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I have a 2006 MTD push lawnmower, model 11A-589B795, with a 6HP Briggs and Stratton engine. This spring, the engine would only start with help of starter spray, run OK for a few minutes, then the speed surges up and down and then dies. The interesting thing is that it will restart almost immediately, and run for another 5 minutes until it dies again, which greatly increases the time it takes to mow the lawn.

Here is what I have done to date:
1) Removed the plug at the bottom of the fuel bowl, cleaned with carb cleaner and blown out with air (multiple times)
2) Drained the gas tank, replaced gas with fresh gas and fuel system cleaner added
3) New spark plug
4) New air filter
5) Bought a carb rebuild kit from B&S - this carb evidently does not have diaphragm because there was not one in the kit, just various gaskets and new fuel metering pin. Removed carb, cleaned with carb cleaner, reassembled with new gaskets and fuel metering pin.
6) New fuel tank cap (loosening or removing the cap when it starts to surge and die makes no difference)

Any other ideas to try before I replace the lawnmower? I know this has been covered already a thousand times. Thanks in advance for your input.

I'm assuming that when you rebuilt the carb you replaced the float needle and seat. Did you reset the float level? What Briggs engine do you have? Model and type numbers please.

Rivets said:

I'm assuming that when you rebuilt the carb you replaced the float needle and seat. Did you reset the float level? What Briggs engine do you have? Model and type numbers please.

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I did replace the float needle; the original one did not appear to be worn or have varnish buildup. I do not believe there was a replacement seat in the kit, I might be wrong. I don't believe the float has an adjustable level, it just pivots on a pivot pin (which I also replaced). The B&S model number of the engine is 122k02-0337-E1. Unfortunately the rebuild kit was set up for a variety of B&S engines and the instructions were on a very tiny piece of paper.

lewist57 said:

I did replace the float needle; the original one did not appear to be worn or have varnish buildup. I do not believe there was a replacement seat in the kit, I might be wrong. I don't believe the float has an adjustable level, it just pivots on a pivot pin (which I also replaced). The B&S model number of the engine is 122k02-0337-E1. Unfortunately the rebuild kit was set up for a variety of B&S engines and the instructions were on a very tiny piece of paper.

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Just to close this out. The engine would run for a few minutes, then the speed would surge, and then die. The problem was the float needle seat, which is made of rubber or a rubber like material. The ethanol in the gas caused it to distort.

Why would it cause these problems? When you first start the engine, the carb fuel bowl is full of fuel. The engine draws this fuel out, but after a few minutes, the gas level drops in the bowl, as does the float, and the float needle. When the float needle drops, gas flows though the rubber needle seat. However, the ethanol has caused the hole in the seat to shrink, so now the amount of gas flowing into the bowl is slightly less than what the engine needs. So for a while, the engine speed will drop some due to the lack of gas, then speed back up as the amount of gas catches up, and then repeats the cycle over and over until the engine finally dies. By replacing the float needle seat, the correct amount of gas is provided to the carb fuel bowl, and no more surging, no more dying after 3 - 4 minutes of operation. Problem solved for $4.

Your explanation may sound correct, but there are a few problems with it. The seat does swell up due to alcohol (ethanol) in the fuel. When it swells it gets larger or fatter, which means that the needle will stop the flow of fuel into the carb bowl sooner. This will result in a fuel level in the bowl to drop creating a lean mixture. The governor will open the throttle valve when the engine slows down, which will draw in more fuel. These two things will cause the engine to surge, now the governor will close the throttle plate and we will repeat the process. Once you replace the float needle and seat, to get the correct float height, problem solved and engine runs good.