Fuel pressure mystery

[vh2q]

Finally made the flight from AZ to TX in my 14-13-2. Long story but 4 shops and about $35K later the bird is in my hangar.

En-route at cruise at about 7500 ft, noticed something strange with fuel pressure. Or right tank full, newly rebuilt gauge showed only 2lbs pressure at first. When tank got to about half full, that moved up to 4lb. Left tank started at 4 and stayed at 4. With aux pump, we were at 5 either tank as checked at runup, or at switchover. What would account for that? Engine ran without a hiccup on either tank. But I do wonder, does the change in pressure affect the mixture setting? We never really checked that. You would think with more pressure the mixture might be richer?

 

[ronm]

Glad your bird arrived at the nest.

Pump and gauge are referenced to ambient, so altitude should not effect reading.

Since all fuel pressures were consistent except the right tank, my guess is a restriction from that tank. One item to suspect are screens at the wing root fuel tank connections. Mine were originally very fine mesh. The consensus, IA, etc, was that they were too easily clogged. I fashioned and replaced with coarser screens for those locations.

Fuel tank vents - check.

I would categorize what you have described as a safety item. At a very minimum, if it was me, I would not use the right tank for anything other than cruise at altitude until I got to the bottom of what is caused the pressure anomaly you spotted.
The tank flow might be checked be connecting a hose to the gascolator and siphoning from each tank. I don't have any specs, but if there is a real problem, it should be evident.

There is a rather common fuel pump leakage problem from the conical cap on top of the fuel pump. Had that happen once. If there is a leak there on the suction side, the pump will pull air and cause the output pressure to drop. But, usually, the fuel pressure indication will be very unsteady, fluctuating from normal to zero. Easy to check and fix.

 

[ronm]

Missed your bottom question: You would think with more pressure the mixture might be richer?

I would think, normally, very little. The carb is sipping fuel from the float bowl. The level in the float bowl will only change a tiny bit with a change in fuel pressure.
But to see for sure, one could adjust the mixture to a sensitive slope on the EGT and turn on the boost pump. Expect no change. If there is a noticeable change, the float/needle/seat/fuel delivery may be off.