ATV Only Runs With Throttle (Top 5 causes)


Riding an ATV on difficult terrain and balancing the throttle to prevent it stalling can be quite the challenge.

The top 5 most common reasons an ATV only runs with 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 can do to fix it.

Idle Speed

Because ATV’s run on rough terrain, it’s understandable if things drop out of adjustment from time to time. Your idle speed adjustment assembly simply 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 effect the fuelling ratio, and should be eliminated as the possible cause before proceeding with fuel ratio adjustments.

Air Fuel Ratio

Your bike needs an exact ratio of air to fuel of 14.7:1, although the mix is set from 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 carburettor 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 carburettors will have 3 fuel circuits.

  1. Pilot – responsible for idling to 1/4 throttle
  2. Needle – responsible for 1/4 to 3/4
  3. Main – responsible for 3/4 to full

Adjusting the air fuel screw, feeds more or less gas through the Pilot circuit. A standard adjusting procedure looks like this –

Step 1

First warm the engine and examine how the throttle responds throughout the range.

Step 2

Turn engine off, find your base setting by turning the mix screw all the way home (clockwise) until it seats, counting the number of turns (about 2 is normal).

Return the screw to its original setting.

Step 3

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.

Step 4

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.

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 carburettor is likely the best approach, some carbs are are easier than others. In some cases it may be possible to remove just the bowl and remove the pilot jet.

If you do remove the carburettor, go ahead and replace the float needle and gaskets, they give trouble too, they aren’t difficult to fit or expensive to buy.

Bad Gas

Ethanol gas is a blended gas which is quite common now, trouble is it turns stale after just one month, regular gas after three.

Stale gas losses its punch, but worse than that. The gas turns to a sticky gummy sludge in the bowl of your carburettor 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 stabiliser will help you sidestep this kind of trouble. If your storing your bike for more than a month, mix some stabiliser into the gas tank and run the engine to get the mixed gas into the carb. You’ll have trouble-free starting next time you hit the button.

Petcock

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 looses its will to live, simply doesn’t open all the way, this causes a shortage of gas.

To eliminate this as a possible problem, by pass the fuel valve and go direct to the carburettor. 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.

Air Filter

A dirty air filter will generally cause a rich condition, symptoms include:

  • Stalling
  • Bogging
  • 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.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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