[KULTULZ]

WHISTING THROUGH THE GRAVEYARD

Originally Posted by bonanza7369

I have a 56 with 292 engine 4 barrel tea pot carb. the carb was rebuilt last year. I am getting some backfiring thru the exhaust when engine braking.

The car has glasspack mufflers.

Could this be caused by a to rich fuel mixture or possibly timing problem.

Engine runs smooth and accelerates well. It also is hard to start after it sits for a couple days(takes a lot of pumping of the gas pedal)

Originally Posted by 40 Deluxe

All true gearheads love that backfire sound! Roll the window down and enjoy! (Or am I giving away my age here?!)

This is caused by the presence of both unburned fuel and oxygen in the hot exhaust manifold. The heat or flame from a cylinder's open exhaust valve ignites this unburned fuel-bam!

Under the right conditions, she'll really cackle!

Correct Analysis!

When the throttle blades close, less air/fuel mixture is admitted to the cylinder, resulting in less cylinder pressure. This results in a slower-burning mixture and part of un-burned fuel mixture tends to cool off the rest of it, slowing down the burn even further. The part of the mixture that still has not ignited when the exhaust valve opens is forced into the exhaust system where it is ignited by the hot pipe.

The popping is eliminated when you use full manifold vacuum because the timing is advanced to the point where the mixture begins burning earlier and has time to burn more or less completely before the exhaust valve opens.

I found this explanation on another forum and as I could not describe it better myself, chose to cut and paste and edited somewhat.

EXCEPT!...

Using only manifold vacuum for the vacuum advance feature is not the way to go IMO.

The 1956 Y had a dual diaphragm vacuum advance canister and the rearmost chamber is attached to full manifold vacuum. This feature allows full vacuum advance (over-riding the LOM signal) @ engine deceleration by burning the mixture with closed throttle plates more completely eliminating the delayed IGN in the exhaust system.

Most likely either the canister is defective and/or the manifold vacuum signal is not reaching the canister.

!!! EDIT !!! - CORRECTION! - GIG!

The 1956 Y had a dual diaphragm vacuum advance canister and the rearmost chamber is attached to full manifold vacuum. This feature allows full vacuum advance (over-riding the LOM signal) @ engine deceleration by burning the mixture with closed throttle plates more completely eliminating the delayed IGN in the exhaust system.

The FRONT CHAMBER is connected to full manifold vacuum, the REAR CHAMBER is the LOM feature signal.

...sheesh...