It sounds like lack of fuel to me also.
Can you spray starting fluid into the carb to see if the RPM picks up?
(without getting chopped to bits, I mean.......)
As a Yamaha motorcycle mechanic in the 80's we used an Allen Test and later a Sun Exhaust Gas Analyzer on the 4-strokes.
It read CO's (burned carbons) and HC's (unburied hydrocarbons).
A lean misfire would have low CO's (burned fuel) and High HC's because it was getting little fuel which caused a misfire and the unburied fuel showed up as HC's. We'd spray Berryman's carb cleaner or WD40 at the intake manifolds and if the CO's picked up we knew they were leaking. If not, we'd spray it into the intake. If the CO's picked up we knew there was a lean mixture. (Sadly, both Berryman's and WD40 were neutered a decade or 2 ago and no longer work as a starting fluid. And lighter fluid no longer works for tennis ball cannons
A rich Mixture would show high CO and high HC.
An ignition misfire would show no CO's but very high HC's.
I have not seen an analyzer like this at an FBO, but I just did a search for "Portable Exhaust Gas Analyzer" and there are a bunch of them on the market.
We also used an in-line spark checker frequently. It provides a gap where you can watch the spark jump inside a clear plastic housing, or watch it fail. You can increase the gap by twisting the housing to the point where the spark can't jump it.
As an aside, I chased a similar issue on my snowblower for a few seasons. When under a big load it would suddenly die. I was sure it was an ignition issue because it cut off without warning, but ultimately discovered that it had a non-vented gas cap on it from another model. The vented gas cap from my lawnmower solved the problem with a twist of the wrist.
