One other thought comes to mind. **** used YF's on everything from the 144 to the 300 (different sizes). If the carb you have is from a smaller engine, additional vacuum signal from your 300 could cause a syphoning effect. What is the ID number stamped on the carburetor (4 digits followed by the letter 's' as in Sam)?

Incidentally, got just a little over 500K on my 300 4-speed with 4 barrel before the rust gremlins ate the body for the last time. Head never removed from engine. Tough engine.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop