Unveiling the Mystery: Symptoms of Improper Air Gap on a Coil

Having the proper air gap between the ignition coil and flywheel is critical for an engine to start and run smoothly. An improper air gap can lead to major performance issues and even engine damage if left unaddressed. When the air gap is not set to the manufacturer’s specifications, the coil may fail to produce an adequate spark to ignite the fuel mixture. This can result in difficulties starting the engine or even complete failure to start.

There are several key symptoms that indicate an improper air gap on the coil. First, you may experience problems getting the engine started, with the engine turning over but failing to ignite. Once running, the engine may misfire or run unevenly if the spark from the coil is weak or inconsistent. Over time, a severely impaired spark can lead to reduced engine power along with increased fuel consumption since the fuel is not burning efficiently. Excess heat buildup and overheating of the coil and engine may also occur.

It’s important to address an improper air gap right away to avoid potentially damaging your engine. The specific air gap measurement varies by the make and model of your engine, so always refer to your owner’s manual for the manufacturer’s specifications. In most cases, the ideal gap is around 0.30mm or 0.012 inches. With some basic mechanical skills, the gap can be adjusted by loosening the coil mounting screws and moving the coil as needed. Otherwise, consulting a small engine mechanic is advised. Catching and fixing a suboptimal coil air gap quickly can save you from costly repairs down the road.

Why the Air Gap Matters

The air gap allows the ignition coil to produce a spark to ignite the fuel mixture in the cylinder. Without the proper gap distance, the coil may fail to generate an adequate spark. This can prevent the engine from starting or cause rough running.

More specifically, the ignition coil contains a magnetized core surrounded by two coils of wire. When electric current flows through the primary winding, it creates a magnetic field in the core. As the flywheel spins past the tip of the core, the rotating magnets pass the magnetic field back and forth over the core. This induces an alternating voltage in the secondary winding, which flows to the spark plug and creates the spark for ignition.

The air gap between the coil and flywheel allows for proper magnetic field fluctuation to generate this voltage. If the gap is too wide, the magnetic field will be too weak to induce enough voltage for a good spark. If the gap is too narrow, the flywheel magnets can actually hit the coil as they pass, shorting the system and potentially destroying the coil. The ideal air gap distance allows for maximum field strength and voltage generation.

Symptoms of an Improper Air Gap

There are several key indicators that your coil’s air gap needs adjustment:

  • Difficult Starting or No Start
    • Cause: Inadequate spark prevents fuel ignition. A weak magnetic field caused by too wide of a gap cannot generate enough voltage for a spark hot enough to ignite the fuel mixture.
    • Fix: Adjust air gap to manufacturer specifications to increase magnetic field strength and voltage generation for a good spark.
  • Uneven or Rough Running
    • Cause: Inconsistent spark leading to uneven combustion. If the gap is slightly too wide, the magnetic field strength will fluctuate, causing voltage generation to be irregular. This produces an uneven spark, resulting in rough combustion.
    • Fix: Adjust air gap to proper alignment so the magnetic field remains stable, generating a steady voltage for a consistent spark and smooth combustion.
  • Decreased Power and Performance
    • Cause: Weak spark reduces combustion efficiency. Too wide of an air gap causes a feeble magnetic field and voltage production. The consequent spark lacks the power to fully ignite the fuel mixture, decreasing combustion efficiency.
    • Fix: Adjust air gap to correct measurement to strengthen magnetic field and improve spark energy for complete fuel ignition and optimal power.
  • Increased Fuel Consumption
    • Cause: Fuel not burning efficiently due to weak spark. Due to inadequate ignition, unburned fuel gets exhausted from the engine. This requires more frequent refueling.
    • Fix: Adjust air gap to manufacturer’s specifications to produce a strong magnetic field and robust spark for efficient fuel combustion, reducing consumption.
  • Engine Overheating
    • Cause: Excess resistance causing coil overheating. If the air gap is too narrow, the flywheel magnets may contact the coil as they pass. This introduces resistance, causing damaging heat buildup in the coil windings.
    • Fix: Adjust air gap to eliminate contact and associated resistance. This will allow proper coil cooling and prevent overheating damage.
Difficult starting or no start– Air gap is too wide, causing weak magnetic field and inadequate voltage generation for proper spark

– Gap is so narrow that flywheel magnets contact coil, shorting out system

– Adjust gap to specifications to increase magnetic field strength

– Widen gap slightly to prevent coil contact

Uneven, rough running– Slightly wide gap leads to fluctuating magnetic field and irregular voltage generation

– Results in uneven, inconsistent spark

– Adjust gap precisely to ensure stable magnetic field and steady voltage generation for smooth spark
Decreased power, poor performance– Excessive gap width weakens magnetic field and limits voltage, reducing spark strength

– Weak spark cannot fully ignite fuel mixture, decreasing combustion efficiency

– Set gap to proper measurement to optimize magnetic field and spark strength for complete fuel ignition
Increased fuel consumption– Weak spark from wide gap causes incomplete fuel combustion

