An article by 'rumbleseat' archived from his excellent website, ca. 2000
FUEL PUMPS
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 to decrease the pressure down to the 3 1/2 psi. I want. They are only slightly larger, but are considerably thicker. On my ‘34, I had 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 came with.
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”. If wear exceeds this amount, 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 they’re worn out.
Ford, Holley, and Stromberg carburetors were designed to run on 2-1/2 psi fuel pump pressure not the 3-1/2 to 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 also. 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 trip across country.
FUEL PRESSURE REGULATING
Fuel pressure is very critical to these early carburetors. Early Ford Motor Co. shop bulletins and manuals state the Ford/Holley/Chandler Grove and Stromberg carburetors are all designed to operate at 2-1/2 psi. Yet the fuel pump delivery pressure spec’s call for 3-1/2 psi ! I don’t understand the thinking behind this. The Strombergs are very sensitive to fuel pressure because of their 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 carbs I install a pressure regulator and set it at 2-1/2 psi. This regulator HAS to be physically located between the fuel pump and carb since the fuel pump is putting out 3-1/2 Psi. If you’re entering your classic car for judging, then carry a spare fuel line without the regulator and replace it when you get to the meet.
STROMBERG CARBURETORS
Applications
- 48’s had a 1.031’ venturi and are rated at 175 cfm. They were stock on ‘34 and ‘35 V-8’s with 221 cu. inches. Main jets were 48 at sea level.
- 81’s had a 0.812” venturi and are rated at 135 cfm. They were stock on ‘37 and ‘38 V-S 60’s.
- 97’s had a 0.969” venturi and are rated at 150 cfm. They were stock on ‘36 and ‘37 V-S’s with 221 cu. inches. Main jets were 45 and power valves were #65 at sea level.
- L’s had a 1.000” venturi and are rated at 160 cfm. They came on ‘36 and ‘37 Lincoln V-12’s.
The Stromberg 97 is the most popular and plentiful at this time and they’re getting pretty scarce. All of the mentioned carbs are quite similar and the following can be interpolated for your specific application.
STROMBERG 97 CARBURETORS
Main metering jet numbers indicate the diameter of the hole in thousandths. Hence a #46 jet has a 0.046” diameter hole. At sea level these carbs came with #45 jets. For carbs used in Denver I use a #43 jet (0.043” in diameter) for starters. These jets are drilled straight through (carbs manufactured beginning in the 60’s have jets with a venturi in the middle and shouldn’t be drilled) . Since they 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 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 carbs come with a #65 (0.0350” diameter) power valve at sea level. In Denver, I solder this hole shut and re-drill it with a #67 drill (0.0320” diameter) for starters. Soldering these and redrilling (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” | #62 = 0.0380” | #63 = 0.0370” | #64 = 0.0360” |
| #65 = 0.0350” | #66 = 0.0330” | #67 = 0.0320” | #68 = 0.0310” |
| #69 = 0.0292” | #70 = 0.0280” | #71 = 0.0260” | #72 = 0.0250" |
The dry float level setting is 5/16” plus or minus 1/32” 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” (plus/minus 1/32”) 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 carb. With the close proximity of the sparking generator commutator, I prefer my carbs stay dry on the outside no guts or faith in my fire extinguisher I guess. On these carbs, the idle discharge circuit supplies the fuel from idle to 25 mph. The main jet circuit operates between 25 and 70 mph. Above 70, 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 are not interchangeable even though the threads are the same. The needle taper extends right up to the threads on Strombergs. On Holley carbs, the needle taper stops short of the threads. Many, but not all, Stromberg idle screws have the screwdriver slot cut only half way 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 basically the same carbs, but manufactured by different companies.
I prefer Strombergs over the Ford/Holley/Chandler Grove carbs for a couple of reasons especially on multi-carb installations. One is in the throttle base and the other is the power valves. 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 besides returning to idle without sticking. The other 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 carbs the manifold vacuum is usually low... . which contributes to premature opening of the vacuum controlled power valve. It’s not uncommon to find these valves opening with less than ‘A throttle applications since almost any drop in vacuum is enough to make them operate. As can visioned, this leads to a rich condition when it’s not needed. It’s 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 richl The Stromberg’s mechanical power valve operates mechanically and is relatively unaffected by low vacuum. This eliminates the over rich conditions that are 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.
When your setting up the carb(s), be sure to check the plugs for indications of leanness and/or detonation for both the main jet range and the power valve range.