My gas trimmer turned toes up to the daisys this year. My neighbor gave me his old trimmer, a Mcullough 385, and told me that it wouldn’t run; it would start if you put the fuel into the carburetor, but it wouldn’t draw fuel from the tank. Investigating, I found that the fuel line had a crack in it, so I replaced it. The problem is this. I can start the engine on full choke, but as soon as I take the choke off even a tiny bit the engine dies. If I leave the coke on full it idles but won’t rev, and dies in 15-20 seconds anyway. Does anyone have any idea what might be causing this, and the solution? Thanks in advance.
My guess is that the carb is not suppling enough gas. This could be due to debris in the jet, or a stuck needle and seat. The engine will run on choke, because the air supply is cut down to what the carb can supply, however when the engine starts to warm up the mixture becomes too rich, and the engine dies.
If it were mine, I would replace the spark plug, and disassemble the carb and clean.
WARNING: the carb may require a kit as some gaskets can be destroyed in the disassembly process, and there may be many small parts that can get lost. These small parts are not spares, they were put there for a reason. If you do not feel comfortable doing this work, take it to a pro.
If there ever were a proper place for a digital camera, this would be it. Besides putting little marks on every single adjustment screw to remember its old position, I’d take loads of photos throughout the disassembly.
Of course, having been present while a friend attempted to rebuild his truck’s carburetter, and then had to buy one at the junkyard, I would hesitate to mess with carburetters.
Thanks for the advice. The entire carb isn’t that big and it’s very easy to remove as a unit, do you think soaking the entire thing in Gumout overnight would be helpful?
Probably not. These carbs have a rubber diapragm that uses crankcase pressure differential to pump fuel. If the diaphragm gets stiff, it won’t pump correctly. It may just have some congealed fuel/oil in the orfices. If it was mine, I’d get it running and open the high speed needle (usually marked with “H”) up until it’d stay running. Sometimes this will help flush the funk out, but a carb kit is pretty cheap and your local lawnmower shop will have one.
Here’s a pic of a typical Walbro (common type) carb.