Is it necessary to install either a mechanical fuel pump or electric fuel pump in a 750 Stol fuel system? Or both.

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Depending on type engine, you will normally have at least a primary and auxiliary fuel pump; auxiliary usually electric.  Carbureted and fuel injected engines fuel systems are different.

With our Viking engine (direct fuel injection), we have two electric fuel pumps inside the header tank supplying low pressure fuel to the engine driven fuel pump.  

Ditto Loren's remarks.  There is no standard fuel system for the STOL 750 - it depends on the engine, not the airframe.  My 750 has a Jabiru 3300.  The engine came with a factory installed mechanical fuel pump for the Bing carburetor and did not require an additional fuel pump.  I've since changed-out the Bing for a Sonex/AeroInjector TBI carb and it doesn't use a fuel pump at all - strictly gravity-fed and works great.  I like the KISS (Keep It Simple, Stupid!) simplicity ... gravity is always "ON"  !  ;>)

Fuel injected engines will definitely have fuel pumps and also return lines to either a header tank or the fuel tanks.  Therefore, you definitely want to determine what engine you will use in time to install the appropriate plumbing during the build.



John, that Sonex/AeroInjector sounds like a nice improvement.  One exception to requiring a fuel return line for fuel injected engines is Viking's new Honda direct fuel injected engines, such as the Viking 130.

Yes, the AeroInjector is dirt-simple, yet it provides a mixture control, which I did not have with the Bing.  I also saved several pounds in the conversion and that helps useful load. I have found that with this gravity-fed system it is critical that there be no high spots in the fuel line firewall-forward so as to prevent vapor bubbles being trapped and causing "burps" or hesitation. So as not to creep this thread, the details of the install are here.  

Didn't know the direct injected engines don't require a fuel return line - that greatly simplifies the plumbing!


when using the 912 series engines I never use the in-line elect. pumps for stand - by or emerg., because they are 1/4" pumps and have a back check valve that hinders gravity feed to the mech. pump . no matter how you plumb it it will need a check valve , also if used WITH the elect. pump they CAN over power the needles in the BING carbs causing flooding when in a critical phase ..BOB 


I modded my fuel system from the 701 plans system some time ago cause it wouldn't reliably gravity feed to the carbs with the small lines and plans routing. Went to 3/8 lines out of the tanks, forward about 6" above the windows into facet pumps(3/8) then down the windshield braces to a L R OFF valve on the glare shield, down thru the glare shield and out the firewall to a gascolator and forward to the engine driven pump. I got reliable(very) gravity flow at the carbs now but still use the electric backup pump on takeoff and go around. The backups cause I have had 2 engine driven pump failures and also it's possible on my plane to get the engine driven pump above the fuel tank outlet in the climb thus preventing gravity flow. I have the low pressure model that makes about 5 psi, about what the mech pump puts out. I've run this system a couple hundred hours and it's been perfectly satisfactory...I'm wondering about the "back check valve" you mentioned being in the pumps. Would you tell us more about that? I been running those pumps for decades and no problems with gravity feed thru them at all. And what's this about we will need a check valve no matter how we plumb it...why is a check valve needed?


Joe Spencer


On both the mech pump failures I turned the backup electric pump on and got pressure back. W/O the backup the failure would have resulted in an off field landing both times. That was on the original plans system design. After the 2nd incident I went to the current system. I also got full control over fuel management now with the tank selector valve. Works good!


Theoretically, you won't need a pump of any kind with a carburetored O-235. You will need 3/8 lines and you will have to insure the line routing doesn't cause any feed problems. Typical to consider is such as the lines if routed forward along the top of the windows, will the line gravity feed over the high spot at the forward corner of the window where it meets the windshield (while in the climb). There's all sorts of ways to screw up fuel systems and they have always said that's the number one reason for engine failures in homebuilts. You gotta do it right, don't build in any traps for yourself

Have fun


How does the check valve work? By that, I mean, all the check valves I'm familiar with are designed to block flow in a specific does one PREVENT flow blockage?

I've done numerous gravity flow checks thru facet pumps and haven't found a problem yet. AFAIK, the facet is pretty much the standard pump used in Van's RVs. I used to be active there and never heard of a check valve in a facet.

OK I just googled the facet specifications. Looks like you guys have got the check valve thing backwards. The only one with a check valve is the one for diesel operations used to prevent drain back and loss of prime. All the other ones are stated as NOT being fitted with a check valve to allow flow in case of pump failure. That makes sense.

Wonder how THAT got started...another one of those things that folks hear and pass along not realizing it's incorrect. Well, one of the good things about forums, huh. We learn.

I'm still wondering why Bob says we have to have a check valve no matter how we plumb it.

looked further and found this link that says built in check valve but only on some models. Looking at the Spruce catalog most don't have a check valve. Believe I got the 40108. 3/8 flare and no check valve

Hi Jeff,

On my CH701sp, I got a Subaru EA-81 with an AeroInjector from Sonex. Since the engine/AI only requires 1.5 - 2 pounds of fuel pressure, no fuel pumps are required. I initially equipped the plane with an electric pump (with a backup) and had to install a fuel pressure restrictor because the pumps were feeding 4-5 pounds pressure... The pumps were flooding the AeroInjector and I turned them OFF after initial startup anyway. Finally, I got rid of the pumps, pressure restrictor and only use GRAVITY and it works great.

Always depends on the type of fuel delivery your engine requires... If your engine is low pressure fed, no pumps required...If high pressure then, you got no choice.



Ch701sp, Ch750

Jeff if you're installing a Lyc O-235 you do not need a fuel pump on either the engine or electric, gravity will work perfectly.  If you want a pump anyway I would opt for the electric since you can control it, whereas an engine mounted pump is always on.  I'm actually not sure that if you have an engine mounted pump and it failed that gravity would be able to take over (you'll have to research this with Lycoming) and in that case you would want an electric back up.  Easiest would be gravity but no harm in adding a Facet pump like the one that Joe Spencer uses...40108, that will allow fuel to flow even if it fails.

The previous comment about always needing a check valve would apply only to certain specific engine installations and would not apply to a Lyc O-235.

Agree with Don.

Here's some info from I think it was Tony Bingelis.

>>>Just keep this in mind. A failed engine driven fuel pump can create a serious fire hazard if the diaphragm ruptures and if the vent has not been connected to a line routed away from the hot exhaust pipes to a safe overboard location. Furthermore, the engine won't run any more . . . not without the help of a back-up fuel pump.

Such an auxiliary pump can keep your engine running even with a failed engine driven pump. This is possible because the AC type engine driven fuel pumps (and the Thompson and the Romec pumps as well) have an internal bypass valve that allows the fuel to flow through the engine driven pump even after it has failed./p>

Obviously he's talking about a non gravity system but the main points still apply.

IIRC the straight thread inlet/outlet fittings for the pump have a significantly smaller I.D. that the 3/8 line. Not only what Tony said above but I wouldn't want any restricted I.D. fittings. I may remembering that wrong...somebody correct it if so. And the Lyc pumps are heavier than the Facet. I definitely wouldn't want that pump on my plane


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