Installing Dual Carburators on a Flathead V8
By Thomas Teixeira. His web site can be found at
Pretty much all the books on flatheads don’t recommend more
than dual carburetors for street use, and many of the newer books
recommend putting on a four barrel carburetor instead of two carburetors.
At the same time, two, three or more carburetors were the only
choices back in the 40’s and early 50’s. Plus the decorative possibilities
for multiple carburetor setups are almost endless: air scoops,
velocity stacks, various types of chromed and polished air cleaners…
is the intake manifold we installed with the carburetors and air
cleaners set up inside before installation. I’ll explain more
about how we chose each of the major pieces and then what we did
to install them.
Choosing the manifold
Some of the other trade-offs involve how and where to mount
a generator or alternator. If the carburetors are put in the ‘ideal’
position (one centered over the front four cylinders and one over
the back cylinders), the generator will not fit in the stock location.
Instead, the generator is mounted on a bracket attached to one
of the cylinder heads. But some of the older intake manifolds
(including the ones still manufactured by Offenhauser) also have
an option where the carburetors are closer together, both towards
the rear of the engine to leave room for a generator.
We were already using an alternator instead of a generator
and have quite a bit more room at the front of the engine, and
had seen pictures in one flathead book of an engine with three
carburetors and an alternator so we thought that we might be able
to install a ‘super dual’ intake manifold and completely satisfy
my fetish for symmetry: alternator in the center of the engine
instead of two one side, and carburetors centered over the appropriate
So, we ordered an Offenhauser ‘super dual’ manifold (supposedly
good for about 10% more peak horsepower than their regular dual),
but the manifold we received did not have any mounting bracket
at the front for a generator or alternator. We knew a generator
wouldn’t fit, but some pictures of older manifolds in books showed
the bracket anyway, and the bracket seems to be their for the
triple carburetor manifolds: on engines from the late 40’s, this
bracket supports the fan which runs on an idler pulley instead
of on the generator.
While we weren’t sure if we had just ordered the wrong version
of the super dual manifold, briefly eyeballing the space available
for the alternator made us nervous. we didn’t actually take the
alternator off and try the fit (particularly since there was no
bracket to fit it to), but decided to exchange the super dual
for a regular dual: we were sure this would have a generator
bracket and also we would be able to use the same manifold if
we decided to go retro and replace the alternator with a generator.
Plus one manifold is specified for all the years 1932 through
1948 in case we ever decide to replace our engine with a 59AB
block. Since then, we have seen used Offenhauser super dual manifolds
at swap meets that do have the bracket, but by then we already
had a manifold.
We briefly considered getting a triple carburetor manifold,
but decided against it for several reasons:
- Although all of Offenhauser’s triple manifold had fan brackets,
it still wasn’t clear if the alternator would fit
- At 221 cubic inches (a little more since the cylinders have
been overbored by about .050), it was questionable if two carburetors
would be too much, let alone three.
- Carburetors are expensive. Even getting our old carburetor
cleaned, rebuilt and plated to match the others would be expensive.
We found out later that the ‘progressive linkages’ for three
carburetors tends to run the whole engine off one carburetor (the
middle one) until you press harder on the accelerator. We did
know that some people also install a block off plate instead of
the middle carburetor, or install the carburetor but don’t connect
the throttle linkage to it.
We also decided that while we were at it, we would get the
manifold polished: the plain cast manifold looked okay, but not
as shiny as our chrome alternator, the air cleaners or the carburetors.
Choosing the carburetors
This was an easy choice for us: Stromberg 97’s, although it’s
perhaps hard to justify. One reason is that the Stromberg was
the factory carburetor in 1935 (although the Stromberg 97 wasn’t
used until 1936). And since they’re older and rarer, there is
more of a mystique to them. Of course, ‘older and rarer’ is another
way of saying ‘more expensive.’
The flow rates are better for the Holley 94’s than for the
Stromberg 97’s and seem to be a better carburetor for a single
carb setup: after all, Ford replaced the Strombergs with Chandler-Groves
and then got Holley to manufacture the Chandler-Groves design.
But for a smaller engine, the smaller flower rates aren’t a problem.
Anyway, while Holley carburetors seem very easy to find, it
wasn’t that hard to find some Stromberg’s: both Hot Rod &
Custom Supply and Red’s Headers were able to locate some, and
I’ve since seen them in Early Ford V8 Sales’ catalog, and heard
about several other sources by inquiring on various mailing lists.
Choosing the Air Cleaners
Although scoops and velocity stacks look cool, we intend to
do enough driving that we wanted real air cleaners. The small
air cleaners, with or without louvers, look nice. Hot Rod &
Custom Supply had some very tempting cast aluminum lids for open
air cleaner elements, but they also had these dome shaped Stelling
& Hellings air cleaners (I’ve also seen them in Flathead Jack’s
catalog). They look even better in real life. We did spring for
K&N filter elements instead of the paper.
