1966 backfires and misses at low vacuum | Ford Thunderbird club group 1955-2005 T-Bird models

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1966 backfires and misses at low vacuum

  • Thread starter Thread starter Sun lover
  • Start date Start date
Sun lover

Sun lover

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Thunderbird Year
1966
I have a 390 with a 4bbl that seems to have suddenly developed a problem. When I attempt to accelerate at anything that would be a low vacuum condition. It will act poorly and try to backfire through the carb yet it will idle fine. I have checked the vacuum advance and it is moving when vacuum is applied to the diaphragm, the centrifugal counter weights feel good and I have the point gap set at .016 The cap and rotor look good and I don't think that the timing would suddenly change. at an idle, my vacuum gauge reads between 15 and 16 inches and when I connect the gauge directly to the distributor line for the advance I show between 6 and 7 inches when I race the engine.. Any ideas?

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Maybe ignition related? How is the condition of the spark plug wires. Maybe a plug wire is shorting out and the spark is jumping across to the plug wire next to it. Also, check the firing order for any crossed wires. Check the ballast resistor and the wire going to the distributor for chafing or a break in the wire that would react when the motor is vibrating as it speeds up.
Also, maybe a restriction in the exhaust system flow? - a frozen exhaust heat riser stuck in the closed position on the exhaust manifold or maybe a collapsed muffler/ collapsed exhaust pipe causing restricted exhaust flow?
 
Recently had similar problem with my 57 Bird
After chasing things for a few hours I put my spare coil on. Problem solved!
It just didn’t act like a coil issue?
Same symptoms, would start and idle fine, but when you accelerated it would back fire through the carb.
 
68ChevyTruck said:
Recently had similar problem with my 57 Bird
After chasing things for a few hours I put my spare coil on. Problem solved!
It just didn’t act like a coil issue?
Same symptoms, would start and idle fine, but when you accelerated it would back fire through the carb.
Click to expand...
68ChevyTruck said:
Recently had similar problem with my 57 Bird
After chasing things for a few hours I put my spare coil on. Problem solved!
It just didn’t act like a coil issue?
Same symptoms, would start and idle fine, but when you accelerated it would back fire through the carb.
Click to expand...
Thanks, I will try a different coil, I am relatively sure that it is something electrical
Iloveoldgrease said:
Maybe ignition related? How is the condition of the spark plug wires. Maybe a plug wire is shorting out and the spark is jumping across to the plug wire next to it. Also, check the firing order for any crossed wires. Check the ballast resistor and the wire going to the distributor for chafing or a break in the wire that would react when the motor is vibrating as it speeds up.
Also, maybe a restriction in the exhaust system flow? - a frozen exhaust heat riser stuck in the closed position on the exhaust manifold or maybe a collapsed muffler/ collapsed exhaust pipe causing restricted exhaust flow?
Click to expand...
I have checked the condition of the plug wires, and the firing order is correct. The exhaust system is in good condition, I will check the ballast resistor
 
It seems to stumble and backfire every time, it does not backfire hard, it just kind of coughs I have not had the time to try a different coil yet
 
Sun lover said:
It seems to stumble and backfire every time, it does not backfire hard, it just kind of coughs I have not had the time to try a different coil yet
Click to expand...
I'd check for a vacuum leak. If you have the stock Holley 4 barrel carburetor 1273-1 or -2 do you know if the chock housing has the gasket? Mine was missing when I bought the car and once I realized that I purchased one for $9, installed it and it has stopped the occasional backfire and stalling.

If you already have that gasket installed then I'd check for other vacuum leaks around the base of the carburetor. There are many YouTube videos on how to check for vacuum leaks.
 
TbirdFan76 said:
I'd check for a vacuum leak. If you have the stock Holley 4 barrel carburetor 1273-1 or -2 do you know if the chock housing has the gasket? Mine was missing when I bought the car and once I realized that I purchased one for $9, installed it and it has stopped the occasional backfire and stalling.

If you already have that gasket installed then I'd check for other vacuum leaks around the base of the carburetor. There are many YouTube videos on how to check for vacuum leaks.
Click to expand...
It is using an Autolite 4 bbl carb that was professionally rebuilt. other than running a little rich, I have not had a problem and I have put at least 1,000 miles on it. My manifold vacuum is running between 15 and 16 at an idle I will check around the base for a leak.
Thanks for the input.
 
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