I stupidly landed my Twin Comanche I-NASA on its belly last November: virtually no damage to the plane, but the props were scrapped and the engines were opened up and inspected for shock loading. The work was done by a highly reputed Italian workshop, whose owner and boss is a recognised authority on Lycoming engines. Both engines were fully disassembled and thoroughly cleaned before reassembly and bench testing (including the fuel injection system)
During the first test flight, the RH engine momentarily lost power in the climb-out, and remained pegged at 2500 rpm for about 40 seconds, and then recovered. It was inspected and the fuel injection system was found to be too rich and recalibrated. After about 5 flying hours, I took off in a very hot day, and it happened again: but I had forgotten to switch on the electric pumps, so I attributed it to a vapour lock. It happened again in Poland, and my copilot quickly leaned the engine, which immediately recovered. So I decided to have the system inspected at the first oil change (50 hours), and flew with great caution, running the engines at FT for 30 seconds before releasing the brakes.
Then it happened again, and this time the engine never recovered. I took off from Reggio Emilia, and the RH engine ran down to 2500 rpm. I leaned it, switched the electric pump on and off, tried everything, but it went on vibrating until landing in Milan after 45 minutes. Curiously, the CHT dropped to about 250°F and EGT remained fairly low irrespective of the leaning. I flew back at 23.5" MP and 2300 rpm, at 130 KIAS instead of the normal 140-142. MP was normal, and therefore there should be no influence of the alternate air door.

So I stopped flying and asked the engine guys in. They did a very careful test and recalibrated again the fuel injection system, which was again too rich. Now the engines tick over at idle at 700 rpm without problems. All the parameters were OK, so we went for a test flight. And sure enough, during engine runup it did it again.

The engine guys suspected a sticking valve, and performed an SB388 inspection: as I was sure, everything was OK. So they tried again to reproduce the phenomenon, and at the end they concluded that it is a vapour lock due to excessive engine temperatures on the ground (as indeed it was, after such prolonged tests). They claim that everything is OK and that this should not happen again if I avoid to run the engine too long at high setting on the ground (as I was doing for prudence with my 30 seconds FT runup before brake release).

I don't feel too happy about this response, and certainly for a while I will avoid short fields. But has anybody any comment to offer?

Thank you all in advance and best regards

Are the fuel lines up to the flow divider covered with firesleeve? Perhaps the fuel lines can be routed better. Just some random thoughts.

I would suspect the servo fuel injector. Problems there will cause similar symptoms. Ask how i know. The other hint is that the mixture keeps changing.

Jim Hiatt

jim,
I also suspect the servo fuel unit, but it has been carefully inspected and nothing was found. Do you have any other info on the matter? what do you mean by "ask how I know"?

I recently had a similar problem with a fuel injected TIO540-C1A in my Aztec. Engine running rough or quitting at high power and idle. Mixture moving all over the place as evidenced by variable EGT settings. After completely troubleshooting the entire system several time I was left with only one component. Call to several fuel system overhaulers led to a concensus that there was probably an internal ressure leak in the servo. An overhaul (and $1200) solved a problem that had been intermittent for months. I used Avstar Fuel Systems 1365 Park Lane South, Jupiter, FL 33458 561-747-0785 ( http://www.avstardirect.com/ ).

Jim,
very interesting indeed. Did in your case the phenomenon occur continuously, or only seldom? In my case most of the flights were absolutely perfect, and the cutbacks occurred only four times in about 45 flights.
Could the pressure leak in the servo fuel unit be so discontinuous?

The problems were intermittant to start with, but became more common as time went on. A call to a good overhaul shop describing my symptoms brought forth the diagnosis, which proved correct.

I bought 4 years ago a Chicago PA30 C Turbo with a fresh annual and the engines only 200h,So I believed everything was fine. But after purchase and on my way over the north atlantic to Europe,I noticed the same problems.
I`m flying mostly high temperature area In Spain and I had to abort 3 times take off because 1 Cylinder didn´t work and one of the the enginees only gave 2500rpms.
After a lot of attempts with changing the plugs and cleaning the nozzles, without remarkable improvement. I send the injection mixers to an authorized shop and they found out that both worked always much too rich. Obviously they haven´t never been changed, maybe the first pieces from 1967 out of the fabric.
I bought with Spruce en exchange new injection mixers and spyders.
Now, it flys great as promised by the books . No problems any more!
Klaus.

I am returning to the item I opened back in 2011, the occasional engine cutback during take-off operation.
I had the whole system rechecked and recalibrated, and everything works perfectly. But occasionally when opening up for take off the right engine falters, with RPM pegged at about 2500 rpm, and FF oscillating wildly around 10 GPH instead of the customary 14 - 15. Abrupt leaning of the engine once or twice normally clears the problem, and I have resorted to a 30 seconds full throttle run before releasing the brakes.
But the other day I was performing a touch and go, and when opening the throttle the thing happened again. I could no longer stop the plane, so I took off, and pumped repeatedly on the mixture lever until the thing cleared itself and the engine ran on again regularly.

At this point I suspect the Romec pump: if everything is correct during the engine runup, I throttle back to taxi and line up and switch the aux pumps on: with engines at idle, the Romec pump bypass valve should open open and might remain stuck, so that when I open the throttle, the fuel flow and pressure cannot build up enough for the engine needs at full throttle.
Alternatively, the relief valve coul also open and remain open: this would be a more dangerous thing, because it might happen in flight (but so far it never did).

The pump has been bench tested and found OK, so I am at a loss. Does anybody have a clue on this matter?

I will try now to take off without the aux pump on: if the event repeats itself, it might indicate a problem with the relief valve, if it doesn't it might point to the bypass valve.

Do you agree on my thinking?