Fuel flow meter “red cube”

[cscotthendry]

If the fuel pressure gets too high it could conceivably overpower the float valves. I'm guessing thought that, that kind of pressure would manifest itself in a number of forms. One of those would be flooding the engine with attendant bad running. Another might be fuel running out of the carby vent tubes. Did you have any of that when you had the proper size restrictor?

As Thruster said, with the Rotax fuel pump, it will determine the fuel pressure based on its internal mechanisms. You can safely assume that unless the pump is faulty, the pressure would be within safe values. More likely from a defective fuel pump, is low pressure.

If your fuel pressure IS too high, the more likely culprit is an oversized boost pump. Check the model of your boost pump and look up its specs against the Rotax specs.

[Kyle Communications]

The savannah system works well..although the kit supplied arrangement is crappy and leaks. But basically it a block with 4 ports screwed into it. One is the inlet from the pump then two are to outputs to the carbs and the last is the return to the tank. This return fitting has the .35mm restrictor in it.  This restrictor will NOT do anything to your fuel pressure from the pump on the motor. You have a issue somewhere else. The electric boost pump is usually a facet and the number is a 40105 for a high wing tanked aircraft and a 40106 for a low wing tanked aircraft. The 40106 has a bit more pressure out to pump uphill

 

 

[K5054]

19 hours ago, cscotthendry said:

I just reread the fc10 manual. It seems like adding a second sensor should be pretty easy.

The cube has 3 wires, 2 are for powering the sensor and the other wire is the signal back to the FC10. The second cube gets its power from the same connections that power the first cube ie, their power connections are paralleled. Then the signal wire for the second cube connects to pin 8 of the FC 10.

But now you have two k factors to adjust. One for the supply sensor and one for the return sensor. The K factor is like a “slippage” or fudge factor for the sensor. 
Since you say you get accurate numbers by turning the return off, I'd guess you have the K factor about right for the supply sensor. If you mount a second return sensor near the first one and in a similar orientation, you could use the same K factor for the second sensor as a starting point.

By leaving the number of engines set at 1 in the FC 10 setup, I'm guessing that it will subtract the second sensor's readings (adjusted by its K factor) from the first sensor's readings.

HTH

The fuel pressure is at the higher end when return line is closed but gets close to limit when boost pump is on. With return line open it sits about 3-4 and 5-6 with boost pump on

7 hours ago, cscotthendry said:

Good stuff.

I still think your return quantities bears investigating. From the figures you quoted, it sounds like more fuel is bypassing the carbies than they're consuming and that doesn't sound like it should to me. I'd check that the restricter is there, that it is the correct size, and is in the correct path.

BTW, the local Rotax dealers suggest a carby jet insert as the restricter. From memory, it is an M5 thread and will fit nicely in the end of a brass T that has been threaded with an M5 tap. Rotax sell a cross connector with the built in restricter, but it's very expensive!

Good luck.

 

5 hours ago, Thruster88 said:

The fuel pressure produced by an engine driven diagram pump is done by the internal spring, it cannot be to high if it is the correct pump. Are you sure of the accuracy of your instrumentation?