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Fuel Pressure Isolator Installation

Fuel Pressure Isolator Installation Instructions:

This isolator is installed in the pressure line between the carb or fuel pump (where you normally pick up fuel pressure) and the fuel pressure gauge. It has a diaphragm that prevents fuel from reaching the gauge (inside the driver's compartment). The driver's side of the isolator and the line to the gauge are filled with a fluid that transfers the pressure to the gauge.

To install this isolator follow the steps here. It must be mounted in the engine compartment at approximately the same height from the ground as the gauge. A bracket is included. Make note of the normal fuel pressure at idle. This will be used at the end to confirm that everything is working correctly.

  1. The rounded end of the isolator faces the gauge. Plan where you will mount the isolator. It must be at the same height as the gauge. Drill a hole in the firewall for the line from the isolator to the gage. Make it just large enough to get the line nut thru. We strongly recommend AN style SS braided steel lines and fittings. Use the correct length line. Extra line coiled up will make it difficult to fill the system. See the Longacre catalog for a selection of lengths. Use a grommet to protect the line.
  2. Install the line from the isolator to the gauge. Run it thru the firewall and install it onto the rounded side of the isolator with appropriate fittings.
  3. You will now fill the system from isolator to gauge with fluid. Use a 50/50 mix of anti-freeze and water (don't use oil as it will not flow properly and slow gauge response). The isolator came with a small plastic bottle and a length of thin plastic tubing. Fill the bottle with the fluid.
  4. Disassemble the isolator and hold it so the gasket surface is level. Insert the small tube into the hole and thru the line to the gauge as far as it will go.
  5. Gently squeeze the bottle. Fluid will be forced into the line at or near the gauge and force out trapped air. As soon as fluid comes out around the small tube continue to gently squeeze the bottle and slowly pull the tube out of the line. As you do this the line will fill completely with virtually no air pockets.
  6. When you have pulled all of the tube out hold the isolator level and fill the cavity to the top. Keeping it level place the orange diaphragm on top with the raised rib facing down. Align the holes and place the other half of the isolator in place. Put the bracket where it needs to be and install the screws. Tighten firmly. Mount the bracket as planned and connect to the carb.
  7. Start the motor and CHECK FOR FUEL LEAKS FIRST! Let the motor run for several minutes to be sure there are none. If there are any leaks repeat step 6 above and recheck for leaks.
  8. Check the pressure reading. If it is lower than before you may still have air in the system. If installed and filled properly it will have no affect on the pressure. Repeat the above procedure starting at step 4. When OK mount the isolator as planned. Check again for fuel and fluid leaks.


Spilled fuel on a motor is extremely dangerous. It can cause fire which may result in injury or death.


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