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2 Stroke Engine Stalls Under Load: Why & What to Do About it


How to fix a two-stroke engine that stalls under load.

Two-stroke engines are a type of internal combustion engine. Like four-stroke engines, they operate using a series of mechanical movements to create pressure that causes the engine to run. If there is an issue with one of these movements, your engine can stall.

If your two-stroke engine is stalling when you apply the throttle, likely causes include filters, gaskets, spark plugs, and vents. If all of these systems check out okay, you likely have an issue with the carburetor which will require cleaning or rebuilding.

The persistent stalling of your two-stroke engine can be caused by a variety of things. In this article, we will discuss why your engine is stalling and what you can do about it. We will also briefly touch on the way two-stroke engines work to better understand our issue.

Two Stroke Engine Basics

A two-stroke engine is an internal combustion engine. That means that the main source of movement in the engine stems from a small explosion that happens inside the cylinder of the engine.

Let’s look at the basic steps of how a two-stroke engine works, so we can better understand why it might be stalling.

Compression Stroke

An air/fuel mixture enters the main chamber of the engine through the fuel inlet port. The piston moves upward and compresses this mixture into the combustion chamber. The spark plug, which is in the cylinder head on top of the combustion chamber, ignites the compressed fuel, causing a small explosion.

Power Stroke

The small explosion puts pressure on the piston, which forces it back down into the chamber. The excess heat and fumes from the reaction are emitted from the exhaust port.

As you can see, the intake and export of air and fuel are crucial to the smooth running of a two stroke engine. If there are issues with either of these routes or with the fuel or spark plugs, your engine can easily stall.

Fuel and Air Filters

One of the possible reasons why your two stroke engine is stalling when revved is because of a clogged filter. Air filters rid the air of debris before it enters the engine and its intricate parts. This debris can clog and damage the air filter over time.

If the air filter is clogged, then not enough air can get into the cylinder for the engine to run. There could be enough air for the engine to idle, but as you put more demand on the engine, it needs more air, and therefore will stall.

Fuel filters cause a similar issue. The fuel filter stops any debris or gunk that may be in the fuel tank from entering the carburetor and engine. If this filter gets clogged, then not enough fuel can be accessed by the engine for it to run.

The Fix

Fixing a clogged filter is simple in a combustion engine, and that’s why it’s our first suggestion for a repair. Remove the air and fuel filters and check them for clogs or damage. If they look used but not clogged, feel free to replace them anyway. Filters are one of the cheapest parts on your engine and should be changed regularly, even if they aren’t the source of the problem.

Gaskets

Gaskets are parts of your engine which help to create airtight seals in chambers. The head gasket, for instance, sits between the cylinder head and the cylinder block. It helps to create the main amount of pressure needed for the combustion of the engine. 

If the head gasket is blown, the seal of the combustion chamber is compromised, and your engine will lose pressure and stall.

The Fix

Gaskets can be simple to replace, as long as you know where they are and what you’re doing. Locate the gaskets and take note of their condition. A blown head gasket is easy to spot, as it will be obviously cracked.

A leaky head gasket is not as easy to spot, as it can appear to be fully functional, but is actually letting some air escape.

Replace any gaskets that appear to be damaged, and take note of any ones you suspect might be leaking. You will most likely have to have a professional take a look to find out if they are or are not leaking.

Spark Plugs

As we’ve learned, spark plugs play an essential role in a two stroke engine. They need to fire consistently to ignite the fuel that enters the compression chamber. This ignition is what repels the piston downwards, and allows the perpetual motion of the engine to occur.

The Fix

To check if your spark plugs are in working order, you can remove the plugs and do a quick visual test. Any black or dark gunk stuck around the plugs is a sure sign that they need to be replaced. 

If there is no gunk, you can set the spark plug down on the metal of the engine and try to start it again. If the plug sparks, then you can move onto another issue. If the plug does not spark, however, then they will need to be replaced.

The Carburetor

It is unfortunate, but the most likely reason your engine is repeatedly stalling when put under a load is an issue with the carburetor. The carburetor is responsible for mixing together the air and fuel needed for the spark plug to ignite the reaction needed for the engine to run.

When you rev the engine, the carburetor increases the supply of gas so that your engine can have enough fuel to respond to the demands of the higher gear. If your carburetor is dirty or broken, it won’t provide the right mixture, and your engine will sputter and die.

The reason why this is so unfortunate is that fixing or replacing a carburetor is a notoriously expensive and complicated repair job. That’s why we suggest testing all of the elements above before diving into the carburetor.

The Fix

If you’ve checked all of the other, smaller components in this article, then by all means, look at the carburetor. If your machine has sat for a long time with fuel in it, then the chances are that that fuel has thickened and is gumming up the inside of your carburetor.

First, before pulling your carburetor apart, try and clean it. Use a specialty carburetor cleaner like Gumout Carb and Choke Cleaner. You may be lucky enough to get away with simply clearing out your carburetor instead of having to replace it.

If the cleaning doesn’t work, you can purchase a carburetor kit, which will include all of the pieces necessary to replace the carburetor in your two stroke engine. 

It would be wise to take photos of your carburetor before you take it apart, either to clean it or to replace it, so that you can ensure that you replace the parts in the proper position.

Also, using a chalk pen to mark any parts that need to be lined up is a good idea, as sometimes these need to be in a specific position to run correctly.

Summary

The most likely reason that your two-stroke engine is stalling under load is that your carburetor is dirty or dead. It is still worth it to run through the other, easier fixes of your engine, before jumping to the rebuild of the carburetor.

Check your filters, gaskets, spark plugs, and vents first, as you may luck out with a simple fix.

Teddy Henderson

Teddy is always fiddling with small engines, picking up thrown-out string trimmers or tearing apart dirt bikes. He shares what he learns along the way. Hopefully, you'll have less headaches than he has had by learning from his mistakes.

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