Red's Engine Talk
Fuel and Carburetors July/Aug 2005
Red Hamilton's guest writer for this column is, Paul Garrigan (also known as "Rumbleseat")
The following fuel and carburetor article was written by Paul originally to assist members of the MSN Flathead List. It is posted on the Flathead Ford Techno-Source site managed by Bill Bentley. We would like to thank both of them for their support of Flathead Ford fans everywhere.
Flathead Ford stock fuel pump pressure is 3- 1/2 psi. The fuel pump pressure can be adjusted by adding or subtracting fuel pump stand gaskets. The gaskets we get in engine gasket sets are quite thin (about 0.010" thick), and we only get one.
What I use are 8BA thermostat housing gaskets to adjust the fuel pump stand height and decrease the pressure to the 3 1/2 psi that I want. They are only slightly larger, but are considerably thicker. On my '34, I have to use two of them to get my new Carter fuel pump pressure down to 3 1/2 psi from the 4 1/2 psi it had originally.
Just because the fuel pressure is within specifications does not mean the fuel pump is good. It can have good pressure but not pump enough volume. It has to pump at least one pint of gas within 30 seconds with the engine running between 500 and 600 rpm.
Fuel pump push rods are supposed to be replaced when wear exceeds 0.010-inch. The new dimensions of the rods are 7 7/8 - inch, 8 7/8- inch or 10 1/16- inch. If wear exceeds 0.010- inch, the pressure/ volume may be insufficient to feed the engine when asked to run flat out, or during a hot summer day. Watch the amount of chamfer on the cam end to determine when push rod is worn out.
Ford, Holley, and Stromberg carburetors were designed to run on 2 1/2 psi fuel pump pressure, not the 3 1/2 and 4 1/2 psi the stock mechanical fuel pump is supposed to deliver. Why Henry made a fuel pump that puts out more pressure than the carb can withstand is beyond me.
I use an adjustable type fuel pressure regulator located between the stock fuel pump and the Stromberg carbs and set it at 2 1/2 lbs. This prevents flooding from too much pressure on the needle seat assembly.
When you're performing your Spring tune up, be sure you tighten the screws that hold the fuel pump together, since they relax with age. Also check the glass bowl bail to make sure it's tight. As these relax, they will suck air instead of fuel and cause you to stall on a hot day. I recommend you check them in the Spring and again before you take a long cross country trip
Fuel Pressure Regulating
Fuel pressure is very critical to these early carburetors. Early Ford Motor Company shop bulletins and manuals state that the Ford/Holley/Chandler Grove and Stromberg carburetors are all designed to operate at 2 1/2 psi. Yet the fuel pump deliver pressure specification is 3 1/2 psi! I don't understand the thinking behind this.
The Strombergs are very sensitive to fuel pressure because of the float/needle valve design. It just doesn't exert enough pressure to overcome the fuel pressure. They have a tendency to flood when fuel pressure is in the neighborhood of 3 1/2 lbs.
On these carburetors I install a pressure regulator and set it at 2 1/2 psi. This regulator must be physically located between the fuel pump and carburetor since the fuel pump is putting out 3 1/2 psi. If you're entering a classic car for judging, then carry a spare fuel line without the regulator and replace when you get to the meet.
• 48's had a 1.031-inch venturi and are rated at 175 cfm. They were stock in 1934 and 1935 V-8s with 221 cubic inches. Main jets were 48 at sea level.
• 81's had a 0.812-inch venturi and are rated at 135 cfm. They were stock in 1937 and 1938 V-8 60's.
• 97's had a 0.969" venturi and are rated at 150 cfm. They were stock on 1936 and 1937 V-8s with 221 cubic inches. Main jets were 45 and power valves were #65 at sea level.
• L's had a 1.000-inch venturi and are rated at 160 cfm. They came on 1936 and 1937 Lincoln V- 12s.
The Stromberg carburetor is the most popular and the most plentiful at this time and they are getting fairly scarce.
All of the mentioned carburetors are quite similar and the following can be interpolated for your specific application.
Main metering jet numbers indicate the diameter of the hole in thousandths. Hence, a 46 jet has a 0.046-inch diameter hole. At sea level these carburetors came with #45 jets.
