The old float looked good but I replaced it anyway. Got a third pump today and will install it in the morning and see what happens. If issue persists a pressure regulator and/or new carb could be in my future.
Put the third pump on and pressure started off at 4 psi and rose to 12 psi at idle before I shut it off to avoid fuel overflow and fire hazard. It can’t be that all 3 pumps were bad. Is there something else that has to do with the carb that could cause fuel pressure to rise beyond the specs of the pump?
What gets me is that the pressure starts out between 3-6 psi but then after about 10 seconds it steadily rises which makes me wonder if perhaps the carb begins to suck fuel causing the psi to steadily increase to 15#. There is a tube off the back of the carb to the valve cover and a tube from the bottom of the carburetor that connects under the distributor. Could either of those tubes be creating a vacuum? I think I will try pinching the fuel line and turn it over to see what the psi is when not connected to the carb.
Everyone has gone radio silent. Ya’ll stumped?
Larger tube is PVC system that vents the crankcase. Smaller tube at base of carb is ported vacuum to the distributor vacuum advance. But the carb is the source of vacuum for both of those, and won’t affect fuel pressure.
If you are seeing 12 psi fuel pressure, that’s your problem. Maybe those pumps all come from a common factory in China that has lost the recipe. Inmanlanier’s sugestion of pressure regulator might be the best solution.
Here are fuel pump specs from the 70 shop manual.
Thanks. I am going to pick up some extra fuel line to give room to install a regulator and the pressure gauge (will need to loop around). It is a shame and dangerous that mechanical pumps are putting out 12-15# when Spec’d at 4-6#. Good to have a regulator but shouldn’t need it. I will let you know how it goes with next test.
I hear ya - that is frustrating and puzzling for sure.
Carbs don’t suck fuel the way you are thinking. The fuel pump simply fills the bowl to the level allowed by the float. You can take the top off the carb and see this in action with the engine running. The carb has no bearing on fuel pressure. That’s regulated by the pump.
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I installed a regulator and problem solved. She purred liked the cat she is. I plan to take back the fuel pump and find one that doesn’t put out 12-15 psi. The only regulator that was in stock said inlet pressure should be less than 8 psi. So I need to get another regulator too.
I appreciate all the tips! Thanks.
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