All the other parts
The intake manifold, carburetors and air cleaners are the big,
expensive sexy parts. But not the most important: you also need
gaskets, fasteners, throttle linkages and fuel fittings and some
of these are a little harder to locate. We also ended up needing
a new fan belt and a new fuel pump push rod. Here’s the complete
- Driver’s side throttle linkage for the carburetors
- Passenger’s side linkage for the choke
- Fuel line fittings for the carburetors
- Carburetor gaskets and mounting studs
- Intake manifold gasket
- 1/4" copper line and a T fitting
- Studs for mounting the fuel pump stand to the manifold
- Gasket for fuel pump stand to manifold
- Some 3/16" brake line and compression fittings for the
- Fuel line fitting for the fuel pump that works with the carburetor
- 57" fan belt (replacing a 58" fan belt with the
- 8.87" long fuel pump push rod
- Fuel pump stand, fuel pump, oil filler/breather cap (although
we could have moved the old ones over)
- Throttle return spring and bracket
Some things we bought that we didn’t end up using:
- Fuel block and fittings for 5/16" flexible hose
- Throttle and choke rods
We’d been told that using hard fuel lines between the fuel
pump and carburetors is asking for trouble, particularly with
copper. We originally were going to go with 5/16" flexible
hose with a fuel block mounted on the firewall, and ordered the
right fittings for the carburetors and the fuel block but forgot
that we would need one more hose barb to go from the fuel pump
to the fuel block: this is an area where the catalogs could be
of more help and remind you that you will need one more fitting
and that the stock fuel pump fitting is the same as the
carburetor. Many of the catalogs steer you towards electric fuel
pumps but even the ones with both mechanical and electric fuel
pumps ignore this fitting. We decided to stay with a mechanical
fuel pump so we wouldn’t have to do more wiring, wouldn’t have
to worry about a pressure regulator (the old carburetors can only
take about 2psi while electric fuel pumps often operate at 6psi),
and wouldn’t worry about things like an inertial cutoff switch
for the fuel pump. If we were planning on drag racing or going
out to the salt flats I’m sure that the fuel pump is a limitation,
but I don’t expect to have any problems on the highways here.
The throttle and choke linkage are just the standard Offenhauser
parts. They may look a little clunky with cotter pins everywhere
holding things in place but they are functional. Most of the fancier
linkages have progressive mechanisms for three carburetors and
are a lot more expensive. There are also some fancy ball bearing
plain linkages too. Maybe next time. Red at Red’s
Headers advised that the choke might not be needed with two
carburetors, but this is New England, not California, and we wanted
to be able to use the truck in early spring and late fall so we
got the choke linkages installed on the carburetors and also a
linkage to connect both chokes to the choke rod into the cab.
We could have moved our current fuel pump and fuel pump stand
over to the new carburetor, but we happened to see NOS replacement
fuel pump and a restored fuel pump stand at a swap meet so we
Removal of old manifold and installation of new manifold
This was all pretty simple. We completely removed the alternator
to make the old manifold lighter and to get better access. Disconnected
the fuel line from the fuel pump to the firewall (we had a can
ready to catch any gas that was ready to drain back, but there
wasn’t much to speak of). Disconnect the choke rod, throttle rod
and accelerator pedal from the old carburetor. There is also a
3/16" metal vacuum line that goes from the distributor to
the manifold (it actually is a vacuum brake rather than a vacuum
advance, but it serves the same purpose of adjusting the ignition
timing to compensate for engine load). Then we went to work removing
the 20 bolts holding the manifold down. The stock spark plug wire
looms are held on by some of the manifold bolts and once they
were removed we just moved the wire looms off to the side and
lifted off the manifold, complete with carburetor and fuel pump.
The left hand picture shows the engine with the intake manifold
removed but the gasket still in place. The middle picture shows
the front intake ports and valves and the yellow arrow points
to the oil pressure relief valve. Since our oil pressure is way
too high when the engine is cold we removed this and checked the
spring which had been stretched some. Although we tried to compress
this back, the oil pressure is still too high when cold, but settles
to a reasonable value once the engine is warm. The right hand
picture shows the rear intake ports and also the push rod which
operates the fuel pump.
The repair manuals always say "installation is the reverse
of removal", and this is all that has to be done. We put
some high temperature RTV on a new gasket, put the gasket in place
and then put the new manifold and carburetors in place.
As you can see, we hooked up the choke linkage as well as the
throttle linkage here. We ended up having to remove the throttle
linkage though. The normal carburetor linkages don’t have any
provision for connecting this throttle rod, but we had an extra
ball joint that we put on the linkage. However, we didn’t try
to make any sliding arrangement so pressing the accelerator pedal
would also try to move the throttle rod, definitely a bad idea.
You can also see that we used 1/4" copper fuel lines between
the fuel pump and the carburetors. We have been told that this
isn’t a good idea since vibration will cause copper to harden
up and eventually crack. I’m not sure how much of a problem that
might actually be since the fuel pump and carburetors are all
mounted on the manifold, but it would be a very bad idea to use
copper or any metal pipe all the way to the fuel line on the firewall.
