I recently bought a truck with a 477 in it.In the process of evaluating its problems I noticed that the vaccum secondaries were not opening.The carb is a holley and is the correct model for this engine,(I know this because it has linkage connected to it that is specific to this engine).The first thing that came to mind was a ripped diaphram in the secondary acuator.After taking it apart I found that the diaphram was not ripped,however there was a small ball like a B.B. blocking the vaccum port.This B.B. was obviously put there by the factory,but why?I removed the B.B.and got a lot more power out of the engine but it seems like it might be getting to much fuel because now it backfires when you let off the gas.(one really loud bang!!!!)What the heck is the deal?
I remember the old man talking about a "check ball" in the 4bbl on his 292 truck motor. This was a governed Holly, with vacuum secondaries. The idle was off the secondaries because of the governer.
If the check ball was missing, the secondaries opened quite easily, resulting in 4 mpg out of the F600 the 292 was in.
The carb is obsolete and you may not even be able to get a rebuild kit and drawing showing how/where etc the ball is supposed to be installed, and how the secondaries are supposed to be allowed to open.
If so, follow the previous advice and get a new carb.....
If you want to save some money on a new carb, call around to some of your local carb rebuild shops. Most have unclaimed carbs they sell for half the price of a new carb. A friend told me this little secret and I found a carb for my old old truck for $65.00. A new carb would've cost over $300. Plus, I've found a rebuilt carb for my old Ford 2-N tractor for $40.00.
"The check ball is a way to meter the vacuum signal. If you look under the ball there is a groove for vacuum flow. It smoothes out the secondary opening. When the primary throttle shuts, the check ball opens and allows the secondaries to close instantly.
If you remove the ball, the secondaries can open more quickly. Maybe too quickly. Remember there is no squirter on the seconday side. Opening the secondaries too quickly can cause a stumble." -Clevelandstyle