Trimmer: Engine Rpm's Surge Up And Down

The carburetor mixes the air and fuel to the proper ratio for a clean combustion. Sometimes a small piece of dirt may enter the carburetor and get in the way of the small parts inside, such as the needle valve, which can impact the flow of fuel, causing the engine to surge up and down. Taking apart the carburetor and giving it a good cleaning on the inside can solve this problem. Remove any covers in order to access the air filter. Remove the air filter housing and air filter, and the bolts holding down the carburetor and gasket. Next, gently remove the fuel lines by twisting and pulling them off, then remove the carburetor linkage. Remove the carburetor. You may want to take pictures during the dismantling of the carburetor, to enable you to accurately put it back together later. Next, remove 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, notice 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 as it is no good anymore. 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, and hook the linkages and fuel lines back up. Reinstall the air filter housing and filter. Re-install any covers 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 start screaming, then 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. 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 initially. Just don’t close the high speed off any more than necessary, no matter how well it runs.

The air filter is located on top of the carburetor and filters debris flowing into the engine, to prevent small particles from entering and damaging the cylinder walls and other 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 and power. Clogged air filters can cause RPMs to surge up and down. Simply remove the air filter cover by the tabs or screws that hold it in place and replace. This should regularly be done as preventative maintenance.

The fuel filter keeps dirt from entering the carburetor. Sometimes a piece of debris can block the fuel ports, closing off the flow of fuel for a bit till it moves out of the way via engine vibrations. Insert a hook into the tank and pull the filter up. Check the filter. Filters are inexpensive, so it’s a good idea to replace yours at this stage, regardless of its condition. Pull it off the fuel line by gently twisting and pulling and install the new one.

A damaged gasket below a carburetor will pull excess air into the engine, causing the air/fuel mixture to be lean. This can cause the engine’s RPMs to surge up and down. Start by removing the air filter housing and filter, then loosen the carburetor bolts. Pinch the fuel line to prevent fuel from spilling everywhere. Remove the fuel line and disconnect any linkages on the carburetor. Check the condition of the gasket; look for tears, rips, or any indication of air entering. Gaskets are a onetime use so you will have to replace it at this point, regardless of its condition. Clean both surfaces where the gasket sits. Re-attach the linkage and fuel line and bolt the carburetor back down, then re-install the air filter housing and filter.