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On a John Deere La120 lawn mower the fuel pump operates on pressure/vacuum impulses from the crankcase. In diagnosing a no start I found that the carburetor had debris(?) and varnish that plugged the inlet but when that was correct and the engine started it ran well until the blade was engaged and the throttle opened, resulting in a sputtering stall. I used a vacuum pump to actuate the fuel pump and the engine started and again ran normally until under load. The fuel pump is new, replaced by the owner in his efforts to get his yard mowed but it seems that it isn’t putting out enough volume or pressure to keep up with the engine’s demand. Is anyone familiar with that type fuel pump? At this point I am looking to install a low pressure electric fuel pump in desperation. There is no definitive method to test such a pump and it does function as seen using an external vacuum source.

The fuel line from the tank is free flowing

there is a new filter installed

the fuel cut off solenoid is operating flawlessly

as long as fuel is available the engine seems to run great…

But I’m stumped.

See if this John Deere manual is of any help.


What engine does it have? Briggs, single cyl, twin? Not that I know anything but the fuel pump on my twin is just a flapper on the carb. I had one that only lasted two weeks until I switched to non-oxy gas with B&S stabilizer. Of course sounds like lack of fuel or lean but those little passages in the carb are really troublesome with bad gas.