My lawn mower was giving me the worst time tonight. The only way I could keep it running was to pull the choke back and the engine would start again. Finally, after 30 minutes of messing with it, the lawn mower kept on. I just put new gas in it. The gas might have been left outside for awhile.

Any troubleshooting tips appreciated.


  • Once the engine is warm, does it run better? From the video, it seems that the choke just has to stay closed longer than normal. Have you cleaned/changed the air filter? Have you cleaned/changed the spark plug? Do you drain the fuel during the off season? If not, have you added any fuel stabilizers or other fuel additives?
    – Tester101
    Commented May 13, 2014 at 12:50
  • Plugged Idle circuit, needs to be given the gumout treatment. Commented May 13, 2014 at 15:32
  • @FiascoLabs can you elaborate?
    – bonhoffer
    Commented May 14, 2014 at 18:20
  • @Tester101, Yes -- after about 30 minutes of pulling, it seemed to run better. I did take out the air filter and dusted it off -- it surged without the filter on. I haven't touched the spark plug. I think we drained the fuel. We didn't add any fuel additives. I do think the fuel might have gotten some water in it, but I'm not sure.
    – bonhoffer
    Commented May 14, 2014 at 18:21
  • 1
    The carburetor has two circuits, a main power circuit and an idle circuit. When the idle circuit gets partially clogged, the governor will drop the throttle shut, the engine partially dies and the governor yanks the throttle back open till the main kicks in. If the idle circuit gets completely clogged, the engine will need the choke partially on to make up for the missing air fuel mixture, will run rich and surge horribly. The carburetor needs to be cleaned out, a can of Berryman's spray can dissolve the crud out and restore operation. 90% of this problem is caused by old contaminated gas. Commented May 15, 2014 at 2:07

3 Answers 3


Many lawnmowers use a plastic vane under the engine shroud that interacts with air moving off fins on the flywheel to work as a speed control mechanism to open and close the carburetor throttle valve in response to the engine speed changing because of loading changes. Lawnmowers often operate in very dusty environments and the linkages associated with the vane assembly can get all gummed up or even stuffed up with dirt. This can lead to the surging type behavior that you are seeing.

The fix for this is to carefully disassemble part of the engine covering and give the vane mechanism and its linkages a good cleaning and re-lube.

Surging can also be caused by dirt particles entering the innards of the carburetor. Virtually all carburetors are equipped with some type of air filter to keep dirt out but sometimes dirt can get in anyway - especially when removing a dirty filter for replacement/cleaning.

The typical fix for that is a complete carburetor removal, dis-assembly, cleaning and reassembly. It is also necessary to fully clean the area where the carburetor mounts and connects to the engine. Any dirt left there can easily find its way into a nicely cleaned unit and set you back to square one. Note that these small engine carburetors can have many small parts that are easily lost so it is necessary to work in a clean area where parts can be found if they try to get away. It is also possible to take a carburetor to a small engine repair shop for servicing if you are leeiry about tackling it yourself.


Usually, it's a quick fix, but you will get fuel on your hands. Tip the mower backwards & weight the handle down on the ground, this drains most of the fuel back into the fuel tank.

Then, you want to remove the carburetor's fuel bowl by removing just its center bolt. Clean the fuel bowl, dump it out & wipe it out. Then, ream & clean out the bolt's 2 holes with a pin or small paper clip. Reinstall the bowl & bolt & you should be solid.

Fuel Bowl And Jet


Often the problem is the use of gasoline with additives such as corn based products that gum up tanks and carbs. As suggested by others, disassemble and clean the carb and tank with carb cleaner. It cost a little more but use premium gasoline.

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