I am running a 1971 Chevy smallbock 400 engine and I have a pressure regulator with a pressure gauge. I am using the standard fuel pump. When I start the engine the gauge reads up at around 5-6 but as the engine warms up, the gauge starts reading less and less
until it is down at almost zero. It is best to run the pressure at around 3 from what I have been told but if I readjust the number to 3 then it ends up dropping down to zero. At 6 is fouls the plugs. Any suggestions as to what may be wrong. Could it be the diaphram in the
fuel pump going bad?

 

I'd change out the pump and recalibrate/re-jet the carburetor. It's running way too rich if it's fouling your plugs. Is your motor burning motor oil? What Carb are you using? Holley, Rochester Quadrajet?

 

I'd also question the gauge.

 

Could be, but a carburetor just needs enough fuel to keep the floats up to specification levels, since it's all vacuum running the damn old things on a car/pickup, thus the vacuum gauge was supreme back then. We didn't even have fuel pumps on our old two-stroke dirt-bikes; just gravity. You just adjusted for altitude, if you had time to do so, or you just " Run What You Brung", as we used to say in old America.

I still can't believe we'd drink the night before a race, but we didn't get real drunk. Good times! LOL!

 

Since it appears the fuel pressure is bleeding off, replace the regulator sounds like diaphram in it has a hole in it. I've never run a regulator on a non electric pump.

 

Since the gauge is glycerin filled, the engine heats up after running for awhile and it heats up the fluid in the gauge which causes the pressure to change inside of the gauge. They sell gauges that have a relief valve to address this issue. After installing one of these, I can push the relief valve and the needle moves back to normal. What got me thinking about this was the fact that the engine did not start running erratically when the pressure reading on the gauge dropped. So my plan is to start the engine cold and adjust the regulator to the pressure I want and then ignore the reading on the gauge after the engine heats up. I think my problem is solved. Thanks for posting.