Generator Troubleshoot

Why is My Generator Misfiring?

Is your generator sneezing, popping, and hesitating when on load or backfiring? If yes, it’s misfiring, a technical term used to describe when there’s incomplete or no combustion in the engine’s combustion chamber. Why is your generator misfiring?

It could be that the engine is running too lean or too rich. It could also mean you’re using a bad ignition spark, or the engine is experiencing early or later ignition. Another possible cause is low compression or air-fuel mixture leak.

All these are possible reasons why your generator is misfiring. Understanding how to diagnose and troubleshoot these problems can save you time and the headache of not knowing what’s going on. That’s why you might want to read more below.

Why is Your Generator Engine Misfiring? Possible Causes Explained

If your generator engine is misfiring, one question that might be running in your head is, why is this happening? Since the possible causes are many, answering this question isn’t that easy. It might even be best to get a professional to check out the unit. But if you’d like to troubleshoot it before calling an expert, here are the potential causes of generator misfire:

1.Ignition System Problems

Almost all the time, when generator owners hear a misfire, they assume the spark plugs are worn out. They don’t know that the misfiring could be caused by something else in the generator ignition system. When the ignition timing is off, there will be incomplete or zero combustion, the misfiring. Here are the three things that can happen to cause a miss-timed explosion:

a) Dirty or Worn-Out Spark Plug

Your generator spark plug is a crucial piece in firing the generator. It’s responsible for creating sparks to ignite the compressed air-fuel mixture in the combustion chamber.

If it’s dirty or worn out, it might produce the sparks late or not make at all. If that were to happen, the generator engine would misfire, sputter before it stalls. It might even become hydro-locked and cause multiple problems.

How do you troubleshoot dirty or worn-out spark plugs? You have to remove it from the generator. Start by removing the top rubber boot first and using a spark plug wrench tool to detach it from the generator engine.

Place it on a clean rag and check if the terminals are covered with soot (carbon deposits.) If it’s, there is a possibility it’s producing sparks but with reduced strength. They are not sufficient for complete combustion.

You can clean it or replace it with a new one, something I recommend since these things are relatively inexpensive. If you decide to clean it, use a clean rag to clean it. Make sure you remove the carbon deposits with a soft wire brush.

While cleaning it, use a gap gauge or feeler gauge and make sure the nodes are gapped properly. Check the generator manual for the instructions on how to adjust your generator’s spark plug gap.

You can test the spark plug before reinstalling it back, checking the strength of the sparks it produced. Start by reconnecting the boot, place its thread to the engine block while holding the rubber to avoid electric shock, and pull the generator recoil slightly. If you’re afraid of sparks, use a spark plug tester.

Does it produce visible sparks? If yes, the spark plug is okay, and you can reattach it to the generator. Remember to remove the top boot, reinstall it and tighten it before reattaching it.

b) Ignition Coil Going Bad

If the spark plug is clean, gapped, and produces strong sparks, but the generator is still misfiring, there could be a possibility that the ignition coil is going bad.

The best way to troubleshoot this problem is to look for black smoke when the engine idles. If you also smell gas from the smoke or experience exhaust backfiring, the ignition coil is faulty.

Unfortunately, there are no repairs for the ignition coil. It’d be best to find a replacement as soon as possible for your generator model. The best place to get it is to contact your generator’s manufacturer.

c) Combustion-Intake Timing Issue

When the air-fuel mixture leaves the carburetor, they are pulled into the combustion chamber by a vacuum effect created during the intake stroke. During the compression stroke, the fuel valve closes to prevent more air-fuel mix from entering the chamber.

If the closing and opening timing are wrong, the spark plug will produce sparks that ignite an uncompressed air-fuel mixture. That is when the generator experiences carburetor backfiring as the air-fuel entering the chamber ignites be entry.

When enough fuel finally enters the fuel chamber and gets compressed, the spark plug mistimes the spark, thus causing late combustion.