Most of the replacement stock fuel lines from the fuel pump to
the carburetor are copper although originally they were copper
plated steel. We do have some 1/4" steel brake line that
we will use to replace the copper at some point. We originally
were going to use flexible hose to a fuel block on the firewall,
but thought that some sort of hard lines between the fuel pump
and carburetor gives a cleaner look.
The intake manifold bolts get tightened down working from the
inside towards the outside and some of these bolts hold down the
spark plug wire looms. The only problem here is that one of the
bolts has very little clearance and a normal socket won’t fit
over the bolt, and there isn’t room to use an open end wrench
or even a crow’s foot because of the write loom. We got an internal
hex-head cap screw since there is room for an Allen wrench. I
guess this means that the hex head manifold bolts from Flathead
Jack aren’t just for show.
You can also see in this picture that the fuel pump isn’t in
place yet. This turned out to be our biggest mistake, or rather
not getting the right size studs in advance was a big mistake.
We could have moved the old fuel pump and stand over and probably
would have moved the studs over. Instead, we got an NOS replacement
fuel pump and a cleaned up fuel pump stand at a swap meet. But
we had lots of problems getting the fuel pump stand attached to
the manifold. The threads in the aluminum manifold are quite soft,
and unfortunately the Offenhauser manifold has very shallow holes
here. So shallow that 1-inch bolts were too long but 3/4-inch
bolts were too short and stripped out the top part of the threads.
It was also difficult to find studs that worked. These were also
on the short side (1-1/4 inches) and needed to have the coarse
threads (going into the manifold) to be quite short or the fuel
pump stand bolts wouldn’t tighten down.
By using the wrong bolts and studs and being impatient, I ended
up stripping one of the holes. Rather than taking the manifold
off and getting a helicoil put in, I used loctite epoxy putty
to hold the stud in (after two unsuccessful attempts, I finally
forced myself to wait a full twenty four hours for it to cure
before trying to tighten down the bolts). If I were to do this
again I would probably drill the holes all the way through the
manifold so the length of the studs wouldn’t be so critical (the
stock manifold has through holes).
We also discovered that we had to get a longer 8.87" fuel
pump pushrod. The catalogs say these are for cast iron intake
manifold, but the Offenhauser intake manifold is taller in the
back. I was able to drive to PV
Antique Ford in Tewksbury over lunch rather than waiting for
another mail order delivery.
These pictures show the fuel pump in place. They also show
some 3/16" steel brake line which I used to connect the vacuum
brake on the distributor to the vacuum port on the intake manifold.
For some reason the Offenhauser regular dual only has a single
vacuum port that would normally be used for windshield wipers
although several of the super dual intake manifolds I have seen
in catalogs and at swap meets have two vacuum ports. Since we
replaced the vacuum wiper with an electric motor we didn’t need
to make any sort of Tee fitting.
After getting the manifold bolts tightened we put the alternator
back. In order to get enough tension on the fan belt we had to
have the bracket very high in the slot. For reasons we still don’t
quite understand, we had had to get a longer fan belt last year
when we put in the alternator (58" instead of 56").
We tried the stock fan belt but it was still too tight, so we
ordered a 57" belt from NAPA which arrived the next day.
The 57" belt seems to be "just right."
These pictures show more of the engine compartment. I’d perhaps
like to find a place for the battery. Maybe a 1940 Ford battery
tray would let the battery stay under the hood (instead of under
the floor), but lower down and out of the way.
How does it run?
Except that we had to crank the engine for about a minute because
it was a new fuel pump and empty carburetors, the motor fired
right up. It didn’t take much choke, but the carburetors seemed
to be set up to run as singles and needed to be leaned back quite
a bit. I think the idle may still be on the rich side.
I got a Unisyn to try to balance the carburetors, but at idle
there isn’t enough downdraft for the Unisyn to work. I tried to
balance them using my hand to gauge the airflow, but realized
maybe I should balance them off-idle and then lower the idle speed.
I had used "California Bill’s Ford Speed Manual 1952 Edition"
reprinted by Fisher Books as a guide to the jet size. He recommends
.046 jets for 1932 to 1948 engines, but was probably talking about
239 or even 255 cubic inch engines. Our engine seems to be bored
.050 over so it should be about 228 cubic inches. Red of Red’s
Headers told me that .042 would probably be better, but suggested
see how it actually runs first. I may just take it to the local
garage with a flathead expert since my house has too much 30 mph
driving between here and the highway to get a good reading on
the plugs at home.
Overall, it does seem to have a little less oomph at 30 mph
in third than it did with the stock intake and dual exhausts,
but it does just fine on the interstate accelerating to an indicated
75 mph. I have 3.55 gears and stock wheels and tires, but the
speedometer gear has 19 teeth instead of 18 and I don’t know how
accurate the speedometer is anyway.
While nothing major is planned, I do have to clean up the old
manifold, carburetor and fuel pump and store them. Follow
the link to see other planned or unplanned activity we’ve
All pictures taken by the author