Cub surges when running.

[Poonie]

What would cause this? I ran my cub up and down the road the other day and it started surging. It surges even at idle. Could it be a clogged or partially clogged jet, choke butterfly opening and closing? Not sure. Any help appreciated.

 

[used red MN]

It is very likely a passage in the carb has become plugged. Grab the rod going to the carb and hold it in one spot, if the engine still surges it is in the carb. If the engine runs steady then it is the governor. There is a buffer adjustment on those, I am not sure what the proper method is to adjust that if needed.

 

[timcasbolt]

I had the same thing the other day with the tractor in the picture (predator style engine). After working it pretty hard for an hour or so it got better on it's own. My best guess would be that the ethanol in the fuel managed to dissolve the offending gunk.

 

[Rodney51Cub]

If you Cub has an IH carburetor pull the main jet and clean it.

 

[Poonie]

If you Cub has an IH carburetor pull the main jet and clean it.

View attachment 93006

My cub has a Zenith carb. I've cleaned the main jet before. I'll pull it again and see if that's the issue.

 

[Brownie450]

With engine off, move the governor to carb rod
to check for any binding or "hard" spots in it.
Jim

 

[560Dennis]

Fuel starve as you thought

 

[Noah H.]

Sounds like either a clogged carb or a fuel delivery issue. If it is a plugged carb cleaning out the jets is a great idea, if it is a fuel delivery issue open the carb drain plug and see that a generously sized and steady stream of fuel flows out, and continues to flow after a minute or two (you'll want a clean jar or jug to catch the fuel in). If not, shine a light down into the fuel tank, the sediment bowl intake in the fuel tank is likely plugged with bits of gunk from the inside of the tank. There could also be gunk clogging up the fuel line or the needle valve could be stuck mostly closed. In any of these scenarios the gunk plugging up the fuel system will have to be removed.
Noah

 

[47fivewindow]

When the point gap gets real tight strange things happen. If the carb doesn't fix it check your point gap.

 

[Poonie]

I cleaned the main jet then rain the engine for about 15 minutes at varying throttle settings. The surging seems to have gone away.

 

[Janicholson]

good solution thanks for posting a conclusion. Jim