This has bugged me for awhile….my fuel pressure gauge doesn’t seem to work. First the details; 59a flathead with dual 97s, 5/16” fuel line routed as follows: from firewall to mechanical fuel pump, fuel pump out to firewall mounted Holley 12-804 regulator, regulator out to rear carb stromberg banjo fitting that has a Marshall brand (dry gauge, 0-15 psi) attached. Shouldn’t this arrangement work just fine? the fuel pressure gauge in this location has never read above 1.5 psi, best I can tell, despite adjusting regulator. So I decided to remove it and setup a bench top test of this gauge with a spare electronic fuel pump, spare regulator and 2 gas cans. In the test I have a Carter p90091 pump (4-5 psi) and a Redline regulator and the gauge is threaded into a 5/16” barb x 5/16” barb x 1/8” npt fitting. Plugged in fuel pump and let her go, adjusting regulator up and down, and gauge registers nothing, nada. Is it fair to say gauge is dead? mean while I’ve ordered a 1-10 psi Vintage Speed gauge to see if it will work. here are some pics , first of fuel line routing minus fuel gauge and second pic is bench test setup.
Instead of inline with the fuel bowl feed lines you can thread it into the unused outlet port of the Holley regulator using a pipe thread bushing adapter. I would be inclined to think your old gauge is no longer good since on the car and bench test showed no difference in operation when playing with the pressure settings. Also if your mech pump is rated for flat head use then a regulator is not required, you could just eliminate it all together.
The gauge is likely damaged. My experience with fuel pressure gauges when mounted on an engine is that a non-oil-filled gauge will have a short life. Engine vibrations will kill it. Hence the need to fill them with oil to dampen the harmful engine vibrations. I mount them outside the p***enger compartment but visible from the driver's seat so pressure can be monitored on high fuel demand WOT situations.
Those small fuel pressure gauges are often not useful. Price point tends to be low, meaning poor quality. If that cool little gauge is the same price as a set of dice valve stem doodads, you are not buying accuracy. Gauge range is often wider than needed. You want the expected operating pressure to be mid-swing, not the first ten percent. They are subject to internal (pressure pulses) and external (temp and vibration) forces that shorten life and accuracy. The smaller gauge means smaller, more delicate parts. Nobody wants a big 12 inch gauge sticking up, but the smaller size forces things to be less ideal. How a mechanical pressure gauge works. https://www.marshallinstruments.com/faqs/detail.cfm?id=22
that’s the plan, move gauge to unused port on the regulator once I get another one and test that it works. I ***ume moving it here there will be less vibration from engine? Or once I get pressure dialed in I’ll remove gauge. my mechanical fuel pump is rated as built and tested for 2.5 lbs but one of the carbs started seeping fuel. the guy who rebuilt carbs said to add regulator because sounds like pressure too high. So trying to get pressure verified with a gauge.
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