Engine: Engine Rpm's Surge Up And Down

The springs in the carburetor allow the butterfly valves to operate correctly, ensuring proper engine performance. If the spring is broken, bent, weak, missing, or installed incorrectly, it can affect the carburetor function. It is recommended that you check the owner’s manual for your engine carburetor, to determine the correct routing of these springs. You will need to make sure they are installed correctly, and you will need to replace any broken, weak, or missing springs.

The carburetor mixes the air and fuel together to the proper mix for combustion. A carburetor that is set up wrong can cause the engine RPMs to surge up and down. To adjust your carburetor, first locate both idle screws on carburetor. One should be marked L (low), and the other should be marked H (high). Next, turn both screws in to shut off the flow off fuel, then back them off 2 turns. Now you can start the engine. Keep in mind that it may not run smoothly at this setting, but it should at least start. Next, let the engine idle for a few minutes, which will bring it up to operating temperature. Start making adjustments by turning the L screw clockwise until the engine starts to slow down. Then, turn it in the opposite direction until it once again starts to slow. Your ideal setting should be at 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 aluminum-cylinder engines 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. Like with the L screw, turn the H screw back the other way until the engine begins to slow. The screw will once again want to be set at the midpoint. Once adjusted, check the engine acceleration by moving the throttle from idle to fast. The engine should accelerate smoothly, but if it doesn’t, repeat the adjustment process until it does.

The air filter is located on top of the carburetor. Its main function is to allow clean air into the engine and prevent 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. This can starve the engine of air and cause a surge in RPMs. To repair the air filter, first locate the air filter cover and remove it. Once the cover is off, simply remove the filter, replace it with a new one, and reassemble.

The purpose of a gasket is to create a seal between two joining pieces, preventing air and fluids from entering areas where they don’t belong. A damaged or worn gasket below the carburetor will pull excess air into the engine, causing the air/fuel mixture to be lean. This could result in the RPMs surging up and down. To replace the gasket, start by removing the air filter and the air filter housing. Next, remove the bolts securing the carburetor down, remove fuel line, and slide the carburetor off. At this point, remove the linkage if necessary. Once you’ve accessed the gasket, check it for damage or leaks. Either way, the gasket will now need to be replaced as they can only be used once. Before you install the new gasket, clean the surface areas of the intake and carburetor. Now you can install the new gasket and hook the fuel line back up. Finally, re-install the carburetor, reattach the fuel line and linkage and re-assemble the air filter housing with filter.

Some engines have a carburetor with a diaphragm that is used to regulate the fuel mixture. Over time, this diaphragm can start to deteriorate, causing fuel delivery problems. Fuel delivery issues can cause the RPMs to surge. To fix the diaphragm, remove the air filter housing assembly and set it aside. Unbolt the carburetor from the engine and remove the fuel line and linkage. Now, remove the gasket under the carburetor and remove the screws on the bottom of the carburetor to pull the bottom off. Remove the gasket and the diaphragm and install both new parts. Install the cover, then re-install the carburetor back onto the engine. Hook the linkage with the fuel line back onto the carburetor and install a new gasket under the carburetor. Finally, re-attach the air filter housing and filter.

The carburetor mixes the air and fuel together to the proper mix for combustion. It is located under the air filter so you will need to begin your repair by removing the air cleaner cap and air filter. Check air filter for dirt and debris at this point and replace if needed. If the air filter was deteriorating, there may also be small dirt particles located inside of the carburetor. If such particles exist, purchasing a carburetor kit, and rebuilding your carburetor will help it run normally. It is wise to take pictures of the carburetor as you’re taking it apart as a reference for when you re-assemble it. The next step in this repair is to remove the bolts holding down the carburetor and gasket. Next, pinch off the fuel line to prevent a fuel spill, then remove the fuel lines by twisting them gently and pulling off. Now you can remove the carburetor and disconnect the linkage. Start the carburetor disassembly by removing the nut on the bowl then remove the bowl and gasket, then 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, and wipe clean. If you notice any corrosion inside the carburetor or the bowl then it will need to be replaced. Reinstall a new emulsion tube and main jet and tighten them down. Install a new metering plug, re-install the throttle stop screw and thread it in 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 into the carburetor by sliding the new pin in. Install the new bowl O-ring, re-install the bowl, and tighten it down. Next, re-install the carburetor with a new intake gasket, reattach the linkage and the fuel line, and remember to un-pinch the fuel line. Finally, re-attach the air filter assembly with the filter. Once you’ve re-attached the carburetor you will need to adjust it properly. See our section on the carburetor for more info on carb adjustment.