ATVs need a certain ratio of fuel and air to run well. When there is too much fuel in the mixture, you have what is called, “running rich.” It can be caused by a variety of issues with the fuel or air systems.
This post will be discussing ATVs with carburetors. If you have a fuel injected ATV running rich, you’ll predominantly want to look at the air filter and air box for dirt or blockages, your spark plug, and your ECU.
You should already be doing this, but we’ll cover it just in case you aren’t because it is the simplest fix.
Turn off the choke when your engine gets warm. If you don’t, you are giving your engine too much gas, and this is why your ATV engine runs rich.
Check your air filter first. It’s a likely culprit, and it’s also an easy fix.
Your air filter may be dirty. If it is, clean or change it.
Intake ducting or a snorkel that is obstructed or kinked can also restrict air flow. Straighten it or remove the obstruction.
Blended gas goes bad in only a month. Pure gas lasts three months. If you haven’t been using fuel stabilizer and or properly storing your gas, it’s probably old and will negatively impact the way your vehicle runs.
Drain the gas and add in fresh. Use fuel stabilizer whenever the gas may be sitting for a month, or every time you fill up.
If you re-jetted your carburetor and your ATV engine is running too rich, the carburetor may not have been tuned correctly. You’ll need to get back in there and re-jet it again.
Or maybe you didn’t re-jet your carburetor when you should have. It needs to be jetted to accommodate wear and tear, altitude changes, temperature changes, humidity levels, and engine modifications.
All carburetors have a fuel mix screw. Some also have an air control screw. You’ll want to tweak them until you get it to 14.7:1 or very close to it.
The needle clip also needs to be adjusted to account for such things as altitude and modifications. It controls the gas supply in the carburetor, and if it is set too low, you’ll have too much gas flow.
You will either need to remove the needle or remove the entire carburetor to be able to manipulate it. You’ll see 5 grooves in the needle. Adjust the clip so that it is in a higher groove to stop ATV running rich conditions.
The carburetor is more likely to block air flow and cause a lean condition, but it is possible for it to malfunction in ways that cause an ATV to run rich. The jet needle, bowl float, needle valve, or needle valve seat seal could be worn out or damaged and allowing excess gas flow.
Check the parts. They might look visibly worn or you can tinker with the individual parts and see how they impact the gas. Whichever part you determine is faulty will need to be replaced. Thankfully this is an easy and inexpensive repair.
We mentioned the choke above. It may be causing a problem all by itself if there is something wrong with it.
Plunger chokes don’t cause rich conditions, so if you have one, move on to another possibility.
The diaphragm inside your petcock can malfunction and restrict gas flow or allow too much gas to pass through. If you’re running rich, it is letting too much gas in.
You can look for gas in the vacuum line connected to the petcock or bypass the petcock and obstruct the vacuum line to see if it runs fine without it.
Your spark plug may be causing a rich condition if it is:
A bad coil, coil wire, coil cap, stator, pickup, or CDI can also cause your vehicle to run rich.
Check your spark plug for signs that it is the wrong one or bad. If that doesn’t explain it, use an inline spark test to find the culprit.
These valves need to be checked and probably adjusted regularly. This is usually every year or whenever your owner’s manual specifies.
When you’ve got an ATV running rich due to carburetor valves, you can adjust or replace them.
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