rebuilt the top end of my Honda TRX 350 and it’s running rich, to the point of a new spark plug turning black after only running it a few minutes. Any advice?

John Talley: Hmm, rebuilding your top end shouldn’t change your air/fuel mixture coming into it, so I’m a little perplexed by that. I would want to go back and just verify that your timing is correct, so if you would, take a peek at that. … Other than that, something must’ve changed in the carburetor for it to be running so rich. Check your timing first and then go back in your carburetor to see if there’s something going on in there. Maybe an air/fuel setting on the side of it. Or maybe that screw got turned not by you but maybe somebody else when they saw the carburetor sitting on the side of the unit.

Watch the video above for tips on whether to clean, rebuild or replace a carburetor.

Polaris ATV Running Rich

I have an 05 Polaris Sportsman 500 and it’s running very rich. The enricher doesn’t work, and I have an erratic idle and oil in the airbox. Would it be a choke adjust or a jet adjust?

John Talley: Well, you gave me one little thing that shouldn’t be going on. It’s spitting oil up in the airbox, and that indicates to me that you’re getting a lot of “blow-by.” Blow-by is when your rings and/or your piston within the cylinder walls become worn and you’re letting too much compression escape past the cylinder, and it’s actually building up pressure in the engine cases and that’s gotta go out through the vent. And the vent goes into the airbox, and if there’s too much pressure in there it’s not just going to release the pressure of the gases in there, it’s going to start spitting oil. But before you jump to conclusions, make sure that your oil level is correct to begin with, because I know with certain Polaris’s it’s easy to overfill them and they start spitting back in there. If you’re sure that your oil is correct, you may want to do a quick compression test to see if it falls into range …

Watch the video above to see how to do a compression test on a Polaris Sportsman ATV.

Want to see more live repair questions answered? See more motorcycle and ATV repair questions answered here.