engine backfire when slowing down

[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)

[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!
If you really want to quiet it down, check that the timing is OK (if you have the original Load-O-Matic vacuum only advance, make sure it is working). Maybe richen the idle mixture a little. May be comparing apples and oranges here, but with my Model A in high school, if I wanted it to backfire more, I would close the mixture rod (GAV) to lean it out and throw the spark lever up to full retard. Result: lots of noise (blown muffler). If I left the timing advanced and kept the mixture knob open, it was much quieter on deceleration.
Finally, add a dashpot to the carb. This slows down the closing of the throttle plates instead of them snapping shut when you back off. They were standard on auto trans cars back then.
For the hard starting, the float bowl may be leaking into the intake. Does it act flooded after sitting a couple of hours? Try looking down the secondaries with the throttle held open. You will get a couple drips from the accelerator pump, but if you see puddles, the carb has issues.

[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...

[dmsfrr]

Originally Posted by KULTULZ

.....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.

Very possible. Because the '54/'55 & '56 distributors use only vacuum to advance the timing it is more critical that the vacuum canister is working correctly to avoid potential damage to the exhaust system or engine, see this link...
https://www.ctci.org/gilsgarage/Crac...stManifold.php

The vacuum diaphragms can be checked fairly simply with a 4 to 6 ft piece of flexible vacuum line.

The '54/'55, '56 and '57 distributors use a different style vacuum canister each year, and are vacuum pressure matched to a carb from that year. It's my understanding the '57 and newer carbs & distributors use a common/interchangeable vac pressure standard.
https://www.ctci.org/gilsgarage/Vacu...Mechanisms.php

Originally Posted by willowbilly3

I'd check the points. Modern points are junk and the rubbing block goes away pretty fast until it gets seated. When the points start loosing their gap, you will get pop on deceleration.

Can the small tubes of points cam / rubbing block lube still be bought at auto parts stores? I remember using it back-in-the-day and my slightly dusty project will be needing some when I can get back to it.
Hopefully it's compatible if the points rubbing block is made from different types of plastic. I've seen some that look like a darker phenolic/bakelite and others are a lighter almost white plastic.

[KULTULZ]

Originally Posted by dmsfrr

Can the small tubes of points cam / rubbing block lube still be bought at auto parts stores? I remember using it back-in-the-day and my slightly dusty project will be needing some when I can get back to it.

Hopefully it's compatible if the points rubbing block is made from different types of plastic. I've seen some that look like a darker phenolic/bakelite and others are a lighter almost white plastic.

Yes...

[KULTULZ]

Sponsored Links (Register now to hide all advertisements)

OH! BTW-

FORD (MOTORCRAFT) points and condenser are still available from parts vendors and/or found on E-bay-

1954-56 Y-BLOCK- Points FAB 12171-B - Condenser FAA 12300-A

1957/ Points B8Q 12171-A Condenser B7A 12300-C

The B7A Condenser seems to be hard to find NOS and to be mainly aftermarket (CHI-COM).

With all of the specialty shops still out there, there has to be those that specialize in LOM distributor rebuild and/or re-curve.

[Lanny]

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)

================================================


If the carb is draining the gas out and down into the engine, and leaving the carb dry,
you possibly have a leaking power valve.







.

[Sid]

Also sometimes if fuel is dripping in the venturi after engine is turned off when warm you may see a smokey white vapor coming out of the carb. Also if you have an exhaust leak near the exhaust manifold it may cause exhaust backfire.

[Johnnydidd]

I am with 40 DELUXE, sit back and enjoy the sound, every car I had after putting on glass packs will pop back, my 56 to slow down I will pull down to low and she will really crackle. also we use to block off the heat risers with pennies on the flat heads to get them to pop back.

[dmsfrr]

Originally Posted by 40 Deluxe

.... add a dashpot to the carb. This slows down the closing of the throttle plates instead of them snapping shut when you back off. They were standard on auto trans cars back then.....

Here's a photo that sort of shows the dashpot, if yours in missing.
.

[dmsfrr]

A bit of popping when decelerating is normal and sounds great. The closed throttle plates and higher vacuum cause the mixture to go rich. Double check the color of the spark plugs or the color of any soot buildup at the ends of the tail pipes.
If it's too dark the normal running mixture could need to be leaned out a bit.
.

[scicala]

It's virtually impossible for a leaking power valve or economizer diaphragm to drain the bowl on a Holley 4000, 2140, 885F or ant teapot style Holley carb, 2 or 4 barrel.
Completely different design than the other Holley power systems. (like the Holley 94 type carbs).

Sal

[Ole Don]

Modern fuel today will evaporate sooner than the old stuff. To solve the hard start after a few days, install an electric fuel pump under the back seat, (on the outside of the floor pan), and use a toggle switch to activate for 30 to 45 seconds before you try to start it. The engine pump will pull through it for normal driving.

[willowbilly3]

I'd check the points. Modern points are junk and the rubbing block goes away pretty fast until it gets seated. When the points start loosing their gap, you will get pop on deceleration.

[dmsfrr]

Originally Posted by willowbilly3

I'd check the points. Modern points are junk and the rubbing block goes away pretty fast until it gets seated. When the points start loosing their gap, you will get pop on deceleration.

Yes, that too.

[34pickup]

If it is an actual backfire and not the popping described by others, it could be a leak in the exhaust drawing cold air into the exhaust. I had that happen on my 56. I fixed the leak and stopped the occasional backfiring.

[Milt/Las Vegas]

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)

I'm having the same problem with a 351w with 2X4s in a 40 ford.I have rebuilt both edelbrock carbs,changed plug wires,and checked for exhaust leaks.

[KULTULZ]

Originally Posted by Milt/Las Vegas

I'm having the same problem with a 351w with 2X4s in a 40 ford.I have rebuilt both edelbrock carbs,changed plug wires,and checked for exhaust leaks.

Read This- https://en.wikipedia.org/wiki/Back-fire

See if it makes any sense and get back.

[Fordestes]

Another source for ignition is Bubbas in Indianapolis . He is great to deal with and Knows his stuff. check on the flathead v-8 side of the Fordbarn.