Pruner: Engine Will Idle But Dies At Full Throttle

If your carburetor isn’t working at its best or not at all, it won’t be doing its job of mixing air and fuel together in the proper ratio for combustion. Locate the carburetor underneath the air filter, remove the air cleaner cap, and look for any obstructions that could block air flow, starving the engine of oxygen. You will also want to check for any adjustment screws; sometimes adjusting these settings to lean out the fuel flow will solve the problem. Locate both idle screws on carburetor, one marked L (low) and one marked H (high). Turn both screws in to shut off fuel then turn them back in the opposite direction 2 turns. Now you will need to start the engine. It may not run perfectly, but it should start at these settings. Next, allow the engine to idle for a minute. Now, turn the L screw clockwise to add more fuel and set the idle. Next, at full throttle, turn the H screw clockwise to set the high idle. When you have the engine running at its best you have successfully adjusted the carburetor (this does require some experience and may take a few times to get it right). Some carburetors are fixed, meaning they have no adjustments. In this case, clean out the carburetor as they can become gummed up easily in 2-stroke engines. This can also result in a pruner that will die at full power.

You can locate your pruner’s air filter on top of the carburetor. Its main function is to allow clean-flowing air into the engine and prevent small particles from damaging internal engine parts. Over time, the air filter will need to be replaced as it can become clogged with small particles and debris. This can eventually starve the engine of air, and cause it to die when you rev it up to full power. To replace the air filter, first locate the air filter cover and remove it. It may be attached by screws, or just a tab, depending on your model type. Once the cover is off, remove the filter, replace it with a new one and re-assemble. This should be done quite often as part of regular maintenance, and to prevent problems with your pruner down the road.

You’ll find your fuel filter inside the pruner’s tank, where it prevents debris from entering the carburetor and engine, causing damage. You can remove the fuel filter by using a hook and pulling it right out of the fuel tank. Check to see if it is partially clogged, as this can limit the flow of fuel to the engine. It never hurts to replace the fuel filter as they are inexpensive and should be replaced anyway, as part of regular maintenance.

The gasket’s job is to seal two pieces together, and prevent air and fluids from entering the engine unexpectedly. A damaged gasket below the carburetor will pull excess air into the engine causing the air fuel mixture to be lean, and this can result in a loss of power. To check the gasket, start by removing the air filter and loosening carburetor bolts. This should give you access to the gasket, and you can now check its condition. You will want to look for tears, rips or any indication of a leak. The gasket will need to be replaced at this time regardless of condition, as they can only be used once.

The purpose of the primer bulb is to prime the carburetor with fuel on cold start ups, which will help the engine run properly. If there is a crack in the primer bulb, it can pull air into the fuel, which can cause an improper air/fuel ratio. This can ultimately result in a loss of power, and can even cause your pruner to die at full power, thus its inclusion as a possible cause of the symptom you are experiencing. A visual inspection will likely be the only step required to determine if you have a cracked primer bulb. If the bulb is cracked, it will need to be replaced. To replace the bulb, simply pull it off with a pair of pliers. Next, grab a small pick and reach into where the bulb came off, grab the clip, and pull it out. Next, clean out the housing. Put the new clip on the primer bulb with the tabs facing out, and use a socket that fits over the primer bulb to push it into the housing. You should make sure it is secure by tapping it gently with a rubber mallet. Make sure the metal clip is as far in as it can go without damaging the primer bulb. This could vary on different models but are all models are relatively similar.

If you’re looking to tear down and rebuild your carburetor, a carb kit has all the components required for this task. It is not listed here because it is a direct cause of this symptom, but because it is a great way to restore your carburetor to new condition, preventing any problems and keeping your engine running smoothly. Fuel that sits for a long period of time, especially when it is mixed with oil, has the tendency to gum up the inside of a carburetor, and re-building the carburetor with a carb kit can help it run like new. To install the carb kit, first remove the air filter housing, the air filter, and remove the bolts holding down the carburetor and gasket. Next, gently remove the fuel lines by twisting them and pulling them off, then remove the carburetor linkage. Disassemble the entire carburetor, laying out all of the parts on a table and making note of where they came from (taking pictures along the way is a great way to remember where they came from and how to re-assemble). Clean the entire carburetor housing with a carburetor cleaner and a soft bristle brush, which will clean out all the gummed-up surfaces. Once you have re-assembled the carburetor you can simply re-install it with a new gasket. Next, set the low and high idle screws by turning them until they shut off, then back off 1 turn. Start the engine, let it warm up for about 5 minutes, then make the necessary adjustments. Start the with the L screw, and turn it until it runs the fastest without engaging the clutch, or about 2,700 RPMs. Now you can adjust the idle screw and set the low speed fuel adjustment. Slowly turn the screw inward until the engine surges or starves of fuel, and make a mental note of the position of the screwdriver slot. Now, slowly turn the adjustment screw out and the engine should run more efficiently. Continue to turn the screw until the engine starts to load up. Make a note of the position of the screwdriver slot and compare it to the position of the lean adjustment. Now, slowly turn in the screw to a position where it idles at its best (it should be about midway between the rich and lean settings). Next, go back and reset the idle speed as the rpm may have changed. At full throttle, adjust the H screw until the engine runs where you like it. This usually takes some experience to adjust, so for an inexperienced DIYer, may take time.

The purpose of the fuel lines is the delivery of fuel from the fuel tank to the carburetor, where the carburetor distributes it to the correct air/fuel ratio for combustion. If the fuel line is blocked or cracked, it can cause your pruner to die at full power. To inspect the fuel lines, first pull them off the carburetor gently, using a twisting motion and do the same on the fuel tank. Some models may require you to remove a rubber grommet on the side of the tank first where the fuel line feeds in. You can simply blow through the lines and look for anything unusual coming out, and any blockages. If you notice anything unusual, you will need to replace the line or lines. A partially blocked line will allow some fuel to flow but when you require more power it will restrict the flow of fuel and this can starve the engine. Lines should also be replaced if they look old or cracked, as they can leak and cause a fire.