Asimov

Not sure what you meant by "XX bravely wrote" but--Tut "bravely writes"
more again.

Briggs makes a 3.5 HP engine matted to a 20 Murray mower sold by
Walmart for $99 US. My experience is that if the air cleaner (which
uses a sponge filter) is dirty--which happens frequently--the engine
rpm will search for neutral idle. Perhaps because the mixture is over
rich and the engine Air Fuel ratio changes on the "fish hook" pattern
as engine speed changes--or perhaps because manifold vacuum fluxuates
excessively causing irratic flapper valve overshoot action--not sure.
Cleaning the sponge is quick, easy, cheap--and requires no $$.

The 3.5 Briggs engine is also sold in a throttle free
configuration--meaning no throttle control for the operator. In this
case, a small thin bar toward the front of the engine holds the idle
springs at the proper tension thus preventing the engine from
unneccessary RPM searching. The bars location is just at the proper
height to be bent rearward when the mower is pushed under low
brush--this reduces the idle spring tension and the mower will RPM
search.

Tuts suggestions/comments.

1. Clean the filter sponge and let us know if the problem exists.
2. Does your mower have an adjustable throttle control on the handle?

The answer may not be here--but a great starting place--short of carb
teardown or flywheel removal--wouldn't you agree?

Tut