Engine: Runs For A While Then Dies

The carburetor mixes the air/fuel to the proper mixture for combustion. Over time, carburetors tend to gum up and may require an internal cleaning. You should clean them with carburetor cleaner and a soft brush on the inside. A quick way to do this is run the engine, revving it up while spraying the inside with carburetor cleaner, and using the brush to get the heavier deposits. Check for any missing, weak, or broken springs that control the carburetor butterfly valves. Check a repair guide for your carburetor routing of these springs, as a lot of carburetors are different. Any damaged ones should be replaced, which is easy to do using a pair of pliers to pull them off and on. Make note of how they come off so you do not install them backwards, as this will cause the carburetor to malfunction. Adjusting the carburetor settings at this time is a great idea. Locate both idle screws on the carburetor, one marked L (low) and one marked H (high). Turn both screws in to shut off fuel then back off 2 turns, next start engine, it may not run well but should start at these settings. Let it idle for a few minutes to get it to operating temperature. Start by turning the L screw clockwise until the engine starts to slow, then turn it in the opposite direction until it once again starts to slow, then bring it to the midpoint of these 2 spots. Using a tachometer to gauge engine speed, set the idle speed screw to bring the engine to 1750 RPM for aluminum-cylinder engine or 1200 RPM for an engine with a cast-iron cylinder sleeve. At full throttle, turn the high speed or main jet screw clockwise until the engine begins to slow. Then, turn the screw the other way until the engine begins to slow. Turn the screw back to the midpoint. Once adjusted, check the engine’s acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly. If necessary, readjust mixture screws.

The air filter provides clean air for the engine to breathe. Although a dirty air filter will not cause the engine to run for a while then die, it can be a contributing factor on engines that have had poor maintenance. Check and change the air filter if needed to bring the engine up to code.

The fuel filter keeps debris out of the tank, and prevents it from entering the carburetor and engine. Check the fuel filter for an obstruction by removing the fuel line on the out-flow side, and check for fuel coming out. If your unit has a fuel pump, turn engine over and look for fuel to come out. If there is no fuel coming out, check the inlet flow by removing the fuel line and once again turn the engine over, looking for fuel to come out of the fuel line. If the pump is receiving fuel at the inlet side but it is not coming out of the outlet side, then the filter is clogged and needs to be replaced.

The fuel pump sends the fuel from the tank to the carburetor. If the fuel pump is weak or worn, it may not be able to keep up an adequate supply of fuel to the engine. Remove the fuel line at the carburetor, then turn engine over and check for fuel coming out. If there is no fuel or little 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 spring back to life. If the line appears fine, then it’s time to replace the fuel pump. Using pliers, remove the fuel line going in and out of the pump, then remove the vacuum line. Remove the screw or bolts fastening the pump to the engine. Remove the pump and install the new pump. Tighten the screws or bolts down. Hook the fuel lines and vacuum line back up.

Gaskets create a tight seal between two joining pieces, sealing off fluids and containing them in areas where they belong. If the engine runs for a while then dies, you may have blown a gasket. The gasket most likely to fail is the head gasket. To test a blown head gasket between the cylinder and another port, do a leak down test. This pumps air into the cylinder and measures how much is lost. You can listen and look for the source of the air. If the engine is liquid-cooled, you can also check the coolant for traces of oil as well as checking the oil for traces of coolant. Pull the spark plug or plugs. If you encounter a clean, shiny spark plug, you have coolant leaking into the cylinder. All these signs point to a blown head gasket. Remove the air filter assembly. Next, remove the spark plug wire and spark plug. Remove exhaust manifold and carburetor from the cylinder head, disconnecting the linkages and fuel line from the carburetor, and then remove the valve cover in an alternating pattern to prevent warping. Loosen the rocker arms and slide them over, you do not have to remove the rocker arms. Next, remove the push rods. Do not mix up the push rods. You may have to rotate the flywheel to get both valves closed. Remove the cylinder head mounting bolts in an alternating pattern to prevent warping of the cylinder head. Remove the cylinder head and pull off the old gasket, clean off both surfaces on the engine block and cylinder head. Install the new head gasket and then the cylinder head. To avoid warping the cylinder head, torque the head bolts in an alternating pattern to manufacturer specs. Next, reinsert the push rods, then rotate the rocker arm back in place and hand tighten the pivots down. Set the gap between the valve tip and rocker arm. They should be 3 to 5 thousandths of an inch. Reinstall the jam nuts and tighten down while holding the pivot stationary. Reinstall exhaust manifold and carburetor, hooking the linkage and fuel line back up, then reinstall the spark plug and attach the spark plug wire. Reattach the blower housing, oil drain tube, and air filter assembly.

