7 Reasons Hedge Trimmers Only Run on Choke

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Hedge trimmers need a specific mixture of air and fuel to run. The choke is that part of the system that ensures this happens, and is typically turned on to warm up the engine. If your hedge trimmer can only run on a full choke, there are issues with the air and gas mixture. To solve this problem, we will take a look at the causes and how to fix them.

A hedge trimmer that only runs on full choke is not getting enough fuel. This is likely due to a dirty carburetor, fuel line or fuel filter. Other possible reasons are a blown gasket, wrong fuel mixture and a clogged vent.

Clogged Carburetor

The carburetor is responsible for mixing the air and fuel so the engine can start. If there is a problem with the carburetor, the fuel will not mix properly and cause the hedge trimmer to run only on choke.

When a choke problem occurs in 2-stroke engines, the carburetor is one of the places you have to look at. There are 3 steps to fix the carburetor: clean, reconfigure the adjustment screws and if necessary, replace it.

How to clean a hedge trimmer carburetor
  1. Turn the hedge trimmer off.
  2. Look for the air filer cover and then remove it. Remove the air filter as well. You will see the carburetor behind it.
  3. Clean the air filter and its cover. Set it aside for now.
  4. Remove the carburetor and clean it. There are cleaning formulas available for this task. The Stabil Fast Fix does a pretty good job on all small egnines.
  5. Before cleaning, check the carburetor for signs of damage. If there is none, proceed with cleaning. If the unit is damaged, get it replaced. No amount of cleaning will fix it. Or you can try rebuilding the carburetor as a last resort before buying a replacement.
  6. Put the carburetor and air filter back on. Put the filter cover back and restart the hedge trimmer. If it still runs only on choke, you have to adjust the carburetor.
How to adjust a hedge trimmer carburetor
  1. Remove the air filter and its cover. Turn the hedge trimmer on and let it run for about five minutes.
  2. You will see the adjustment screws. One is marked L (low speed), H (high speed) and idle mixing.
  3. Turn the L screw once and turn again until it is almost dead.
  4. Open the throttle and turn the high speed screw to the max, then dial the high speed screw back down.
  5. Keep adjust the screws until the hedge trimmer is able to run with the choke closed. Make small turns only.

Be careful when adjusting the screws. Turn each one slowly and observe the changes. Too rapid turns can drastically alter the way the trimmer works.

Cracked Fuel Line

The fuel line moves fuel from the reservoir and sends it into the carburetor for combustion. For various reasons the line can develop tears. If this happens, air gets through and this can affect how the hedge trimmer runs.

The biggest problem is that a damaged fuel line might pull air and affect the ignition. When that happens the engine will have serious issues working.

Check the fuel line for any signs of tear. Replace it right away. Do not use your hedge trimmer until the fuel line has been replaced.

Dirty Gas Cap

The fuel cap has a simple function, to keep fuel in the tank and debris out of it. Some hedge trimmers have a valve connected to the cap so air gets through. This is done so the tank does not produce a vacuum.

If the gas cap is blocked, do the following.

  1. Activate the hedge trimmer and keep the choke on.
  2. While the hedge trimmer is running, loosen the cap screw, but do not remove it. Take the choke off.
  3. If the engine keeps running, the cap is clogged. Wipe it clean with a brush and wash with water. If that does not work, replace the cap.

This part of the hedge trimmer is often overlooked when doing repairs. However the cap is an integral part of the engine. It is also one of the easiest to fix so it is worth checking out if you are having issues with the choke. This problem can also occur on pole saws since they are on 2-stroke engines tool. For more about the similarities and differences between pole saws and hedge trimmers, check this guide.

Plugged Fuel Filter

The fuel filter prevents dirt from contaminating the fuel. A dirty or clogged filter will not be able to do this, forcing the hedge trimmer to rely on the choke to run. Once the fuel filter is clogged, the fuel will have a difficult time getting to the engine. To fix this, insert a hook in the gas reservoir and remove the fuel line. You should be able to tell if the line is damaged or clogged.

Either way you should replace the fuel filter. They are widely available and replacing means you do not have to do repeated cleanings. Check your hedge trimmer manual for guidance on how often to replace it.

Fuel filters play such an important role for hedge trimmers including the Wen40415, but it is easy to overlook them. When it comes to filters, we usually think of air filters. But fuel filters are also crucial when it comes to maintaining performance.

Wrong Fuel Mixture

Most gas hedge trimmers use a 50:1 or 40:1 fuel mix. It is important that you get this right because it will adversely affect the way the engine functions. Use the wrong mix ratio and the trimmer may only run on choke.

The good news is this potential problem can be easily avoided. All you have to do is consult the manual and you will see the right mix. If it says 40:1 for example, that means 3.2 oz. of engine oil per gallon of gas.

2-stroke engines like those on hedge trimmers run on regular unleaded gas. The octane rating should be 87. You can opt for a higher rating but it really does not make much of a difference in most cases.

Blocked Fuel Vent Valve

The fuel tank vent enables air to filter in the tank as the engine expends fuel. If the vent is clogged, air will not get into the hedge trimmer.

To check the vent, look for the fuel line as the valve is connected to it. Turn on the engine and keep the choke on.

Take the vent valve off the fuel line. Close the choke. If the hedge trimmer keeps going, the vent valve is blocked. Just like the gasket, vent valves are for a single use only and must be replaced when no longer functioning.

Damaged Gasket

The gasket limits the amount of air in the engine, which keeps the fuel lean. If the gasket is damaged or blocked, there would be too little or too much air getting in. Either one of those can cause a hedge trimmer to run on full choke only.

How to fix the gasket

  1. The gasket is under the carburetor, so you have to remove the carburetor first.
  2. Remove the air filter.
  3. Hold the fuel line to avoid spillage and then take the fuel line off. Disconnect the link to the carburetor.
  4. Take off the screws holding the carburetor and lift it up. After you remove the carburetor you will see the gasket under it.
  5. Gaskets are not reusable, so if it is damaged or too dirty, replace it. Before putting on a new gasket, clean its housing first.

Install the new gasket. Put the carburetor back on and all the screws. Reattach the fuel line, air filter and cover in the reverse order that you took them off. When shopping for a replacement, look for one that is fully compatible with your unit. This is also applicable for any engine part that has to be replaced.