Fuel return Line for a 912UL

Yes, the 0.35mm orifice is too small for 5psi and too large to 45 psi.

But it is an over the counter item and "Close Enough" for most purposes.

Because no one officially is willing to put a number on it,  I descided that 5 gph is a reasonable number to target.

The pumps typicall move 30 gph at Zero psi and can move half that amount at 70% of their peak pressures.

Assumiing 15gph avalable, the engine can use up to 5, if we bypass another 5 that leaves the last 5 in reserve with the pump operating at 1/3 peak capacity.

Anywhere from 2 to 8 will be close enough also.

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On a Historical Note, the Orifice first came into use on the Fuel Injected engines with the intention of allowing the fuel hoses a way of relieving their 45psi operating pressures while stopped.

It was then found that the extra flow, especially in a hot engine operating at low power during approach, was benificial at keeping the fuel hoses cooler and free from vapor locks so the baypass lines were add to all the engines.

The 0.35mm Off-The-Shelf, Repurposed, Fuel Jet Oricfice just came along for the ride.

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I made my own.

I took a Cap-Head Screw sized that it would thread into a Tee Fitting and drilled an 1/8" hole clean through the middle.

The hole was then backfilled with solder because it is a lot easier to drill Solder than it is to drill steel with a #78 Jewlers Drill bit.

(Side Note: Solder machines even cleaner than brass!  Really nice Stuff!!!)

This also has the advantage that you can use a larger hole because the orifice is not sharp edged but a passageway with real length adding to the restriction.

The larger hole is less likly to become blocked with micron sized debris.