The likely cause of this problem is a sheared flywheel key and faulty spark plug or ignition coil (discussed above.) Anyone with basic DIY engine troubleshooting skills can replace the sheared flywheel key, but I recommend you leave the work to a professional.

2. Engine Running Too Lean or Too Rich

The air-fuel mixture balancing is crucial for complete combustion. If your generator engine receives an imbalanced air-fuel mix, there is a chance it will misfire or backfire.

What Causes Generator to Run Too Lean?

a) Carburetor misconfigurations

One of the few adjustments you can make in a carburetor is the fuel screw tightening or loosening. The screw meter the amount of fuel leaving the carburetor.

When you screw it in or turn it clockwise, you’re leaning the air-fuel mixture. When you loosen it out or turn it counterclockwise, vice versa happens. You want to make sure you tighten it in or all the way out as you listen for the engine.

Normally, it is 2-2.5 clockwise turns from the lowest point. And since this can be tricky for an inexperienced user, I recommend leaving this part to a professional.

b) Plugged fuel jet

The fuel jet allows controlled fuel to go out of the carburetor, mixed with air, and pulled to the combustion chamber. A carburetor has a main jet and a pilot fuel jet. The pilot has a very tiny passageway that can easily be blocked by small debris.

If these two jets were plugged by debris or dirt, the amount of fuel getting mixed with air on its way to the combustion chamber would be less than normal, a condition that can cause the generator to run lean.

Disassemble the carburetor and hold the jets under a bright light to see if debris or something is blocking them. If there is, clean using carburetor cleaner and compressed air. You can also get a carburetor cleaning set with thin wire strands for dislodging the debris.

C). Low Float Height

The float controls the amount of fuel that enters the carburetor. Its height opens and closes the float needle valve.

When it’s low than normal, the amount of fuel in the carburetor is also low. That means, when it’s leaving to the engine, there is more air than fuel in the air-fuel mixture, another cause of generator misfiring.

Disassemble the generator carburetor, open the float bowl and hold it horizontally with the float facing you. Gently vibrate the float with your finger to make it rest on it.

You want to make sure it’s level with the float bowl line. If not, you will have to adjust the small metal tab holding it to push it in or out.

D). Stuck Chock Valve

If by any chance you left some fuel in the carburetor during the last storage, there could be varnish and gunk that’s making the valve chock get stuck on the off position. It’s like you’re running the generator with the choke in the off position.

Open the air filter cover, remove the cleaner cartridge or foam, and the air filter support to gain full access to the choke valve. Try opening and closing it with the choke lever while watching its movement.

What do you see? It has to open on the open position. If it’s not, it’s dirty and stuck. Get some carburetor cleaner and spray on it. Wait a couple of minutes for the varnish to loosen up before spraying some more carburetor cleaner to rinse it out.

Also, read my article on ‘how to clean a carburetor‘ and ‘how to store generator properly‘ if the problem was caused by you leaving fuel in the generator last time you store it.

What Causes Generator to Run Too Rich?

1. Dirty, Clogged Air Filter

The air filter cleans the air entering the carburetor before getting mixed with the fuel. With time, the air cleaner does get clogged with dirt and debris filtered out of the air.

If that were to happen, the amount of air would reduce, thus not meeting the desired ratio, causing the engine to run too rich, thus causing the misfiring.

Open the air filter cover and check the condition of the cartridge or the form. If it’s too dirty, you can decide to clean it with a mild dishwasher and running water in a sink. If it’s beyond cleaning, replace it with a new cartridge.

2. Carburetor Misconfiguration

The same fuel jet adjustments apply here. If you release the fuel jet more out, the carburetor will flood the engine with more fuel causing imbalanced air-fuel mixtures resulting in misfiring and backfiring.

You have to make sure you adjust the fuel jet screw accordingly before ruling out that this is the cause of the problem. It can be challenging for an inexperienced person, and that’s why I always recommend getting an expert to help out.

However, you want to open it out till the idle speed starts to hang and tighten it in until the speed sounds normal. Normally, it’s 2-2.5 turns from the speed-hanging point of adjustment.

