Trimmer: Engine Misfires

The carburetor mixes the air/fuel to the proper ratio for combustion. Fuel that sits for a long period of time, especially in a 2-stroke engine, has the tendency to gum up the inside of a carburetor, which will restrict the flow of fuel, resulting in an engine that is burning too lean. This can trigger a misfire. You can disassemble the carburetor and clean the internal parts on the inside, or purchase a carburetor kit and replace all the internal parts, restoring your carburetor to optimal performance. Remove the covers to access the air filter. Remove air filter housing and air filter, the bolts holding down the carburetor and gasket, and the fuel lines by twisting them and pulling. Then remove the carburetor linkage and carburetor. Next, remove the screws on the bottom of the carburetor and pull the bottom off. Remove the gasket, diaphragm, primer bulb, and the cover. On the bottom of the carburetor, take 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 needed. If you notice any corrosion on the inside you must install a new carburetor. 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. Install the top cover and primer bulb. Re-install the carburetor back onto the engine with a new gasket, hook back up the linkages and fuel lines. Re-install the air filter housing and filter. Re-install any covers on the unit you removed. 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. Back this out until it starts to run rough or slows down, then 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. This setting of 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 little 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 initially. Just don't close the high speed off any more than necessary, no matter how well it runs.

You will find the air filter on top of the carburetor. It allows clean flowing air into the engine and prevents small particles from 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. Saturated air filters can choke the engine of oxygen. Locate the air filter cover and remove it. It may be attached by screws or just a tab. Once off, remove the filter, and check to see how heavily soiled it is. Replace as needed and reassemble. This should be done regularly to prevent problems.

The gasket performs a seal between two joining pieces, sealing off air and fluids from entering areas they do not belong. A damaged gasket below the carburetor will pull excess air into the engine, causing the air/fuel mixture to be incorrect. This will cause an engine to run lean, which can lead to a misfire. To check this, start by removing the air filter, then loosen the carburetor bolts. Pinch off the fuel line to prevent a spill. Remove the fuel line and linkage. Check the condition of the gasket; look for tears, rips, or any indications of leaks. Clean both surfaces where the gasket is located. The gasket will need to be replaced regardless of its condition, as they can only be used once. Re-install the carburetor, hooking the fuel line back up and reconnecting the linkage. Re-install the air filter housing and filter.

The spark plug ignites the air fuel mixture in the combustion chamber. An oily build up on a spark plug will cause the spark plug to misfire. Oily build ups are very common on 2-stroke engines from improper mixture of the oil and fuel. To check this, remove the spark plug, inspect it, clean it off, gap, and re-install. If the spark plug has deposits, corrosion, erosion, melting, or oxidization, replace it as these can impact the operation of the spark plug as well as cause misfires. If the engine spends most of its time revving high, install a cooler running plug to dissipate heat more quickly.

The ignition module creates the electrical current to provide the spark plug with power to ignite the air/fuel mixture. Remove the engine cover and locate the ignition module by following the spark plug wire to it, and disconnect the 2 smaller wires to check the on/off switch operation. Check with an ohmmeter and turn the switch off and on. If you see the ohmmeter change, the switch is good. Check the spark plug with a spark plug tester or by removing the spark plug and grounding out the end on the engine block, and pull the start rope to look for a spark. If there is no spark, remove the bolts holding the module in place. Replace with the new module. Next, you will need to set the gap between the module and the magnets on the flywheel. You will need a feeler gauge for this as well as the manufacturer specs for your machine, if you do not have these specs then a good general rule of thumb is to use a business card as it is usually the appropriate thickness. Insert the business card between the module and flywheel, then spin the flywheel around so that the magnets are in the direction of the module. Allow the magnets to pull the module into place, then tighten bolts down on the module and remove business card. Hook the wires back including the spark plug wire and reinstall the engine cover.