The spark plug is used to ignite the air/fuel mixture inside the combustion chamber. Badly worn plugs can fail, causing all kinds of engine problems. Spark plugs should always be replaced yearly as routine maintenance to prevent problems, especially since they are so inexpensive and play a major role in engine operation. Remove the spark plug and check. Brown or light grey indicate normal operating conditions. Spark plugs with corrosion, deposits, breakage, melting, or oxidizing indicate something is going on inside the combustion chamber. You may need to run a higher-octane fuel or adjust the carburetor, or install cooler running plugs. If the engine runs for awhile and then dies, especially at higher RPM, try using a cooler running spark plug to dissipate the heat more quickly.

Some engines contain a carburetor that uses a diaphragm to regulate the fuel mixture. Over time, the diaphragm can start to deteriorate from the fuel, causing fuel delivery problems to the engine. It may be able to keep up enough fuel flow, but under certain loads it may starve the engine of fuel, causing the engine to die. To check the diaphragm and fix this, remove the air filter assembly. Remove the carburetor from the engine by unbolting it. Remove the fuel line, remembering to pinch it off first to prevent a fuel spill, then remove the linkage at the carburetor. Remove the gasket under the carburetor. Next, remove the screws on the bottom of the carburetor and pull the bottom off. Remove the gasket and the diaphragm, and install the new gasket with a new diaphragm. Install the cover back onto the carburetor and reinstall the carburetor back onto the engine, with a new gasket below it. Hook the linkage back up with the fuel line. Un-pinch the fuel line to allow fuel to flow again. Reattach the air filter housing and filter.

The carburetor kit contains new parts for the inside of your carburetor, restoring it to new condition and restoring its performance. Locate the carburetor underneath the air filter, remove air cleaner cap and air filter, check the condition of the air filter at this point, and replace if needed. There may also be a buildup of dirt inside of the carburetor if the air filter was deteriorating, so purchasing a carburetor kit and rebuilding your carburetor is a good idea. Take pictures of the carburetor as you’re taking it apart for your reference later. Remove the bolts holding down the carburetor and gasket, and pinch off the fuel line to prevent a fuel spill. Remove the fuel lines by twisting them gently and pulling them off. Next, 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. Unscrew the main jet screw and dump it out with the emulsion tube, then remove the plastic throttle stop and the metering plug out of 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 on the bowl, you will need a new carburetor. A carburetor with corrosion on the inside is no longer good. Reinstall the new emulsion tube and new main jet, and tighten. Install the new metering plug down tight and reinstall the throttle stop screw. Thread it in till it sticks out the other side about 1/16 of an inch. Next, install the new spring on the needle valve and snap the new needle valve into the new float. Reattach the float into the carburetor by sliding the new pin in. Install the new bowl O-ring, then reinstall the bowl and tighten down. Reinstall the carburetor with a new intake gasket back onto the engine, reattach the linkage and fuel line, remembering to un-pinch the fuel line. Reattach the air filter assembly with the filter. For tips on adjusting your carburetor to get it running at peak performance, see the “Carburetor” section elsewhere on this page.

The ignition module produces the current for the spark plug via magnets on the flywheel, rotating past the module. If the engine runs for a while then dies, you may have a module that is slowly dying, or weak. Hook up a spark plug tester to the spark plug wire coming off the module, then turn engine over, looking for a spark. If there is no spark, check that the connections at the module are tight. To replace the module, un-plug the wires going to it, including the spark plug wire. Remove the bolts securing it in place, and discard. Install the new module, hand tightened, and insert a business card between the flywheel and the module. Rotate the flywheel into place so that the magnets line up with the module. Let the magnets on the flywheel pull the module into place. Tighten the bolts securing it in place and remove the business card. Hook all the wires back up to the module.