Engine: Engine Will Idle But Dies At Full Throttle

The purpose of the carburetor is to mix the air and fuel to the correct mixture for combustion. Over time, carburetors tend to gum up and may require cleaning. Cleaning the carburetor requires the use of carburetor cleaner and a soft brush on the inside. A quick way to clean the carb is to run the engine, and rev it up while spraying the inside with carburetor cleaner. Check for any malfunctioning, missing, or weak springs that control the carburetor’s butterfly valves. Any damaged ones should be replaced. To replace the springs, simply use a pair of pliers to pull them off. Make note of how they come off, as you do not want to re-install the springs backwards; this will cause carburetor to malfunction. It is always a good idea to adjust the carburetor settings at this time. To do this, locate both idle screws on the carburetor. One will be marked L (low) and the other will be marked H (high). Turn both screws in to shut the fuel off, then turn them back in the opposite direction 2 turns and start the engine. The engine may not run perfectly under these settings, but it should at least start up. Next, let the engine idle for a few minutes, allowing it to reach operating temperature. Then, begin the adjustment by turning the L screw clockwise until the engine starts to slow. Next, you will want to turn it in the opposite direction until it once again starts to slow. Now, bring it to the midpoint of these 2 spots. Using a tachometer to gauge the engine speed, set the idle speed screw to bring the engine to 1750 RPM for an aluminum-cylinder engine or 1200 RPM for an engine with a cast-iron cylinder sleeve. Next, at full throttle, turn the high speed or main jet screw clockwise until the engine begins to slow. Then, turn the screw the other direction until the engine begins to slow. Finally, turn the screw back to the midpoint and your carburetor adjustment will be complete. Once adjusted, check the engine’s acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly.

The air filter blocks dirt and debris from entering the engine. A clogged air filter will starve the engine of oxygen, causing the engine to die at full throttle. A clogged air filter will often cause your engine to idle rough as well. As part of your normal maintenance routine, you should check the air filter for heavy build-up, and replace if necessary. A good way to check the filter for a clog is to shine a light behind the filter. If you cannot see the light, or the light is very dim, the filter needs replacing.

The fuel filter prevents contaminants from entering the carburetor and engine, causing damage. If the fuel filter becomes clogged, it will impact engine performance, as the rate of fuel is restricted. The engine may idle normally, but at higher RPMs it may not get enough fuel, causing the engine to stall. To inspect or replace the fuel filter, follow the fuel line from the tank to the carburetor and locate the filter. Remove the filter and check for an obstruction by blowing through it. If you suspect it is obstructed, you should replace it. Install the new filter with the arrow pointing in the direction of flow. If the filter is installed backwards, it can restrict the flow of fuel. Some engines have the fuel filter located inside the tank. In this case, remove it with a hook and check for obstructions on the outside. It is recommended to replace a fuel filter as a part of regular maintenance.

The fuel pump is designed to pump fuel from the tank to the carburetor. If the fuel pump is weak or worn it will not be able to keep up an adequate supply of fuel to the engine, especially at higher RPMs, and this can cause the engine to die. To inspect the fuel pump, first remove the fuel line at the carburetor, then turn the engine over and check for fuel coming out. If there is little to no fuel coming out, check the vacuum line going to the pump for cracks. If a vacuum line is cracked, the pump will malfunction, as it works on vacuum pressure. If it is cracked, replace the line and the pump should work properly again. If the line appears fine, then it is time to replace the fuel pump. Using pliers, remove the fuel line going in and out of the pump, and remove the vacuum line. Next, remove the screw or bolts fastening the pump to the engine. Now you can remove the pump and install the new one. Tighten the screws or bolts down, then hook the fuel lines and vacuum line back up.

The purpose of a gasket is to create a seal that prevents air and fluids from entering or exiting areas where they do not belong. A damaged gasket below the carburetor will pull excess air into the engine, and this can lead to the engine dying at full throttle. This problem may allow the engine to idle, but even if it does, it will typically idle rough. To replace the gasket, remove the air filter and bolts holding down the carburetor. If necessary, pinch it off to prevent a fuel spill, then remove the line. You will want to disconnect any levers as well. Now, you can check the gasket for damage or leaks. At this point, the gasket will need to be replaced, as they are a one-time use. Finally, install a new gasket and re-assemble the carburetor back onto the engine with the air filter assembly.

Spark plugs ignite the air/ fuel mixture inside the combustion chamber. Badly worn plugs can fail, causing the engine to die at full throttle. To determine if the spark plug is your issue, remove the plug and check it thoroughly. If it is not brown or light grey in color (which is normal), then replace the plug. Spark plugs should generally be replaced once a year as a part of regular maintenance.

Some engines use a carburetor that contains a diaphragm inside of it, which regulates the fuel mixture. Over time, this diaphragm can start to deteriorate and cause problems when delivering fuel to the engine. If the diaphragm is damaged, the engine may idle properly, but at higher RPMs it may not be able to keep up. This will starve the engine of fuel and cause the engine to die. To fix the diaphragm, first remove the air filter housing assembly. Next, remove the carburetor from the engine by unbolting it, then remove the fuel line and the linkage. Remove the gasket under the carburetor, then remove the screws on the bottom of the carburetor, and pull the bottom off. Now, remove the gasket and the diaphragm, and install a new gasket with a new diaphragm. Install the cover back onto the carburetor, and re-install the carburetor back onto the engine with a new gasket below it. Hook the linkage back up and re-connect the fuel line to the carburetor. Finally, re-attach the air filter housing and filter.

The carburetor kit contains all-new parts for the inside of your carburetor, so you can allow it to run like new. To install the carb kit piece by piece, first locate the carburetor underneath the air filter, and remove the air cleaner cap and air filter. While you have the air filter out, it is a good idea to check it for dirt and debris, and replace if needed. This is a part of regular engine maintenance. There may also be small dirt particles located inside of the carburetor if the air filter was deteriorating, so purchasing a carburetor kit, and rebuilding your carburetor is a good idea as well. Take pictures of the carburetor as you’re taking it apart as a reference tool for when it comes time to re-assemble it. Next, remove the bolts holding down the carburetor and gasket. Pinch off the fuel line to prevent a fuel spill and remove the fuel lines by twisting them gently and pulling them off. Then, remove the carburetor and disconnect the linkage. Start disassembly by removing the nut on the bowl then remove the bowl and gasket. Remove the float pin and the needle valve and unscrew the main jet screw and dump it out with the emulsion tube. Next, remove the plastic throttle stop and the metering plug from the carburetor. Spray the carburetor, fuel bowl and all the small orifices with carburetor cleaner, then wipe clean. If you notice any corrosion inside the carburetor or the bowl, you will need a new carburetor. Now, re-install the new emulsion tube and the new main jet and tighten them. Install a new metering plug, re-install the throttle stop screw, and thread it until it sticks out the other side about 1/16 of an inch. Next, install the new spring on the needle valve, snap the new needle valve into the new float, and re-attach the float to the carburetor by sliding the new pin in. Install the new bowl O-ring, re-install the bowl, and tighten down. Re-install the carburetor with a new intake gasket back onto the engine, then re-attach the linkage and fuel line, remembering to un-pinch the fuel line. Re-attach the air filter assembly with the filter. At this point, you will need to make any necessary adjustments to your carburetor. Please see the carburetor section, to determine how to properly adjust your carburetor.