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| Joined: | 05-01-2005 |
| Location: | Minneapolis, Minnesota USA |
| Posts: | 575 |
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The Z-S carbs like 1.5-2.5 psi... ie, less than pump pressure. But flow losses from the pump at the back of the car to the carbs at the front, and pressure loss due to the difference in elevation between the pump and carbs, reduces the pressure at the carb inlet... but it's typically still too high.
Therefore, the factory added a bleed orifice and fuel return line that acts as a very basic "poor man's" pressure regulator. The orifice diameter was experimentally chosen ('tuned') for the J-H system, in order to drop the pressure to the correct value at the carb inlets. The orifice calibration is so specific that changing anything in the system (a non-standard pump or pump location) will throw the calibration off.
I don't recall the exact orifice size at the moment, but it's small, on the order of 0.015" (0.381 mm). It's so small, that it is easily plugged by any crud that gets past a dirty fuel filter.
I'm not pointing a finger at any one thing, I'm just pointing out things to check.
Start by measuring the fuel pressure at the carb inlets, looking for 1.5-2.5 psi. Even if the pressure is correct with the engine not running, a tired pump with reduced flow capacity may not be able to keep up with flow demands at high speeds.
If you seem to have a built-in speed limiter (engine runs fine at idle and low speeds, but falls flat on it's face at some consistent highway speed), then the fuel filter may be dirty/ plugged, or the pump may be tired or suffering from a small perforation in the diaphragm.
Do the easy thing first, replace the fuel filter and make certain the bleed orifice is not plugged. In older cars, there is a greater probability of rust issues inside the tank; so I like to install the largest fuel filter available that will fit.
Good Luck,
Tim Engel
Last edited on 05-21-2018 06:50 pm by Esprit2