Fuel Starvation in Flight

[edclee]

Hi Gordon, I've put another 1.5 hours since moving the fuel vent. Still no sign of any issues. I'm hoping it was just the fuel vent in disrupted air from the cowl exit. I'm using a MA3-SPA and I don't think we get the recommended head pressure with our Sonex tanks and the short downhill run. Any disruption to the marginal head pressure we get might be enough to cause the problem I was seeing.

I have had exactly the same problem, same symptons...and strangely, the same engine. I think the engine has nothing to do with it except that the MSA-SPA carb is a common item on Corvairs. I am thinking the issue is a sticking float needle. I have a fuel pump on mine because I thought it best for a back up. As soon as I turn on the pump, the starvation problem goes away and may or may not return some time after I turn the pump back on. Also on mine the problem seems to occur only when the fuel level gets down to 6 or so gallons. It happens on the ground or in the air. Taxiing on the ground with that level of fuel, then engine may simply lean out and quit. If I catch it starting to run lean and turn on the pump, the issue goes away.
Sonex #1212, 3100cc Corvair

Found my problem and it is likely the cause of yours. After reading about burp tubes I thought the problem could be a buildup of bubbles in the fuel at a high spot in the hoses somewhere. After a examination of all the plumbing I could see there was loop in the hose from the fuel pump to the MSA carb. The hose was about 2" too long and there was a vertical hump in that hose from the pump to the carb because of it. I removed and shortened the hose so it flowed virtually horizontally from the pump to the carb and the problem went away. I have not had any further issues with erratic fuel flow. Whenever I used the back up pump, the problem went away, but at times the fuel flow would lean somewhat then return to normal. That loop was collecting bubbles and reducing the fuel flow because the Sonex has very little head for gravity flow.

[Direct C51]

I have since completed phase 1 testing and moved my Sonex from Shafter, CA KMIT to Gillespie Field, KSEE. I have had zero problems with fuel starvation since. I contribute the issue 100% to fuel vent location. My fuel vent is now directly in to the free stream air. The Sonex simply does not provide very much head pressure. I believe the MA3 carb is more sensitive to low head pressure than the aerocarb. With my current setup, I have no problems, even at high pitch attitudes and low airspeeds. Please let this be a lesson to everyone to watch the fuel vent location and any disturbances in airflow around it.