Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 3:58 pm
by 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?
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 4:04 pm
by 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
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 7:10 pm
by 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?
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 7:50 pm
by 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
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 8:18 pm
by 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.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 8:35 pm
by 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.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 9:29 pm
by 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
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 9:40 pm
by 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.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 9:48 pm
by 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.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 10:10 pm
by 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.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 10:38 pm
by Rick524
Could the problem be carb ice? Those carbs originally had carb heat available.
Re: Fuel Starvation in Flight
Posted: Mon Feb 05, 2018 11:38 pm
by Direct C51
I don’t think so. It happened at a high cruise power setting, without any reduction in throttle. Also, I don’t think carb ice would cause the increase in fuel flow above max engine consumption. This pretty well points to refilling the float bowl and/or fuel lines.
Re: Fuel Starvation in Flight
Posted: Tue Feb 06, 2018 9:04 am
by ldmill
I had the same thing happen with the same carb in my Pietnpol. It ended up being a partial restriction in the carb. A piece of something made it into the bowl and partially blocked the pickup in the bowl. It happened first on my dowwind leg at Brodhead, then again on base - then nosed over during taxi after landing. I goosed the throttle, no help, punched the choke - and it backfired and blew my exhaust seat loose in the A75.
Lorin
N81YX - 250+ hrs and having fun!
Re: Fuel Starvation in Flight
Posted: Tue Feb 06, 2018 4:48 pm
by WaiexN143NM
hi all,
One more item to consider, after re-reading all the posts in this thread, is what type of fuel are you using? Auto? , 100LL?, or a combo of those two? If its just auto fuel, id go back to the vapor lock scenario.
WaiexN143NM
Michael
Re: Fuel Starvation in Flight
Posted: Sun Feb 25, 2018 3:08 pm
by tx_swordguy
any update?
Re: Fuel Starvation in Flight
Posted: Sun Feb 25, 2018 3:59 pm
by Direct C51
I’ve only flown it on two quick flights since my first post. I’ve been rather busy with work. I didn’t like how my fuel vent seemed to be disrupted by the cowl exit. I thought maybe under the right conditions, a low pressure area was developing. I have moved the fuel vent away from the cowl exit and now face it directly in to the wind. Like I said, no issues thus far, but I should be able to fly it a lot in the coming week. I am using 100LL for whoever had asked.
Re: Fuel Starvation in Flight
Posted: Wed Mar 14, 2018 8:47 am
by GordonTurner
Did this issue get resolved? Or still too early to know…
Eagerly awaiting your findings…
Gordon
Re: Fuel Starvation in Flight
Posted: Wed Mar 14, 2018 9:53 am
by Direct C51
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.
Re: Fuel Starvation in Flight
Posted: Wed Mar 14, 2018 4:18 pm
by GordonTurner
I am going with the same set up, 3.0 corvair and ma3 carb. Would you mind describing your Fuel Flow installation?
Re: Fuel Starvation in Flight
Posted: Wed Mar 14, 2018 4:49 pm
by Direct C51
I’ll see if I can find some pictures when I get a chance. My fuel system is as follows;
Finger Strainer
Fuel Valve
6AN braided line to firewall passthrough
Summit Fuel Filter
6AN braided to Red Cube
6AN braided to carb
Re: Fuel Starvation in Flight-Update!!!
Posted: Mon Jul 09, 2018 7:36 am
by edclee
Direct C51 wrote: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.
Re: Fuel Starvation in Flight
Posted: Tue Aug 28, 2018 2:56 pm
by 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.