Try disconnecting the muffler.

I had a similar problem with my lawnmower.

I went to the local handyman's shop and talked with the owner.
He suggested (the above), and hot dang if that didn't do it!

He could have taken in the mower and charged me "whatever".
Instead he got to sell me a new muffler, and won a loyal
customer who has been back many times.

I have a lawn mower with a Briggs & Stratton 5HP Quantum I/C engine
that suddenly developed problems. It's run great for the several
years that I have had the mower (bought it used) and the engine has
been well maintained during that period of time (regular oil/air
filter changes, stored inside and out of the weather).

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