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Hi,
Long time member of ICS, and on my second Twinkie.
This one has an unusual problem - The indicated fuel flow (Factory gauges) indicates nearly double what it's actually burning. The gauge reacts to throttle and mixture adjustments, and the two sides are fairly closely matched. Where to start?
Thanks so much!
Justin
Justin, what I observe is that you are comfortable with the problem being the gauge reading, and not the engines.
Let me explain how the system works. Off the spider on the top of each engine there is a tube, a pressure line, that runs from the spider, back though the aft engine baffle, through the engine firewall, and through the wing into the cabin and into the back of the fuel flow gauge.
On the Pipers, the lines are empty of fuel, while Cessnas are wet lines and full of fuel. So, your lines should not have fuel in them. I have found many Twins that have fuel in them, probably from a mechanic misunderstanding that they are air pressure lines. As the fuel flows the air pressure in the lines builds, and the gauge on your panel has a bellows in it, and the pressure bends the bellows outwards a little and the needle responds accordingly. If there is fuel in the line, it may give a false reading.
If there is a break in the line you will get a high fuel flow reading, and I've seen that. I annualled a PA30 where the owner complained of the left side burning more fuel and, in flight, the gauge showed it burning 2 to 4 GPH more. I located the break in the line at the rear engine baffle and the sharp edge of the aluminum baffle had eaten through the line. I repaired the line and the fuel flow went to normal.
If you want to try blowing out the lines, you remove the line from behind the gauge (remove the gauge and that exposes the lines) and remove the line at the spider, attach a tube to the line in the cabin and blow, and have a helper tell you if fuel comes out (it will be a volume, not a bubble). Be sure that you do this with an A&P since it must be done properly and signed off (you don't want a fuel leak from an improperly connected line). Your fuel flow lines should have only air in them.
Good call Pat, I'll disconnect the lines on both sides and blow through them.
When I purchased the plane, the previous owner said the problem appeared after he had the fuel system (injectors, nozzles, manifold, lines) replaced and that maybe the gauge isn't compatible. I'm not sure if I buy that, but maybe the mechanic did something wrong, like fill the lines with fuel.
The gauge may simply need an adjustment, and this has to be done by an instrument shop.
Where the pipes attach at the back of the instrument you will see the receptacles on the back of the gauge ..... these are the adjustment, and if someone has fiddled with these then the reading would be off. At this point, if I was with you and working on the problem, having just learned that the prior owner did all of this, I would take the unit to a professional instrument facility and have the calibration checked. It could be just that simple.
If there is any leaking fuel anywhere there should be some blue staining from the av-gas evaporating and leaving the dye behind. If you see any of that do not leave it alone until you get is sealed up. The blue stuff must stay inside the engine...
