Trimmer: Engine Will Idle But Dies At Full Throttle

The carburetor mixes the air and fuel together to the proper ratio for combustion. Locate the carburetor underneath the air filter. What follows is a guide to fine-tuning your carburetor, as a poorly adjusted carburetor can contribute to your symptom. If you’re looking for a guide to stripping down and rebuilding your carburetor, see our “Carburetor Kits” section elsewhere on this page. Remove covers to gain access to the carburetor. Remove the air cleaner cap and filter, checking for any obstructions in the way that could block air flow, starving the engine of oxygen. Locate the adjustment screws. To adjust the carburetor, start with both screws backed out 2 turns from the closed position. One should be idle mix and the other should be high speed. Start the engine and let it warm up for 5 minutes. You may have to keep the throttle open a bit. Slowly turn in the low speed jet, until it starts to die, then back it out another 1/4 to 1/2 turn. Next, hold the throttle wide open, and slowly turn in the high speed, until it really starts to smooth out and rev high, and starts screaming, then back this out until it starts to run rough or slows down. Turn it back in to midway between these two positions. Keep playing with the low speed needle until you get rapid immediate response from the throttle, and good idle with the idle adjustment. Next play with the high-speed needle, but always back it out from the setting about 1/4 turn or so, from where it runs the smoothest. Backing it out will allow more fuel when under a load, and keep your 2-cycle motor from running too lean. Put a load on it and just tweak the settings a hair at a time. Also make sure your oil/gas is mixed at the proper ratio. Most will start and run with both screws open 2 turns. Don't close the high speed off any more than necessary, no matter how well it runs. Some carburetors are fixed, meaning they have no adjustments. Clean out the carburetor with carburetor cleaner. They can become gummed up easily in 2-stroke engines.

The air filter’s main function is to allow clean-flowing air into the engine and prevent small particles from entering, damaging internal engine parts. Over time the air filter will need to be replaced as it becomes clogged with small particles and debris, starving the engine of air, causing a loss in power. Locate the air filter cover on top of the carburetor and remove it. It may be attached by screws or just a tab. Once off, remove filter, replace, and re-assemble. This should be done regularly to prevent future problems.

The fuel filter is found in the tank and stops debris from entering the carburetor and engine, causing damage. You can remove the fuel filter by using a hook and pulling it out of the fuel tank, visually inspect the fuel filter to see if it is partially clogged as it can limit the flow of fuel to the engine. Remove the fuel filter and install a new one. Fuel filters are inexpensive and need to be replaced regularly as a part of routine maintenance. If your unit does not have an internal fuel filter, follow the fuel line from the tank leading to the carburetor and locate the filter. Remove the filter by twisting and gently pulling the lines free and install a new filter. Make sure you check for an arrow on the fuel filter indicating the direction of flow. The arrow should point towards the carburetor.

The purpose of a gasket is to seal pieces tightly together, sealing off air and fluids from entering areas where they are not supposed to enter or exit. A damaged gasket below a carburetor will pull excess air into the engine, causing the air/fuel mixture to be off and result in a loss in power. To check this, start by removing the air filter then loosen carburetor bolts. Disconnect the fuel line and remove any linkages depending on your clearance to remove the gasket. Check the condition of the gasket; look for tears or any indications of leaks. Clean off both mounting surfaces. The gasket will need to be replaced regardless of condition, as they can only be used once. Re-install the carburetor and hook up any linkages you removed, and reconnect the fuel line. Re-install the air filter housing with air filter.

The spark plug’s job is to ignite the air and fuel mixture in the combustion chamber. Check the spark plug by carefully removing the spark plug wire, as they are fragile. Remove the spark plug and check its condition. Sometimes on a 2-stroke engine spark plugs get oily build up from improper oil/gas mixture. You can clean off the plug and re-install it. Make sure you also check for any deposits, breakage, melted parts, or corrosion. If you see any of these symptoms, replace the plug and gap it to the manufacturer’s specs. If the engine spends most of its time running at high RPMs, try installing a cooler running plug to dissipate heat more quickly.

The primer bulb primes the carburetor with fuel for easier cold start ups, to get the engine to run. If there is a crack in the primer bulb it can draw air into the fuel, causing an improper air/fuel ratio, which can result in a loss of engine power. Seeing leaking gas is a good indicator as well hearing air rushing in when pushing on the bulb. To replace it, simply pull it off with a pair of pliers. Next, reach back into where the bulb came off, grab the clip and pull out. Clean out the housing. Put the new clip on the primer bulb with tabs facing out. Then use a socket that fits over the primer bulb and push into the housing, giving a gentle tap with a rubber mallet if necessary. Make sure the metal clip is as far as it can go without damaging the primer bulb. This procedure could vary on different models but are all similar.

This part allows air into the fuel tank as the fuel level drops during use. If it gets blocked, it will cause a vacuum and stop the flow of fuel. The fuel vent is located just off the fuel tank. It will be attached to a fuel line, entering the tank through a grommet with the fuel delivery and return lines. Once located, simply pull off the clamp that connects to the fuel lines. You will need a pressure checker to check for vacuum, this is rare tool which most people will not have on hand. The part is inexpensive and easy to replace.

A carburetor kit has everything you need to tear down and rebuild your carburetor; it is a great way to restore your carburetor to new condition. Fuel that sits for a long period of time especially mixed with oil has the tendency to gum up the inside of a carburetor causing fuel delivery problems. The following is a guide to tearing down and re-assembling your carburetor. If you want to fine-tune your carburetor instead, see our breakdown of this process elsewhere on this page under the “Carburetor” heading. It is a good idea to take photos as you dismantle your carb, to make reassembly easier. Simply remove the air filter housing and air filter, then remove the bolts holding down the carburetor and gasket. Next, gently remove the fuel lines by twisting them and pulling, then remove the carburetor linkage. Remove the carburetor and the screws on the bottom of the carburetor, and pull the bottom off. Remove the gasket and the diaphragm. Remove the primer bulb and the cover. On the bottom of the carburetor, make note of how the lever, spring, and hinge pin fit together inside the carburetor. Remove the screw and pull these parts out of the carburetor. Clean the carburetor with carburetor cleaner, making sure to get inside all the ports. Use a soft bristle brush if required. If you notice any corrosion on the inside, you must install a new carburetor as this one is beyond repair or cleaning. Install the needle and lever on the bottom of the carburetor and re-install the screw to secure them in place. Install the gasket, diaphragm, and cover. Make sure to install them in the correct order, by looking back at the photos you took whilst taking it apart. Install the top cover and primer bulb. Re-install the carburetor back onto the engine with a new gasket under the carburetor on the engine, hook back up the linkages and fuel lines. Re-install the air filter housing and filter. Finally, re-install the covers on the unit.

The fuel lines deliver fuel from the tank to the carburetor, where it is mixed with air for combustion. Check the fuel lines for cracks as over time they can become weather cracked and may leak. This can reduce the amount of fuel to the carburetor. Pull the fuel line off the carburetor gentle in a twisting motion and do the same on the fuel tank. On some models, you may need to remove a rubber grommet on the side of the tank where the fuel line feeds into. You can simply blow through the lines looking for anything unusual coming out, or any blockages. If you notice anything unusual, 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 starving the engine.