Tiny Bubbles

[alpinelakespilot2000]

Just did my fuel flow test today on one tank and noticed a few bubbles. Let me explain...

My tailwheel RV-9 sits at a deck angle of 12.4 degrees. In order to simulate a 20-degree climb angle, I connected some clear tubing to my fuel line where it enters the carb and then raised the tubing 27 inches above the carb before letting it drop down into the empty fuel can. Anyway, b/c I was using clear tubing, I could see the fuel as it flowed into my gas can. Of course, as soon as I turned on the pump I got a lot of big bubbles as the fuel system purged itself of air. However, even as I got close to done pumping the 2 gallons I had put into the tank, I was still seeing a few very tiny bubbles flowing with the fuel. I assumed I must have had a leak somewhere that allowed the pump to pull in air, but everything was dry around all the fittings, from tank to carb. Is it normal for this to be so? If so, where might this air be coming from?

FWIW, I was still getting about 26.8 gph for flow and only need to have 18.25 on my 0-320 to meet the 125% requirement.

Thanks for any thoughts.

[airguy]

Pinch the hose off to supply a little back pressure on the pump and look for leaks. If you've got the line running uphill 27 inches and then back down to the floor to a recovery bucket, you can actually have a slight vacuum (normal to atmospheric) on the fuel system which would allow you to pull air in from any leak. If you pressurize and find a leak, fix it. If you pressurize and still see bubbles, then you're either sucking air in from somewhere on the suction side of the pump or you're still clearing air from the system.

Run the pump with the return back into the tank for several minutes and see if that clears up the bubbles, if so it's most likely just air trapped in the system clearing itself.