pa30 takeoff fuel flow issues

[George Babcock]

Greetings fellow flyers,

1964 pa30, stock engines. Mechanic thought we had glazed cylinders. Inboard cylinders on either side were introducing the bottom plugs to quite a bit of oil. Compression all good. I acceded to removal to rabbit turd and re-ring for an estimated $200 per jug. Just got a bill for almost 3k from local engine shop. Cylinders were instead overhauled. Was allowing airplane to be used for ME training. Works ok when I have time to instruct, not so well when relatively inexperienced MEI's fondle my friend. Think the poor girl was overheated and think I may have discovered the culprit. Take off fuel flow, with electric boost pumps on is between 14 and 14.5 gph at 1000'. As soon as I turn the boost pumps off, the fuel flow drops to 10.5 to 11 gph. There is a corresponding rise in egt across all 8 cylinders. The egt's are as high as at this point as I would see were I aggresively leaning at cruise altitude and power. Fuel flow rises, engine surges, and temps drop as soon as I bring aux pumps back on. The probelm is bilateral and almost identical on either side. Egt's are manually checked for individual cylinders. Consistent rise and and drop.

Both selector valves have been overhauled recently courtesy of Webco. The west coast fuel pump stc was also recentely installed. Everybody jumps to the mechanical pumps as the culprit, but seems like a test of probability that both would fail simultaneously. The sole theory posited thus far that makes sense to me is some kind of obstruction, perhaps of the filter, that the auxillary pump is able to overcome to establish proper prsessure at the servo.

If anybody has any theories or troubleshooting suggestions they can share, it would be greatly appreciated.

Thanks in advance

[N3322G]

George,

In my twin I get 16/15 FF at take-off at 605' and 13/12 FF int he 1700' pattern with pumps off.

Have you already checked the flow divider? and engine driven pump?

[Zach Grant L1011jock]

The Bendix RSA fuel injection, if operating properly, us not fuel pressure dependent. An input pressure of 12-40psi will yield the same results on the injector end. I would hazard a guess that you have some issue with the servos.
Zach

[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
ICS member

[Zach Grant L1011jock]

I believe the max FF spec is 16.5 gph for the Lyc IO-320. You are running way to rich. I dont have the manuals in front of me but remember allowable FF range at max power to be 13.4 to 16.5. The only way to fix this is to send it bck to the overhauler, and tell them to fix it. Tell them what the other side FF is and they can match it on the bench.
-Zach