NA - Mower problem - surging, backfire

[ateamer]

I have a John Deere E120 with a Briggs & Stratton 20 HP engine. I used it for three years, and then it mostly sat for the last three years because I hired a lawn service . I’m about to fire them because we pay for weekly service (barely often enough in the Florida summer) but they only show up every 10 days or so and don’t refund or waive 50% of the price. So I’m getting ready to go back to doing it myself.

I replaced the battery, it has fresh oil and air filter, spark plugs are good, and I put in fresh ethanol-free gas. The engine surges at idle - cold or warm, even after 15 minutes of running at full throttle mowing. I removed the carb, cleaned it thoroughly - all jets and passages are clean and clear - replaced the gaskets, reinstalled, and it surged a few times and then settled down. The various rods and springs are connected as they were before.

Today, I fired it up and it ran pretty smoothly for a few minutes, and then went back to surging. It’s also backfired a couple times on shutdown. The fuel filter is clear and still looks new, but only fills up slightly when running. The fuel is clean - no debris visible - and fresh.

What else should I be looking at?

 

[Flatiowa]

The governor might be gummed up from sitting that long. If it were mine I put a healthy shot of marvel mystery oil in the oil and a bit in the gas then run it under load for a while. A half hour or so after reaching full temp, change the oil and add a bit more MMO. (It has worked wonders smoothing out a few different engines for me with similar symptoms/ circumstances).

 

[Racerx]

Spray some carb cleaner around the carb while it's surging. If it clears up, it's sucking air.

I have had an ongoing issue with my Kohler on my welder. Been getting worse. Have ruled out fuel. Seems to be an issue with the egov system. Either the e gov itself or the magneto's not sending the proper pulses to the governor.

When you say plugs are good, are they new?

 

[Dan Thomas]

Spray some carb cleaner around the carb while it's surging. If it clears up, it's sucking air.

Common problem now. The carb's manifold is often plastic, and it distorts with heat and the weight of the carb so that it no longer seals properly at the carb or cylinder head. Some of these had O-rings instead of gaskets, and the O-rings take a set and can't seal when the manifold distorts.

I had that problem on my lawn tractor. 17 HP engine. I had surging and backfiring and poor idle. I found that the manifold no longer had nice level mating surfaces, so I ran those faces across my disc sander and got them flat, replaced the O-rings, and made gaskets to go between the manifold and carb and head, too.

I also found that there is a small plastic plug in the top of the carb that meters air into the idle circuit. Really tiny hole in it that plugs up with old fuel and upsets the idle mixture. That plug pulls out and you can run a wire through that hole. Make sure the O-rings on the plug are still useable.

Runs great now.

 

[AvNavCom]

Common problem now. The carb's manifold is often plastic, and it distorts with heat and the weight of the carb so that it no longer seals properly at the carb or cylinder head.

I was thinking about this the other day. I'm really getting tired of everything being made out of plastic. I get that it's cheaper and lighter, but it generally doesn't hold up to heat and time. I'd rather pay a little more and get metal that has a reasonable chance of holding up.

 

[Mikey52]

Does yours ha the electric solenoid that closes when you turn key off. Mine went out, and it afterfires 3 or 4 times on shutdown.