PA-39 servo over fueling after overhaul?

[Gary Johnson]

I have a 1971 PA39 turbo twin Comanche. At the annual we pulled the heater connection to the right engine servo and found rust in the lower servo. We had the system cleaned, checked and send the servo and flow divider out for overhaul. After the re install the engine run up checked out. On takeoff the fuel flow went to 18.5 GPH on the overhauled side. Prior to the overhaul we were getting 16.1 on this engine and 15.8 on the left. We put a calibrated fuel pressure gauge inline before the flow divider on both engines and ran them at full power. The overhauled servo side read 8.4 the left 7.8. I read that the maximum fuel pressure should be 7PSI. We pulled the injectors and found no contamination. The factory pressure gauge has been replaced with a Shadden which has been dependable. Thanks Gary
Gary Johnson
ICS member

[N3322G]

Gary,

Last month at annual had left servo out for maintenance. Repair shop adjusted it and the engine never ran up so strangely. Sent it back a second time - more repairs and now all is OK after mechanic had to adjust idle.

Might start by asking O/H shop what changes they made and share with them what you saw on take-off.

[Zach Grant L1011jock]

The simple answer here is that the servo is calibrated for too high of a fuel flow (where you put the fuel pressure gauge, between the servo and divider, is simply reading output pressure from the servo, and is a variable pressure based on mass airflow. Lower air flow, lower fuel pressure, and thusly lower fuel flow. If you had the original fuel flow gages in the plane, you would find that they are nothing more than fuel pressure gauges calibrated to read ff based on the variable pressure pick off of the flow divider). The lycoming spec for the IO320 engines is that the servo be set for a sea level std day full power fuel flow between 13.4 and 16.5 gph. I dont know where you read the max 7 psi at the flow divider, but based on my calculations, that should only yield about 15gph, certainly in the range, but no where near the max fuel flow allowed. The servo needs to be returned to the overhaul facility, and you need to instruct them to calibrate the servo for whatever the left engine does at full power if you are happy with that setting. That will make it easier for you to rig and square the engines in all phases of flight. The fuel flow adjustments, except for the idle mixture, can only be done on a flow bench and are not field serviceable items.

Zach

[Gary Johnson]

Thank you Zack for thinking about this and taking the time to answer my post. I agree with your answer. There have been no other changes to the fuel system and the shop overhauled the servo and flow divider at the same time.

Thanks again

Gary

[Gary Johnson]

[quote="N3322G"]Gary,

Last month at annual had left servo out for maintenance. Repair shop adjusted it and the engine never ran up so strangely. Sent it back a second time - more repairs and now all is OK after mechanic had to adjust idle.

Might start by asking O/H shop what changes they made and share with them what you saw on take-off.[/quote

Thanks Pat