The Ford Barn

ling102 · 05-31-2026, 03:52 PM

I have a 52 Mercury Flathead V8 in my 48 Ford Coupe.
The car in new to me.
It has an Offenhouser intake.
It has a Holley four barrell carburator.
It has Reds headers.
The motor runs rough.
I have 100lbs compression.
No overheating.
Good oil pressure.
No vacuum leaks.
The plugs look ok.
However, I have one backfire
when I start the car.
Could this be timing?
Any help would be greatly appreciated.
Thanks guys.

mfirth · 05-31-2026, 05:32 PM

Could be timing, but we need to know what distributor you have. 12 or 6 volt ? Plug wires in the proper firing order etc. Others here much smarter than me will jump in and you'll get it sorted.

hueyhoolihan · 05-31-2026, 06:56 PM

i had a leak somewhere that drew enough oxygen into the exhaust system to combust unburnt fuel passing through it. lots of pops and occasional bangs once the engine and exhaust system warmed to a critical temp.

it was very difficult to diagnose as there can be many causes, include fueling, electrical and mechanical that produce the same symptoms. it was exacerbated by a rich mixture and timing issues.

rotorwrench · 06-02-2026, 09:36 AM

All the flathead Fords of the 8BA era (1948 - 1953), had Holly integral fuel delivery/ignition systems. The carburetors had vacuum systems that controled the spark timing of the distributor. If the carburetor is replaced with a unit that is not set up for Holly Load-O-Matic ignition, then some other design distributor must be used. Mallory all centrifugal distributors or modified early GM vacuum advance distributors are the easier ones to use. The GM would use manifold vacuum with an adjustable vacuum advance can installed. The flyweight springs and advance unit can be taylored to work well with any flathead. There are still a few folks that do GM distributor conversions. The old Mallory types for flatheads are harder to source now days but they are out there.

ling102 · Today, 11:54 AM

I truly appreciate the response.
My car is still a 6 volt.
Question: If I have a sticking or stuck valve would that show up on a compression check?
Also, how and where do I check the vacuum, and what should it be?

markdtn · Today, 11:57 AM

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What Holley 4bbl is on it? Too big can be an issue. Also using the Load-o-matic distributor with it as mentioned.

tubman · Today, 12:08 PM

Until we know exactly what he is running for an ignition, we can't be of much help. As above, the exact model of Holley carburetor would also be helpful. A "Load-A-Matic" will run with a mismatched carb, but not well.

"rotorwrench" mentioned the old Mallory distributor being an option, but said they are "harder to source". Charlie Schwendler can convert SBC Mallory's for flathead use, and they're a lot easier to find. Plus, they come in both mechanical advance and vacuum over mechanical versions, plus a few have tachometer drives built in.

05-31-2026, 03:52 PM	#1
ling102 Member Join Date: Mar 2012 Posts: 48	52 Mercury Flathead runs rough. I have a 52 Mercury Flathead V8 in my 48 Ford Coupe. The car in new to me. It has an Offenhouser intake. It has a Holley four barrell carburator. It has Reds headers. The motor runs rough. I have 100lbs compression. No overheating. Good oil pressure. No vacuum leaks. The plugs look ok. However, I have one backfire when I start the car. Could this be timing? Any help would be greatly appreciated. Thanks guys. Last edited by ling102; 05-31-2026 at 05:26 PM. Reason: New information

05-31-2026, 05:32 PM	#2
mfirth Senior Member Join Date: Aug 2017 Location: ohio Posts: 1,165	Re: 52 Mercury Flathead runs rough. Could be timing, but we need to know what distributor you have. 12 or 6 volt ? Plug wires in the proper firing order etc. Others here much smarter than me will jump in and you'll get it sorted.

05-31-2026, 06:56 PM	#3
hueyhoolihan Senior Member Join Date: Dec 2024 Location: central coast california Posts: 594	Re: 52 Mercury Flathead runs rough. i had a leak somewhere that drew enough oxygen into the exhaust system to combust unburnt fuel passing through it. lots of pops and occasional bangs once the engine and exhaust system warmed to a critical temp. it was very difficult to diagnose as there can be many causes, include fueling, electrical and mechanical that produce the same symptoms. it was exacerbated by a rich mixture and timing issues.

06-02-2026, 09:36 AM	#4
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 18,007	Re: 52 Mercury Flathead runs rough. All the flathead Fords of the 8BA era (1948 - 1953), had Holly integral fuel delivery/ignition systems. The carburetors had vacuum systems that controled the spark timing of the distributor. If the carburetor is replaced with a unit that is not set up for Holly Load-O-Matic ignition, then some other design distributor must be used. Mallory all centrifugal distributors or modified early GM vacuum advance distributors are the easier ones to use. The GM would use manifold vacuum with an adjustable vacuum advance can installed. The flyweight springs and advance unit can be taylored to work well with any flathead. There are still a few folks that do GM distributor conversions. The old Mallory types for flatheads are harder to source now days but they are out there.

Today, 11:54 AM	#5
ling102 Member Join Date: Mar 2012 Posts: 48	Re: 52 Mercury Flathead runs rough. I truly appreciate the response. My car is still a 6 volt. Question: If I have a sticking or stuck valve would that show up on a compression check? Also, how and where do I check the vacuum, and what should it be?

Today, 11:57 AM	#6
markdtn Senior Member Join Date: Jun 2016 Location: Chattanooga, TN Posts: 191	Re: 52 Mercury Flathead runs rough. Sponsored Links (Register now to hide all advertisements) What Holley 4bbl is on it? Too big can be an issue. Also using the Load-o-matic distributor with it as mentioned.

Today, 12:08 PM	#7
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 12,137	Re: 52 Mercury Flathead runs rough. Until we know exactly what he is running for an ignition, we can't be of much help. As above, the exact model of Holley carburetor would also be helpful. A "Load-A-Matic" will run with a mismatched carb, but not well. "rotorwrench" mentioned the old Mallory distributor being an option, but said they are "harder to source". Charlie Schwendler can convert SBC Mallory's for flathead use, and they're a lot easier to find. Plus, they come in both mechanical advance and vacuum over mechanical versions, plus a few have tachometer drives built in.