Vacuum reg setting question

[PoAdeleted5]

Go the new engine on my Cherokee, complete with a new vacuum pump ( Rapco 215). Cleaned and checked all the hoses, installed new filters, checked all the connections.

The old pump was pretty worn, but it would make acceptable suction on the gage as long as it was above 1500 rpm. (looked inside of it once it was off, the vanes are near the end!).

The new pump will pull 5.2 at 800 rpm. Once we got airborn, saw that the suction comes all the way up to 6 on the gage at flying RPM levels.

I'm figuring that someone cranked up the regulator to compensate for the weak old pump along the way. Now I'm just curious as to what the acceptable range of adjustment is?

The thing that has me wondering is that it seems to hold steady at around 5, until the RPM goes over 2000 rpm and goes up with engine rpm from there. My mechanic is going to adjust the setting, I'm just wondering if there might be a problem with the regulator that's showing up now with a brand new pump.

 

[Tom-D]

Now I'm just curious as to what the acceptable range of adjustment is?

4.8 to 5.2 it's on most gyros .

My Gyros require "0" hg to run properly.

 

[Let'sgoflying!]

"I'm figuring that someone cranked up the regulator to compensate for the weak old pump along the way. Now I'm just curious as to what the acceptable range of adjustment is?"

Your theory sounds quite plausible.
And I would want it back down, I'd imagine the gyros would spin too fast if offered excess vacuum.

 

[gismo]

The thing that has me wondering is that it seems to hold steady at around 5, until the RPM goes over 2000 rpm and goes up with engine rpm from there. My mechanic is going to adjust the setting, I'm just wondering if there might be a problem with the regulator that's showing up now with a brand new pump.

From what I've read, this is an indication that either the regulator's bypass intake is clogged, the regulator is dirty/sticking inside, or both. The regulator should be able to handle far more airflow than the pump can supply at any RPM the engine is designed to operate at. IMO something's wrong with the regulator and/or it's air supply, not the pump or the rest of the system.

Assuming you have a suction system (vs a pressure system), it should work like this:

The pump pulls air through the regulator and dumps it overboard. The regulator has a port that lets in enough air (from wherever the regulator is located) to keep the pressure in the line to the gyros at the set amount below ambient. The air to the gyros comes from the cabin, through a filter (either a common one for all gyros or individual ones on each gyro). The air coming out of the gyros passes into the regulator.

Because of differences in how the gauge is plumbed into the system the nominal and limit values for suction are different for different aircraft. The limits information should be in your POH in the limitations section, and you can probably set the nominal reading at halfway between the limits.