The choke restricts airflow and makes the fuel mixture richer to help start up a cold engine.
When the engine gets warm, the choke is turned off to allow the engine to run on its normal fuel and air mixture.
If your Honda engine runs only on full choke, it is either getting too little fuel or too much air causing it to stall when the choke is turned off. This means that something is wrong internally and needs to be fixed as soon as possible.
In this article, I share the common reasons your Honda engine runs only on full choke, simple tips to help you diagnose the problem, and steps you can take to fix these problems.
Reasons Why Your Honda Engine Runs Only On Full Choke
Your Honda engine runs only on full choke as a result of air leaks or blocked fuel passages. This could be a result of one of the following:
- Clogged carburetor jets
- Carburetor adjustment set too lean
- Stale fuel
- Split fuel line
- Blown head gasket
Tools You May Need For This Job
Some tools you may need to troubleshoot and diagnose the problem include:
- Work gloves
- Empty container
- Carb cleaner
- Clean rag
Honda Engine Runs Only On Full Choke: Causes, Diagnosis, and Fixes
1. Clogged Carburetor Jets
A clogged carburetor is the most common reason an engine runs only on full choke. A carburetor has small jets with tiny holes that deliver fuel to the combustion chamber.
These holes are tiny and can get clogged easily with debris and old fuel residue. When they get clogged, the fuel supply becomes too lean meaning that it is too little compared to the air supply and this can alter the combustion process.
The engine will run only on full choke because the choke restricts the airflow and delivers more gas to the engine, enriching the fuel mixture.
If your engine stalls after starting it normally, the carburetor jets might be blocked. Take out the carburetor and inspect the jets for debris and dirt.
- Take out the carburetor
- Spray carb cleaner all over the carburetor jets and bowl
- Allow it to sit for a few hours
- Clean the carburetor and allow it to dry
- Reinstall the carburetor
2. Carburetor Screw Set Too Lean
The carburetor adjustment screw controls the Air Fuel Ratio(AFR) that goes into the engine for combustion. The engine requires a particular amount of fuel and air for it to start and run smoothly.
When the screw is set to allow only a small amount of fuel in, the fuel mixture becomes too lean, and the engine stalls. Putting the engine on full choke will enrich the mixture by restricting airflow and will allow the mower engine to run.
Check the exhaust temperature. If the exhaust is overheating or glowing red, your carburetor adjustment screw might be set too lean.
Adjust the screw to allow more fuel into the engine.
3. Stale Fuel
Another reason your Honda engine runs only on full choke could be old or stale fuel in the carburetor bowl.
Fuel that has stayed in the engine for over 30 days will get stale and leave behind a sticky residue that can affect fuel delivery in the carburetor.
Stale fuel rusts the carburetor and clogs the fuel lines and carburetor jets. This reduces fuel supply and stalls the engine. The fuel mixture will need to be made rich by putting the engine on full choke.
Drain out the fuel tank and pour the engine into an empty container. A sticky or slimy residue in the fuel is a sign that the fuel is old and has clogged the carburetor.
- Drain the fuel tank and carburetor bowl of old fuel
- Clean the carburetor with carb cleaner
- Refill the tank with new fuel
4. Split Fuel Line
A split fuel line can also be the reason your engine runs only on full choke. Fuel lines deliver fuel to the carburetor.
The fuel bowl of the carburetor might not be getting enough fuel as a result of a split or crack in the fuel line.
Excess air might also be getting sucked into the engine through the split causing the fuel mixture to become too lean. As a result of this, the engine will run only on full choke.
Inspect the fuel lines thoroughly for signs of fuel leakage, cracks, or splits.
Split fuel lines should be replaced.
5. Blown Head Gasket
If the carburetor jets, fuel lines, and fuel condition are okay, the problem might be from the head gasket.
The head gasket seals the combustion chamber to enable it to build enough pressure needed to start the engine.
It also prevents oil, coolant, and exhaust gases from entering the cylinders and interfering with the combustion process. Simply put, a head gasket seals the cylinders to ensure maximum compression is obtained.
A blown head gasket will affect the combustion process by allowing gases into the cylinders. This can make the fuel mixture run lean and result in the engine refusing to start without putting the choke on full hold.
- Open the engine and inspect the head gasket. If it has oil leakage or has a hole in it, the head gasket is blown
- White smoke blowing through the exhaust is another sign that the head gasket is blown
Replace the blown head gasket with a new one.
Should I Run My Engine With The Choke On?
The choke helps to start a cold engine. A cold engine requires more fuel supply than air to ignite the engine and warm it up. The choke restricts the airflow to help the engine start and get heated up.
After the engine has warmed up, the choke should be returned to allow the engine to run on the normal air-fuel ratio.
Allowing the engine to run on full choke for more than 5-10 minutes will result in a rich fuel mixture and can cause the following problems:
1. Diluted Oil
An engine with an excess fuel supply will not be able to burn all the fuel. With time, the excess fuel makes its way to the crankcase and dilutes the oil.
Diluted oil is thin and unable to coat and protect the engine components. It can result in overheating, excessive friction, and damaged internal components.
2. Engine Backfiring
Backfiring is a loud bang by the exhaust of your engine caused by extra gas pushed out to the exhaust coming in contact with the hot manifold/exhaust pipe.
Constant engine backfiring can cause serious damage to the engine if the root cause is not identified and fixed.
3. Excessive Fuel Consumption
In addition to causing harm to the internal engine components, running your engine with a full choke will lead to excessive fuel consumption by the engine.
The choke allows more fuel to be supplied to the engine. Running the engine with a full choke for extended periods will result in the engine using up more fuel than usual.
Your Honda engine running only on full choke could be a result of clogged carburetor jets, stale fuel, split fuel line, or a blown head gasket.
Follow the troubleshooting guide in this article to identify the root cause of the problem and then, apply the recommended fixes to get your engine running as it normally should.