Why Can I Only Run My Mower With The Choke On?


You’ve been using your lawnmower for a while now, carefully following every step outlined by the user manual and the dealer you bought the machine from. Everything’s been working perfectly…until now. Why won’t my mower run without a full choke?

At some point, some lawnmowers experience the problem of not being able to run unless the choke is engaged. The problem persists even when the gurgling mower gets warmed up. What’s the cause? More importantly, can it be fixed?

Let’s find out.

What Is The Problem If A Mower Only Runs On A Full Choke?

Most mower owners know the drill when it comes to cranking one up. Check the spark plugs, prime the carburetor, open throttle, choke on, start the mower, let it run…then choke off once the mower is warm. 

But what if the last step turns the mower off instead? Then you know you have a problem.

The purpose of the choke mechanism is to literally restrict the air intake into the carburetor…hence “choke”.  

When you start the mower for the first time in days, the engine is cool, which is not great for the evaporation of fuel and combustion. The choke mechanism constricts air intake valves to prevent even more cold air from coming into the carburetor. 

As a result, the engine cylinders get just enough air for the combustible air/fuel mix. Less air in the carburetor means less internal pressure, which facilitates the easy flow of fuel into the carburetor and combustion chamber.

Essentially, the choke helps speed up the engine’s warm-up process by limiting the amount of cool air coming. The carburetor’s role is to maintain the delicate balance of air and fuel regardless of engine temperature.

Of course, once the engine gets warm, fuel will burn much faster, and the combustion process will be at an optimum level. Fuel will be evaporating at a rate that can accommodate regular air intake. 

That’s how it’s supposed to work at least.

If your mower engine cuts off when you try to disengage the choke, there are a few potential causes. I recommend that you check all of the causes outlined below because they can all cause this problem individually and collectively.

A Dirty Carburetor Can Cause Mowers To Need A Full Choke To Run

A dirty carburetor is the most common root cause in this regard, especially for gasoline engines. Most gasoline variants (especially ethanol blends) contain many impurities and bits of debris that can cause serious damage to the engine. This is why the carburetor has a protective mesh screen.

However, over time, this debris can build up on the mesh screen and block the free flow of fuel into the carburetor.

If this happens, the only way to get enough fuel into the carburetor and combustion chamber is to keep internal pressure low by restricting air intake (engaging the choke). 

Even when the mower gets warm enough to carry on with the choke off, the poor flow of fuel will hinder the combustion process. 

This is a situation where you can either…

  1. Run the mower with the choke on (which uses fuel inefficiently) or
  2. Have the mower peter out on you because of restricted fuel intake while the choke is off.

The carburetor’s “balancing act” becomes virtually impossible.

I will go over a fix for the problem further on. Let’s move now to the next issue that could cause the problem.

Blocked Fuel Filter And Fuel Lines Can Require Full Choke Operation

The fuel filter also gets clogged up with use. Using lower-grade gasoline or ethanol blends will accelerate this process because of all the impurities in them. Grass clippings can wind up in the fuel filter and these can form clumps that block the flow of fuel through the protective mesh screen.

This problem can also affect the fuel lines. Fuel lines are small pipes that transport fuel from the tank to the carburetor. Fuel impurities, as well as dirt and grass clippings, can cause blockages in the lines, which also restricts the flow of fuel.

Fuel lines are also susceptible to wear and tear. Low-grade gasoline and ethanol blends (E10 and E15) contain elements that may have a corrosive effect on fuel lines. 

E15 blends are especially dangerous for small engines because of the volumes of water their chemical reactions release. Metal fuel lines without anti-corrosion measures could even end up leaking if exposed to this kind of fuel for too long. 

Ultimately, a blocked air filter and compromised fuel lines mean less fuel reaches the carburetor. Engaging the choke will force the carburetor to draw in fuel…unless you have leaking fuel lines, which is a whole other problem.

Is It Bad To Run A Mower With The Choke On?

Running a warm mower with the choke on wastes fuel because restricting carburetor air intake creates an internal vacuum that is filled by fuel. In simpler terms, the lack of air forces the carburetor to draw in more fuel than it needs for each combustion cycle. 

The choke is only meant to aid the combustion process as the engine warms up. Keeping the choke engaged after this stage is very bad for your lawnmower’s longevity, as well as your wallet. This is fine for kickstarting a cold mower but unnecessary once the engine gets warm.

The warmer the engine gets, the less fuel it needs for the ideal fuel-air mix.

By keeping your choke engaged, you are essentially giving your engine an unneeded fuel “boost” with every combustion cycle. Do this for long enough and you will find yourself filling up your lawnmower much more frequently.

These high volumes of fuel per (combustion) cycle can also wash the oil away from the cylinder heads and pistons. This has a negative impact on lubrication and, ultimately, the engine’s efficiency and longevity. Your oil changes will also become more frequent.

Let’s not forget about the environmental impact. The inefficient combustion of choked engine results in higher CO2 emissions. Carbon deposits may also be formed in the engine cylinder, and some may escape with the combustion exhaust.  

How Do You Fix A Lawn Mower That Won’t Say Running?

If you’re having this problem with your walk-behind or riding mower, your best bet is to open it up and check for the causes we discussed above. Remember to check for everything (fuel filter, fuel lines, and carburetor). In fact, we recommend routine checks before problems arise.

Be very careful not to rush into DIY repairs if your lawnmower is still under warranty. Consult the manufacturer or dealer to go over your options before opening up the machine.

If you identify the fuel filter as the problem, you will have to replace it. Likewise for the fuel lines. These processes are different from mower to mower, so you should consult your owner’s manual. You can also get a trusted mechanic to help you out. 

How To Clean A Mower Carburetor

If the carburetor is the problem, you will have to clean and service it.

Step 1: Find a carburetor cleaning agent of your choice and mix it into a small container or bucket. Use the mixing ratio specified on the label (e.g., 3 parts water and 1 part cleaning agent).

If you would like a quick and easy carburetor cleaner that you can simply spray on, I recommend this one found on Amazon.

Step 2: The next step is to access the carburetor. You must remove any coverings, connecting wires, and lines. Use pliers to remove any clamps and take out the carburetor. Wipe off any dust and use compressed air to remove dirt from tight spaces on the component.

Step 3: Next, detach the carburetor float bowl that sits at the bottom of the carburetor. Safely dispose of the fuel remnants in the bowl and detach the float bowl pin. The float bowl is where a lot of dirt piles up so it needs a thorough cleaning. 

Detach any of the carburetor’s other removable parts and keep them in a safe place. 

Step 4: The next step is to soak the components in the cleaning agent solution for about 10-15 minutes. Use a cloth or an old toothbrush to scrub off resistant dirt. You should then rinse the parts and leave them to dry before reassembling and replacing the carburetor.

Final Touches On Why A Mower Only Runs With The Choke On…

Barring a leak of some kind in the the fuel line or other area, the main reason for a mower that will only run with the choke fully open has to do with the proper amount of fuel and air intake. The fixes can be simple cleaning or small part replacements.

This shouldn’t be that expensive or time consuming if it occurs and can even be presented by proper, regular maintenance. Just be sure that any self cleaning or repairs won’t void any of your remaining warrantee requirements.

References

https://simplegreen.com/cleaning-tips/rooms/garage/carburetor-small-engine/

https://www.youtube.com/watch?v=rz-EoINKQig&ab_channel=GardenTools%26Instruments

Mathew Booe

Mathew has worked in landscaping professionally for over 10 years. He is a grandpa and frequently interviews other experienced landscapers and lawn care experts who are also grandpas for these articles.

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