I had this same problem one time with my original '52...heck, back in the late 70's. I found my problem to be junk in the float bowl that didn't start messing around with me until I put it in gear and got the RPM's up.
Being a young, stupid kid I tried downshifting from 3rd to 2nd going @ 45 or 50 and guess what? The tach jumped right up (D-uh!) no sputtering or spitting....until I put it back into 3rd and started trying to accelerate back to cruising speed.
Did it one too many times apparently. Blew the snot out of my tranny. Felt like an utter fool after I replaced the tranny then decided to learn how to rebuild a carb. I have no idea how a chunk of whatever it was got in there, but sure as heck, cleaned it all up, finished the rebuild, back on and ran like a charm for years after that!