For carburetors in Denver, I use a #43 jet (0.043 - inch in diameter) for starters. These jets are drilled straight through. Carburetors manufactured beginning in the 1960's have jets with a venturi in the middle and shouldn't be drilled.
Since the early carburetors are drilled straight through, they can be soldered shut and re-drilled. The solder is softer than the brass, and the jets will probably have to be re-soldered and drilled about every 15,000 miles or so.
The power valve used to be called the "high speed jet." The Stromberg power valve is all brass and does not have a vacuum diaphragm like the Holley/Ford/Chandler Grove carburetors do.
The power valve has one small hole drilled in the side for fuel flow. Unlike the main jets, the numbers on a Stromberg power valve are NOT the diameter of the hole. They refer to the numbered drill used to drill the hole. (Numbered drills are backward: the larger the number, the smaller the drill bit.) These carburetors come with a #65 drill (0.0320" diameter) for starters. Soldering these and re-drilling, like the main jets, is the way to go since power valves are getting impossible to find.
Below are the numbered drill bits and their diameters:
#61 = 0.0390-inch #62 = 0.0380-inch
#63 = 0.0370-inch #64 = 0.0360-inch
#65 = 0.0350-inch #66 = 0.0330-inch
#67 = 0.0320-inch #68 = 0.0310-inch
#69 = 0.0292-inch #70 = 0.0280-inch
#71 = 0.0260-inch #72 = 0.0250-inch
The dry float level setting is 5/16-inch plus or minus 1/32-inch measured without a gasket. This is close enough to start the engine, but the floats should be set with the engine running so the fuel level is 15/32-inch, plus or minus 1/32-inch, from the top of the fuel bowl without a gasket.
Be careful while making the run setting since raising the float will cause fuel to overflow onto the engine. With this wet setting, there should be no problem with gas soaking through the float bowl gasket and running down the outside of the carburetor.
With the close proximity of the sparking generator commutator, I prefer my carbs stay dry on the outside - no guts nor faith in my fire extinguisher I guess.
On these carbs, the idle discharge circuit supplies the fuel from idle to about 25 mph. The main jet circuit operates between 25 and 70 mph. Above about 70 mph, the power valve cuts in.
The idle screws are different appearing from Holley carbs, although I've seen Holley screws in Strombergs. Each has a different taper angle and they are not interchangeable, even though the threads are the same. The needle taper extends right up the threads on Strombergs. On Holley carbs, the needle taper stops short of the threads.
Many, but not all, Stromberg idle screws have the screw driver slot cut only halfway across , whereas the Ford / Holley/Chandler Grove idle screws all have their screwdriver slots cut all the way across.
Incidentally, the Ford/Holley/Chandler Grove carbs are all basically the same carbs, but manufactured by different companies.
I prefer Strombergs over the Ford/Holley/Chandler Grove carbs for a few reasons, especially on multi-carburetor installations.
One reason is the throttle base. The Stromberg throttle base seems to have better machine work on the throttle valves and the throttle bores, in that they don't stick when coming off idle at a stop light. This makes for smooth throttle openings and in addition they return to idle without sticking.
Another reason is that you can access the three body to base throttle screws from the top.
This allows you to change jets without upsetting the throttle linkage adjustment.
A third advantage, in my opinion, is the Stromberg 's power circuit. It uses mechanically operated brass power valves instead of vacuum operated diaphragm power valves as found in the Ford/Holley/Chandler Grove carbs.
When using multiple carburetors the manifold vacuum is usually low, which contributes to premature opening of the vacuum controlled power valve. It's not uncommon to find these valve s opening with less than adequate throttle applications, since almost any drop in vacuum is enough to make them operate.
As can be envisioned, this leads to a rich condition when it's not needed. It is impossible to compensate for this over rich condition by reducing the main jets, because when these vacuum power valves open it's the same as increasing the main jet size 10 whole numbers - no wonder they always run rich!
The Stromberg's mechanical power valve operates mechanically and is relatively unaffected by low vacuum. This eliminates the over rich conditions caused by the power valve opening too soon, or when it's not needed. Also, the Stromberg power valves can be drilled to suit your needs and driving habits, whereas the vacuum type cannot since they are sized during manufacturing.