I'm thinking it's probably caused by running on the ragged edge of too lean.

1. Check for any loose intake/carb mount bolts.

You might try running a STRONG dose of carb cleaner through it for 1/2 tank of fuel and see if that helps.

There's several parts that could contribute, such as a diaphragm being on the brittle side and not flexing enough to properly do its job or a check ball.

You might download the IPL for that engine from the Briggs website and see the parts I'm referring to.

Does the problem happen under load?
If not, it might not be worth worrying about.

I have a Briggs engine, on a push mower, that does the same thing! I have changed everything on it, to no avail. Still does it. It now resides under the big pine tree in my side yard, a light cover on it. I gave up! I don't do that very often, but that one deserves the sledge hammer trick!

Thanks Bill, i'll try the carb cleaner and then ultimately put a diaphram kit through it. All carb bolts etc are ok coz i torqued them all. It does it under load but to no detriment to the power, just misses a beat once in a while. When all else fails it'll stay the way it is because it runs fine. The only ball valve in this model is in the primer and sometimes they stick, but the symptoms are different.
I'll test it out over the next few days and report my finding back here.
Thanks again.