3. High Float Height

Again, when the float is out of position, the amount of fuel in the carburetor will increase or reduce. When the float height is high, it means the carburetor will accept more fuel than normal before triggering the float valve to close.

If that were to happen and the fuel jet is at the wider open position, the air-fuel mixture will be out of balance with more fuel entering the engine.

You want to do the same thing you did before to ensure the float is sitting right. If it’s inside the float bowl line, loosen the metal tab to release it out slightly until it’s parallel with the bowl line.

4. Stuck Chock Valve

The choke valve can get stuck open or closed. Either way, the generator could run on an imbalanced air-fuel mixture, a state potential of causing misfiring. In this case, the Valve is stuck closed. Apply the same troubleshooting tips I’ve discussed above.

5. Leaky Gasket

Another problem that could be allowing more air in the combustion chamber, causing an imbalanced air-fuel mixture, is a blown-out gasket. The gasket lies between the combustion chamber and the engine head.

Mostly, this thing blows internally, causing the engine to burn oil. However, if it were to blow to the outside, it could create a leak that allows air to pass through the engine or lose compression, two possible causes of misfiring.

Even though you can check for a fuel leak around the gasket, the best way to troubleshoot this requires opening the engine head to access the gasket. If you’re not confident of doing this, leave it to an expert.

You want to make sure there is no crack or gap. If there is, even the slightest, consider replacing the gasket.

Low Compression

Incomplete combustion could also occur when the compression of the air-fuel mixture doesn’t reach the ideal compactness. The compression raises the mixture temperature before it’s ignited to create an explosion required to move the crankshaft. These are the possible cause of these problems:

Blown Gasket

As I mentioned above, the gasket sitting between the engine valve head and the combustion chamber can be blown out by the high temperature and pressure conditions around it. It can be blown internally or to the outside.

Either way, there will be a compression leak when the compression stroke tries to compact the air-fuel mixture for ignition. If that were to happen, the fuel wouldn’t burn or explode as it’s supposed to, causing incomplete fuel combustion.

As suggested above, you’ve to take the engine top apart to access the gasket. You want to check for cracks and gaps. If there is any, even the tiniest, consider replacing the gasket.

Low-Quality Fuel

The air-fuel mixture entering the combustion chamber has to be in vapor form and without any impurities, including water or oil

If by any chance the fuel was to get mixed with water or there is an oil leak in the combustion chamber, the air-fuel quality can reduce, causing low compression and incomplete combustion.

You want to make sure the fuel leaving the fuel tank is clean and without water. Pour some fuel in a jar and check for water. You have to drain the fuel tank content completely and refuel it with clean, fresh gasoline if there’s water.

While at it, make sure you’re not operating the generator on stale gas. Stale gas has a different, dirty color and smells bad; you cannot miss it. If you’re running on stale gas, there could also be incomplete combustion and loss of compression.

Can Generator Misfiring Damage the Engine?

Well! Engine misfiring is a sign of a problem that needs to be addressed as soon as possible. The aftermath of misfiring can cause catastrophic damage to the engine interior. As you can tell from the signs I mentioned early, misfiring can cause backfiring at the carburetor and exhaust, blue or black smoke, and more. Those are the things you want to avoid by troubleshooting and fixing a misfiring cause as soon as possible.

How to Prevent Generator Misfiring?

  • Clean or change the spark plugs as often as possible. They don’t get dirty that easy unless there is a problem inside the combustion chamber.
  • Inspect the ignition coil early when you start experiencing engine starting or running problems like stalling.
  • Get a professional to adjust the carburetor properly or use this ‘carburetor adjustment‘ guide always to keep it optimally adjusted.
  • Clean the carburetor after a while and use clean fuel to avoid plugged fuel jets. Avoid storing your generator with fuel.
  • Keep an eye on the condition of the engine and inspect the generator smoke. If you start to see white smoke, find the cause as soon as possible as the head gasket could be blown.

About the author

Sharif Gen

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