Right now the best "it could be ..." is the head gasket, Paul.
So far as spraying fuel into the carburetor is concerned, I don't see it as having much different effect from partly closing the choke, which we already know, made it run better. However it also made it wet its spark plug and plaster its air filter with puffed-back fuel. As a general rule, don't spray fuel into the intake of an engine that is puffing back through the carburetor, for safety reasons. A faceful of burning petrol is definitely not something to look forward to.
As an aside, when you are opening the throttle is one of the times engines are most likely to puff burning fuel back through the carburetor. It is called a "spit", and is a symptom of lean mixture. The old guy may or may not know what he is talking about in general, but in this case, simple prudence should tell you not to do it.
At the moment the engine will not run at steady speed, so doing tests while opening the throttle simultaneously is rather irrelevant. It mixes two variables: steady state running conditions, and acceleration conditions. His test would be relevant (though excessively dangerous) if the engine would run at steady speed, but would not tolerate an opening throttle situation. Poor running then would indicate insufficient acceleration enrichment. By the way, the classic sign of mildly inadequate acceleration enrichment is the engine hesitating. Severely inadequate acceleration enrichment is often indicated by a spit. Spits are a bit disconcerting when the air cleaner is in place, but with it removed, they are rather frightening. No matter how well an engine is running, never stand in front of the air intake with the air cleaner removed. Have you ever seen anyone who's had a faceful of burning petrol, even several decades before? It's a sight you won't forget.