– Unburned fuel expelled results in reduced fuel economy

– Adjust gap to specs to produce strong spark for efficient fuel combustion and reduced consumption
Engine overheating– Extremely narrow gap causes flywheel magnets to contact coil

– Direct contact introduces resistance and heat buildup in coil windings

– Widen gap slightly to prevent contact between coil and flywheel

– Eliminates resistance and overheating

Diagnosing and Adjusting the Air Gap

If your engine exhibits any of the symptoms above, an improper air gap may be the culprit. Follow these steps to set the proper air gap:

  1. Consult manufacturer’s guidelines for the gap measurement. Your owner’s manual or engine specs will provide the ideal distance, typically 0.25-0.40mm.
  2. Use a feeler gauge to measure the existing gap between the coil and flywheel. Turn the engine over slowly and slide the gauge between them to check the current alignment.
  3. Loosen the coil mounting screws. There are usually two bolts holding the coil in place. Back them off a few turns so the coil can move.
  4. Move the coil as needed to achieve the correct gap. If the gap is too wide, slide the coil closer to the flywheel. If it’s too narrow, move it away.
  5. Tighten the screws and recheck the gap with the gauge. Turn the engine again and confirm the measurement remains correct before fully tightening.
  6. Start the engine to test performance. It should crank smoothly and run cleanly if the gap is now properly set.
  7. Let the engine fully warm up and recheck the gap. Heat expansion may alter the measurement slightly requiring more adjustment.

Consequences of an Improper Air Gap

It’s critical to address an improper air gap promptly before significant issues occur:

  • Difficult Starting or No Start – As explained above, too wide of a gap prevents adequate spark voltage for ignition. This leads to hard starting and potential failure to start as the spark becomes weaker over time.
  • Premature Wear – Uneven combustion and ignition stress from a poor coil gap can accelerate wear on engine components like the pistons, rings, cylinders, and bearings.
  • Increased Emissions – Incomplete fuel combustion results in more unburned hydrocarbons and other emissions expelled from the tailpipe. This impacts air quality and fails emission standards.
  • Reduced Fuel Economy – Wasted fuel from poor combustion requires more frequent fill-ups, increasing operating costs. Fuel inefficiency worsens over time as the spark deteriorates.
  • Carbon Buildup – Inconsistent ignition and combustion allows fuel residue to condense on internal components as carbon deposits. This can hinder engine operation.
  • Coil and Flywheel Damage – As noted above, if the gap closes entirely, direct contact between the coil and flywheel can destroy the coil winding insulation and demagnetize the flywheel. Replacement of both parts would be required.
  • Costly Repairs – If the issues above are left unchecked, they can lead to larger mechanical problems and engine failure over time. Major repairs or even a total rebuild may eventually be needed.


How can I tell if my coil’s air gap is set improperly?

Symptoms such as hard starting, rough running, reduced power, increased fuel use, and engine overheating can indicate an improper air gap. Using a feeler gauge to check the gap against manufacturer specifications can confirm if it needs adjustment.

What is the ideal air gap distance for most small engines?

The optimal air gap distance depends on the specific engine make and model. In general, gaps between 0.25-0.4mm (0.010-0.016 inches) are common. Always consult your owner’s manual.

How do I adjust the air gap if it’s misaligned?

Loosen the mounting screws securing the coil, use a feeler gauge to measure the gap, move the coil to achieve the correct distance, tighten screws, and recheck the measurement. Review the steps outlined in this article.

Can a small air gap cause problems?

Yes, if the gap is too narrow the flywheel magnets can contact the coil directly. This will damage the coil windings and flywheel magnets. The gap must be wide enough to prevent contact.

Is fixing an improper air gap difficult for a DIYer?

Adjusting the air gap does not require advanced mechanical skills. Using the appropriate feeler gauge and referring to the manufacturer specifications allows most homeowners to complete the job themselves.

How soon should I address an improper air gap?

It’s important to adjust the gap as soon as possible to avoid potential engine damage from a weak or irregular spark. The symptoms will only worsen if left uncorrected.

Does temperature affect the air gap measurement?

Yes, heat expansion from normal engine operation can alter the gap slightly. Always recheck the gap after the engine reaches full operating temperature.

Will fixing the gap resolve all the engine issues?

In many cases, yes. But other problems like worn parts or mechanical defects could also contribute. Fixing the gap should be the starting point before exploring other repairs.


Properly setting and maintaining the air gap between the ignition coil and flywheel is essential for optimal small engine performance and longevity. An improper gap can lead to difficult starting, rough running, power loss, increased fuel consumption, overheating, and potentially serious engine damage if left uncorrected. By identifying symptoms like hard starting and misfires early and adjusting the gap precisely to manufacturer specifications, you can restore a strong, consistent spark and proper combustion. With this simple maintenance and attention to the ignition system, your equipment will start easily time after time, maximize fuel efficiency, produce full power, and avoid unnecessary wear and repairs. Don’t underestimate the importance of the coil air gap, and be diligent about regularly checking and tuning this critical distance for reliable, long-lasting engine operation.

Leave a Comment