Skymaster Forum - Excessive Fuel Flow Indication

I have been getting an excessive fuel flow indication on the front engine of my 76 T337G at full throttle. The needle actually goes past the top of the gauge and around to the other side. However, at 65% power, the fuel flow indication is close to where it should be.

My mechanic first decided to trouble-shoot the problem by switching the lines on the back of the gauge to see if the problem moved. Unfortunately, this caused further problems. The front side of the gauge quit indicating at all depending on which lines were hooked up to it.

We sent it out for overhaul. When it came back and we hooked it back up, we still got no pressure indication on the front side. We started trouble-shooting that and found that the fitting that penetrates the firewall was plugged with some very fine material similar to very finely ground tobacco. Incidentally, seeing this fitting out of the airplane made me feel a little better about having the fuel pressure lines penetrating the firewall (I never did like this idea). However, the orfice is so tiny (smaller than fine gauge safety wire) that not much fuel could come through it in the event of a leak.

After we got the line cleaned out, we were back where we started - excessive fuel flow indication at full throttle. Next, we began checking the unmetered fuel pressures according to the service manual. Both idle and high throttle pressures were in the correct range. On the other hand, it was not clear to us how from the service manual how to set the metered pressure at high power. The book gives a range for metered pressure (which is about half the unmetered pressure) but it doesn't say how to set it.

We are trying to decide what to do next. I have a few thoughts and wanted to see if anyone had prior experience with this.

1. The fuel flow gauge has a reference line to upper deck pressure on each engine. If there was a leak in this line, or it was stopped up, then the gauge would read too high after the turbo spooled up. This is a possibility that needs to be checked.

2. The idle mixture is probably set a little bit rich. I base this on the fact that we get more than a 25 RPM rise when we pull the mixture control from full rich back to lean at idle. However, I don't know if the idle-mixture adjustment would also affect the metered fuel pressure at full power. We are reluctant to mess with this adjustment unless we have to because we don't want to introduce more variables into the equation.

3. I vaguely recall that there is a "circuit" in the fuel metering apparatus that causes the engine to go to extra-rich at full throttle. However, I don't recall how this works or if there is any adjustment for it. I didn't find it in the service manual.

4. In general, I don't find anything in the service manaul that pertains to setting high-power metered fuel pressures. But maybe I am just overlooking it or something.

That's about all I can think of. Anyone have any other ideas? What is the most likely problem?