Bob,

I have a Mallory MAL-4305M fuel pressure regulator installed and had the following experience with it.

The fuel pressure regulator is an adjustable aftermarket racing unit that also modifies the fuel pressure according to the intake manifold pressure. It does this in order to keep a constant difference between the fuel pressure in the fuel injectors and the pressure inside the intake manifold.  This should make the injector flow rates very predictable.  After the regulator was used with avgas, applying a vacuum to the compensation port when checking the regulator gave no response.  Adjusting the overall fuel pressure also gave erratic results.  Upon taking the regulator apart, I found that the x-ring seal between the piston and movable cylinder that acts as the valve was very tight and made the parts hard to move.   It took 4 lb of force (not psi)  to move the piston in the seal.  The piston diameter is 0.5”.   I lubricated the seal with Fuel Lube and the regulator seemed to work better this way, at least for a while.

During the first flight after lubricating the parts, the fuel pressure changed with manifold pressure, although not as much as it should as indicated with my data logging system.  At the end of the flight, the fuel pressure didn’t follow the manifold pressure at all.  Probably the Fuel Lube had washed off the x-ring seal during the flight and the parts again weren’t moving as they should.  I could get the engine to run alright by manually adjusting the mixture for the conditions at any particular time and, during cruise at least, this isn’t hard to do.

I called the regulator manufacturer and they told me that the x-ring material should be unaffected by avgas, auto fuel or even alcohol.  They said that maybe the avgas had different lubricating properties since the regulator works in other applications.  They had no suggestions as to how to make the regulator work better so they were of little real help.  Figuring that I couldn’t make things much worse, I machined off the piston x-ring land and made a Teflon cup seal like those used in hydraulic jacks, only smaller.  This eliminated the X-ring seal.  The bore of the cylinder was somewhat rough, so I tried to polish that too.  The regulator works much better now.  I tried to attach a picture of the parts after modification, but wasn’t able to get it delivered to the list.

Steve Boese