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Carburetor Fuel Pressure Regulators

By Steve Grosekemper

There are several solutions for your problem. The easiest solution is to run a fuel loop through the carburetors. To do this you hook the pressure line up to one carburetor and the return line to the other carburetor. Then attach a balance hose between the two units. Next you will need a fuel pressure gauge attached to the pressure side of the carburetors.

You now have a fuel loop with lots of volume and no pressure. To add pressure you must have a restriction on the return side of the system. The easiest way to achieve this is to add a clamping device to the return line until you get your desired pressure. (This is what was done in the early PMO conversion kits.)

There are a few problems with this solution, however. First of all, clamping a fuel line, if done incorrectly can damage the line. Secondly, when you set the pressure to 3 psi at idle you do not have constant pressure. As the carburetors demand more fuel under peak H.P. the pressure drops dramatically. (Just like loosing shower pressure when the toilet gets flushed). If you increase the pressure so you maintain 3-4 psi under load, the chances are the pressure at idle will be excessive and overpower the needle & seat assemblies.

The solution is to install a standard Bosch fuel pressure regulator. (A regulator maintains a set fuel pressure no matter what the pump volume or engine demand) The problem is where to get a regulator like those used in later fuel injected engines that operates at a lower pressure level. While this regulator doesn’t exist, it can be easily manufactured from a standard Bosch fuel pressure regulator.

The first step is to locate a regulator with the proper fuel fittings for your particular application. The regulator I have used for this application is an early 944 regulator, Bosch # 0 280 160 214. A new one will set you back about $60, but used ones can be had for a fraction of that cost.

The modification for this regulator is actually quite simple. All that needs to be done is to open the top of the regulator and replace the high-pressure spring with a much smaller one. Here is how it is done –

•  Using a pipe cutter cut the top half of the regulator off. Don’t use a saw as there is a large spring in the center of the regulator and you are going to want a very precise cut.

•  Open the top and remove the heavy spring.

•  Using a file dress the two pieces best possible so the mating surfaces are a flat as possible.

•  Install the new spring (part number 930.105.535.01). This perfectly matched spring (from a 911 chain tensioner) will provide a steady 3.5-4.0 psi of regulated pressure. Its cost is only $1.33

•  Re-sealing the regulator is simple as the spring is so weak. Just place the top of the regulator back into position and secure it down. It can be secured with aluminum tape, epoxy or be tack welded.

•  With the regulator installed you will have perfect pressure at all times.