Engine John Deere L120 Engine Surging

[mattymax]

John Deere, Home Depot Special, L120 with the Briggs engine
Full tune up done last may. Plugs, filter, fuel filter, oil change will filter. Fuel treated each winter with stable, never a problem

This season the mower has developed a problem. Full tank of gas, starts right up and mowes perfectly. Get done mowing. Next time with the fuel a bit low the engine surges and will NOT stay running unless the choke stays on. If the fuel tank, under the seat, is topped off, it doed NOT do it. It starts right up, no choke, and mowes perfect.

The machine has 62.0 hours on it.

What do you guys think ? When it happes the see through fuel filter does seem a bit empty, and if I pump it with my fingers, it immediatly starts up and runs for a few minutes. Obviusly its a fuel delivery issue.

Thanks guys
-matt

 

[mr rojo]

i would check and see if the fuel tank vents are open and breathing properly... If vents are clogged up a vacum gradually builds up till no more fuel can flow to the carbs... vacum is almost non existent in a full tank compared to one that is half full!!

 

[Rivets]

I would check two things. First I would look to see if I have some type of foreign substance in the tank that is blocking the tank pickup tube or drain. When you fill the tank with fresh fuel, you move the debris around and keep it in suspension while you are cutting. While the unit sits it is allowed to settle down and start the problem over. Second, I would check for water in the fuel. I have seen the same symptoms as I described above caused by water which you cannot see.

 

[DaveTN]

Taking it to be a fuel delivery problem, have you thought about putting a small tank, say off a push mower with some gas in it ABOVE the engine so as to let gravity feed into the carburetor, bypassing the fuel pump? Just to see if it is the fuel pump failing to deliver enough fuel. It could be weak and delivering enough to idle, but not enough for full blade engagement. I call this my MASH IV test. We used to have one of those rolling IV units when I worked for Suzuki cycle shop and also at SEARS where we would roll that elevated gas tank and line to an engine to test for that very thing. It's an easy test and would let you know if the fuel pump is bad. You could also try and enrich the main mixture screw...give it more gas on the high idle side. You definitely need more gas at that point when you engage the blades.

 

[prlaba]

I have fuel delivery problem in my JD L120. The engine will start, but I have to keep the choke almost on full to prevent it from stalling. There's barely any fuel showing in my fuel filter. All the symptoms point to a fuel starvation problem.

I confirmed that there was no blockage in the fuel filter and there's fuel flowing from the gas tank. I removed the fuel pump and confirmed that I'm getting pressure out of the pump's air line . I suspect the fuel pump is shot.

With the fuel pump removed, if I blow hard into the pump's air input line, should I be able to feel the impeller turn? When I blow into my pump's line (or suck on the line), nothing happens and the air is s completely blocked.

Bad pump?

 

[Rivets]

I don't know what you mean by impeller? Your pump should be a diaphragm style, which works on a constantly changing vacuum and pressure created in the crankcase. Yes it could be the pump, but I would do the check as posted by DaveTN, by trying to hook up a gravity feed tank and see if the engine will run that way. If it runs, I would suspect the pump.

 

[prlaba]

Thanks, I assumed there was some sort of plastic impeller inside the pump, driven by the vacuum line.

I don't have gravity feed tank handy. There must be an easier way to test the pump directly.

I don't know what you mean by impeller? Your pump should be a diaphragm style, which works on a constantly changing vacuum and pressure created in the crankcase. Yes it could be the pump, but I would do the check as posted by DaveTN, by trying to hook up a gravity feed tank and see if the engine will run that way. If it runs, I would suspect the pump.

 

[Rivets]

Do you have a small funnel that you can attach the fuel line to? It may take two people to do.

 

[Rivets]

Forgot to say plug the output side of the pump or put a second fuel line into a second container and if the engine does start, you will see if the pump is working. I hope you understand what I am saying.

 

[prlaba]

Perfectly, thanks for the great suggestion!