ATV Idles Up and Down (This will fix it!)


An erratic idling atv engine is so annoying, but could point to a far more serious problem.

The 4 most likely causes of an ATV idling up and down include:

  1. Vacuum leak
  2. Bad gas
  3. Fuel restriction
  4. Tight Valves

In this post you’ll learn why your ATV engine idles erratically and what you can do to fix it.

1 Vacuum Leak

Air/fuel mix is drawn in through the carburettor under a vacuum which is created by the piston as it moves down the bore.

The air and fuel are mixed by the carburettor very precisely. The afr (Air Fuel Ratio) is 14.7:1 for gas engines. Any interruption of either gas flow or air flow causes the ratio to change and that will effect how the engine performs.

Symptoms Of Erratic Idling

The classic symptoms of a vacuum leak include an erratic idle, engine does its own thing revving up and down but once on the move, the problem isn’t noticeable.

Your spark plug may be a grey whitish colour and the engine may misfire.

What is a vacuum leak? Your carburettor is a clever bit of kit. It takes air in through the intake where it passes over the venturi speeding up the air flow in the process and creating a vacuum that lifts gas from the fuel bowl.

The carb is designed to supply about 1 part gas for every 14.7 parts air, adjuster(s) on the carb allow for fine tuning the ratio.

So whats the problem? Air that doesn’t pass through the venturi, isn’t calculated (un-metered) and that causes a lean condition inside the engine.

A lean condition is a situation where there’s too much air in the mix or another way to look at it – not enough gas.

But why is the idling erratic? Erratic idling is likely caused by the erratic nature of the air leak.

Sources Of A Vacuum Leak

The most common sources of a vacuum leak include:

  • Loose carburettor
  • Damaged intake gaskets
  • Cracked intake manifold
  • Adjusting mix screw seal worn
  • Faulty carburettor
  • Damaged/misadjusted valves
  • Headgasket failure

Locating Vacuum Leak

Check for obvious signs of damaged gaskets or loose carburettor bolts. A can of wd40 helps seal off vacuum leaks temporally.

With the engine idling, systematically spray around the carburettor to engine union and around carb adjusting screw. When the surging stops, you found the location of the leak.

2 Old Gas

Old gas causes lots of problems in small engines. Regular gas starts to go stale after just one month and old gas just doesn’t have any Omph

Erratic idling and poor running are symptoms of old gas. This is an easy problem to solve. Just drain the old gas out and fill with fresh gas.

A gas stabiliser additive will help keep your gas fresh for up-to two years and is advisable for all small engines, especially those that lay up.

3 Restricted Gas Flow

A restricted gas flow will cause lean erratic running, common causes of gas restriction include:

  • Blocked idle (Pilot) jet
  • Float out of adjustment
  • Faulty float needle
  • Restricted gas filter
  • Restricted in tank

4 Tight Valves

Your engine valves are responsible for allowing the gas mix in and the spent gases out. The valves are opened by rockers which are driven by the crankshaft. The valve tip to rocker gap is known as lash and this gap is important.

If your intake valve lash is too tight, it may not seat. This will allow extra air into the cylinder, lean out the mix and cause erratic idling.

A tight valve will cause your engine to run hot and as the valve isn’t closing completely, it can’t transfer heat to the cylinder head. In addition a tight valve is liable to float reducing power at the top end.

Checking valve lash is preventative maintenance and is often neglected. It’s a job you can easily take care of yourself, just some basic tools and an inexpensive tool called a feeler gauge.

A tight valve if not adjusted will burn the valve and seat.

Engine Damage

Erratic idling in most cases is caused by a vacuum leak and that means your engine is running lean. A lean engine runs hot and that can spell trouble, blown head-gaskets, seized bearings, seized piston etc.

If your valves are tight that can result damaged valves, damaged valve seats, valve guides, cylinder-head damage etc.

John Cunningham

John Cunningham is an certified mechanic and writer on 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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