found cause for losing manifold pressure

[GJ Humphrey]

I've just come through a very lengthy, very frustrating and very, very expensive trial in dealing with a rear engine manifold pressure problem on N3CU, a 73 T337G.

At altitudes above 10,000, I could not bring RPM below 2500 without the manifold pressure dropping suddenly by a couple of inches. At altitudes aove 10,000 the problem was even worse.

We went through all the usual tests looking for exhaust and/or induction leaks, blocked orifice, bad waste gate controller, sticking waste gate, bad turbo, bad Absolute Pressure Controller -- you name it, we did it. What an agony!

Finally, I think we found the problem. The rear fuel pump pressure was set too low. To increase (or decrease) pump output pressure is the simplest, fastest, cheapest operation you can think of. A slight twist of a setting screw.

The strange thing is, before the adjustment of the fuel pump I never had any problem in getting full take off power. I could even cause an overboost if I wasn't careful. Yet, using a pressure gauge attached to a T-fitting while the engine was at take-off power, the mechanic found the fuel pressure was below that specified in the service manual.

There is a procedure in the Aircraft Service Manual that details how to set the fuel pump pressure while the engine is runing at take-off power.

On the test flight to 10,000, RPM came back to 2300 with no problem. After the test flight, I flew N3CU home from Fort Pierce, FL to New Hampshire into fierce headwinds and climbed only to 11,000 on each leg. Again, the rear engine ran happily at 2350. The airplane went in for it's annual th next morning, so I haven't had the opportunity to take it to a higher altitude, but I think the problem is fixed.

Gordon Humphrey