I fixed an issue with my lawn mower but was surprised not to easily find
many references to the problem in my google searches. So I'm posting the
symptoms I experienced, some details on how the fuel system works and how
I fixed it. I'll be trying to use key words I can remember searching on.
It'll help me in the future if I forget, or maybe help someone else when
they go searching.

I have a fairly cheap MTD mower with the Briggs 4hp motor on it. People
powered with no self-propelled equipment attached. Last season the mower
started acting up where it would seem to surge, or the engine would slow
down, the speed up. It seemed that keeping the fuel tank full helped.
Then it got worse. I didn't find any help on google so I gave it a shot
myself.

What I found was I obviously didn't have a clue what I was doing. First I
took things apart and didn't pay attention so when it went back together,
it was missing a part. Ah yes, the parts.

How these briggs and stratton motors get fuel.
Apparently on many (if not all) of there cheaper models, briggs now uses a
carburetor setup that is attached directly to the fuel tank. The fuel
tank has machined into it's top various ports almost like a carburetor
valve body. The bottom of the carburetor has similar ports and orifices.

The assembly is:

Carb on top
Fuel Pump Spring
Diaphragm
Gasket
Fuel Tank.

Of course I didn't realize that the diaphragm was separate from the
gasket. This because after 2 years of use the diaphragm had become fused
to the gasket, making it all just look like a gasket. So I didn't
understand that the diaphragm is actually the "fuel pump" of this carb.

The carburetor uses engine vacuum in concert with a spring to move a
section of the diaphragm creating a pumping action to pull fuel into the
carburetor. Over time this diaphragm will become stiff and not pull as
much fuel. This can lead to a lean condition that creates the surging
that I was experiencing.

The fix is to replace the diaphragm. Unfortunately what I did was just
clean things, bolt it back together and forgot to put the spring back in.
As you can imagine, the mower didn't work any better, though it didn't
appear to work much worse. My wife thought it needed more oil and
overfilled the crankcase causing it to blow oil out the exhaust. So we
limped through the fall and figured on buying a new mower this spring.

I gave it another shot on google and finally hit the right search terms to
give me a clue on that I might be dealing with a lean condition. I then
searched Briggs website and found out that what I thought was a gasket was
really the diaphragm. The guy at the hardware store explained how it
works to me, and sold me a carb kit that included the diaphragm, gasket
and spring (along with a o-ring, and plastic thing that connects the carb
to the intake tube on the engine).

So that's the story. From an engine that was speeding up, slowing down,
surging, and idling odd to a motor that now is purring as only a 4 cycle
small engine can.

Of course now that I can fix this mower, I've since fixed a in-law's power
washer that had a similar engine on it with a similar problem.

I have a 4 hp BS engine that is doing the same thing. I removed and cleaned the carb, replaced the spring (after i bent it) but I didn't even notice the diaphram. I want to thank you for your posting. A carb kit is my next stop.

My mower is over 10 yrs old and has been faithful to me every year and i would hate to part or replace her.

My mower had been doing exactly the same thing and I also struggled to find information regarding the cause. Even a book I had regarding repairing Briggs and Stratton engines never explicitly detailed this problem / remedy.
It was the diaphragm and replacing it solved the problem. Many thanks for your posting. Spot on !

Another satisfied customer. thankx for the post!