Fuel Starvation in Flight

[Direct C51]

I am having a fuel starvation issue and was hoping to get some ideas from the collective knowledge of the group. 3 times in the last 5 or so flights, I have had my engine nearly die in cruise flight. This happens randomly in cruise, without changing anything. Looking over the data, my fuel flow will slowly decrease from about 5.5 GPH (normal cruise flow) to about 3GPH. This happens over about 30 seconds. My RPM will drop drastically and the engine will nearly die. I go full rich and throttle back to about half throttle and the RPM eventually comes back. Once this happens, my fuel flow will go to 10GPH+ for 6 or 7 seconds, presumably refilling the float bowl and maybe fuel lines. I'm using a Marvel Schebler MA3-SPA carburetor. My fuel vent line is per Sonex plans, using brake line and exiting under the airplane with a 45 degree cut, in to the wind. My fuel plumbing is as simple as it gets. Just a fuel filter and red cube, no gascolator. The last 2 times this happened, I have had over 10 gallons in the tank. Has anyone had similar happen? Any ideas?

[kmacht]

Sounds like either the fuel vent line is getting plugged up or maybe the finger strainer in the tank is getting blocked. I would pull the line from the carb and do a fuel flow test and not fly again until you have it figured out. What engine?

Keith
#554

[sonex892.]

What Keith said and a couple of other things.

I'm guessing its a float bowl carb, if so could be the float sticking? Do you run a fuel pump? Could the fuel flow sender be restricting it?

[WaiexN143NM]

Hi guy,
Just throwing out a couple more ideas. Please confirm what engine your using also. The floats in those carbs have had some ad's over the years. Fuel lines insulated? Sounds almost like vapor lock. Also what kind of air induction? Filter? Clean? Airbox? And just gravity feed? Good luck, be safe, hope you find out and let us know!!

WaiexN143NM
Michael

[cliffrunkle]

My first thought was your fuel tank vent. Take a hard look at that and see if a vacuum
is not being created at cruise flight.

[fastj22]

Just my two cents and worth every penny, but seems like a vent issue. On my aircraft, I have the fuel vent taking ram air like the pitot. IE, bent into the airstream.

[WaiexN143NM]

Hi all,
Yes john, i keep thinking the vent too....., we origionally had the vent on top,and bent 90* into the wind. We then took that off worried about the tank full , and having some fuel vent overboard and on to the lexan. Peter anson has a good check valve vent now to take care of that. We then installed a steel brake line ( in case ever of a fire in engine compartment) vent line that goes down the firewall, and exits out the bottom, bent 90* forward into the wind. We also drilled a small hole on the aft backside , just above the 90* bend, just in case a bug or debris would clog up the front opening.

WaiexN143NM
Michael

[Direct C51]

Thank you everyone for the discussion. Yes it is a float bowl carb. Marvel Schebler MA3-SPA. Same as a Cessna 152. Rebuilt by an airplane carburetor shop in the last few months with only 10 hours on it so far. Engine is a Corvair.

I was looking at my fuel vent and it is possible that the ram air is being disturbed by the cowl exit. I am going to relocate my vent and face it directly in to the airstream. Of course I am going to check the fuel filter and finger strainer first. The odd thing is that this seems random and not associated with a certain airspeed or angle of attack. Perhaps it is just the perfect storm that causes it.

If that is not the problem, then I might look in to the float possibly sticking. I would not assume vapor lock as the fuel flow was never less than 2.8 GPH, and the conditions did not seem conducive.

[fastj22]

My vent line is an Autozone brake line with the ⅛ NPT fitting on one end, inserted into the tank on the top fitting with a 90 degree bend. I route it through the fuel tray on to the PX side, bent around the shape of the outer hull down to the bottom of the firewall at the gear exit. Its then bent 90 degrees into the wind. I never liked the loops above the filler some builders have done, wanting a concealed vent instead. The only issue is if I fill the tank to the top and let it heat soak, I will get some overflow through the vent to the ground, but its dumping under the plane.

[Direct C51]

John, I like that idea. I think I'm going to move mine to that same area. Mine exits closer to the centerline of the airplane under the firewall, which probably has some majorly disrupted airflow because of the cowl exit.