Riding an ATV on difficult terrain and balancing the throttle to prevent it from stalling can be quite the challenge.
The top 5 most common reasons an ATV only runs with the throttle include:
1 Idle speed needs adjusting
2 Air fuel ratio needs adjusting
3 Dirty pilot jet
4 Petcock faulty
4 Air filter dirty
5 Bad gas
In this post, you’ll learn about all the most common reasons your ATV won’t idle without throttle and what you need to do to fix it.
1 Idle Speed
Because ATVs run on rough terrain, it’s understandable if things drop out of adjustment from time to time. Your idle speed adjustment assembly holds the throttle open slightly at idle, which prevents the engine from stalling.
Try adjusting the idle screw (if applicable) or google your particular model for the location of your ATVs adjuster.
The idle adjuster doesn’t affect the fuelling ratio but should be eliminated as the possible cause before proceeding with any fuel ratio adjustments.
2 Air Fuel Ratio
Your bike needs an exact ratio of air to the fuel of 14.7:1. Although the mix is set from the factory, it doesn’t remain set for life. Wear and tear, adding mods like big air filters or fast flow exhausts will upset this mission-critical ratio.
Luckily your carburetor has a fuel mix screw. After every service, a technician will check how the bike is running and adjust the mix if necessary. Adjusting your mix is a little technical but not brain surgery. Most ATV carburetors will have three fuel circuits.
- Pilot – responsible for idling to 1/4 throttle
- Needle – responsible for 1/4 to 3/4 throttle
- Main – responsible for 3/4 to full throttle
Adjusting the air-fuel screw feeds more or less gas through the Pilot circuit. A standard adjusting procedure looks like this –
- First, warm the engine and examine how the throttle responds throughout the range.
- Turn the engine off, find your base setting by turning the pilot screw (mix screw) all the way home (clockwise) until it seats, counting the number of turns (about two is normal).
- Return the screw to its original setting.
- Start the engine and have a helper balance the throttle while you turn the mix screw anti-clockwise until the engine runs by itself.
- Keep turning until it stumbles and stalls.
- Restart, idle the engine and turn the screw (clockwise this time) until it stumbles and stalls.
- Within that window of both stalls, set the screw where the revs are at their highest.
- Now reduce the idle screw (not air-fuel mix screw) to where the engine idle is smooth, anywhere from 1000- 1500 rpm.
3 Dirty Jet
Your Pilot Jet is responsible for supplying the gas at idle. Any blockage here, and you’ll have difficulty idling. Old gas blocking up the pilot jet is a common problem, and the only fix is to remove the jet and clean it. Removing the carburetor is likely the best approach.
Gumming of the carburetor, caused by stale gas is a real problem suffered by all small engines.
Some carbs are easier than others. In some cases, it may be possible to remove just the bowl and remove the pilot jet.
4 Bad Gas
Ethanol gas is a blended gas that is quite common now. Trouble is under certain conditions gas begins to turn stale after just one month, regular gas after three. Stale gas loses its punch, but worse than that. The gas turns to a sticky, gummy sludge in the bowl of your carburetor, and that’s where your pilot and main jet live.
When this crap gets into the tiny orifices of the jet and emulsion tube, your engine either won’t run or just won’t run right. Using a fuel stabilizer will help you sidestep this kind of trouble.
If you’re storing your bike for more than a month, mix some stabilizer into the gas tank and run the engine to get the mixed gas into the carb.
You’ll be trouble-free starting the next time you hit the button.
Faulty vacuum-actuated fuel valves have been known to cause fuel flow problems. The valves employ a simple diaphragm and engine vacuum hose to open the valve and allow fuel to flow. The petcock diaphragm gets old and hard and loses its will to live. It simply doesn’t open all the way, which causes a shortage of gas.
To eliminate this as a possible problem, bypass the fuel valve and go direct to the carburetor. If this solves your issue, replace it with an OEM or a standard valve and seal off the vacuum hose.
But you’ll need to remember to turn your gas off.
Other Possible Causes
A dirty air filter will generally cause a rich condition, symptoms include:
- Black smoke
- Black plug
Checking your air filter is an easy job, and most won’t require any tools. Try removing your air filter, run the engine and see if that helps.
If it does, try cleaning your air filter and using the recommended air filter oil.
If, after reinstalling the filter, the problem persists, your bike is running rich at idle and needs to be leaned out.
You may find the following posts useful:
Can I ride ATV with a choke on?
Can I use WD40 on my air filter?
- About the Author
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John Cunningham is a technical writer here at ATVfixed.com. He’s a Red Seal Qualified Service Technician with over twenty-five years experience. He’s worked on all types of mechanical equipment, from cars and trucks to ATVs and Dirt bikes.