low fuel pressure on main pump

Are you testing the pressure with the engine running and the power bus at 14+ volts?

It is not uncommon for the pumps to strain with only the 12 volts from the battery.

The pumps are slightly modified and relabeled Pierburg Fuel Pumps.

They most closely match the specs of the Pierburg 7.212787.53, 7.21565.70.0, and 7.21565.71.0 pumps.  (~$150 USD)

These three pumps differ only in the Fuel Input connector.

They match the Rotax pumping specs but vary in the input Hose Connection and the Electrical Connector.

I do not know of any source of a direct match.

Coincidence???

My Main pump is also marginal on the battery. Aux is solid.

It makes me want to question the Supply power.

Try swapping the pump power connectors and see if the weak pump becomes the AUX.

Thanks Bill for your fast reply.

The test done on 4000 rpm. I didn't payed attention to the voltage during running. 

When I tested the pumps before running the engine each pump gave the same pressure, but the noise is different. 

Will go through the voltage question before blaming the pump. 

Thanks. 

Just out of curiosity what is the Serial number of your pump? 

I would guess that if one pump gives 2.8 Bar (really it should give 3.0 bar if to spec) and the other 2.6 It likely not the electrical system.  You cannot give one pump different power.  The power is regulated by the fuse box and when the engine isn't running it would be battery voltage.  If the engine is running and Regulator A is powered (after 2500 is reached for 3 seconds) it would be supplied with 14.2 volts +/- 0.2.

Bill I would be interested in your serial number as well.  I smell problems ever since the ASB on the fuel pumps came out checking for leaks I have been suspicious of that serial number group. 

Keep your head on a swivel.

Our pumps are S.N 12.0086

At the end of the day we found that the analog pressure Gauge was pressure loaded in between (the glycerine inside was under pressure.) after the inner pressure released, the gauge shows 3 Bar. According to that, we set the pressure in the computer system and everything looks good. 

Thanks for sharing your thoughts. 

Micha