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I'm having a problem with fuel delivery. My system should use the mechanical pump on my Corvair engine and an electrical "Facet" pump available at the click of a switch.
Here's a picture of the system:
I'm not sure why this is happening, but if I put a "L" in place of the "T" after the check valves the Facet pump works and I get a fuel pressure reading of about 2 or 3 psi. I'm use to electric fuel pumps from my experience with MGs and on these cars the pump would only pump when there was a need. Those would slow down and stop pumping when the float bowl was full and only pump once in while when there was a need for more fuel. This Facet pump continues to sound like it's pumping (making lots of noise) even when the system is full of gas all the way to the float bowl and the bowl is full. Is that normal?
Why should there be any difference between having a check valve right above the "T" and bypassing the mechanical pump with an L. Here's a sketch of the firewall forward part of the system:
You must have the two pumps in parallel, Jim. If you have them in series as William has them set up, no check valves are needed. One pump runs and the other pump is static and the fuel just flows right through it. If you power the pumps with a single pole, double throw switch with a center off position, you can change pumps with the flick of a finger. William uses a double pole, double throw switch that also has the two ignition systems on it. If the engine farts, one flick of the finger replaces both the ignition system and the fuel pump - if the engine starts running well again, you can sort out which component is bad on the ground. Airborne is a lousy time to troubleshoot and make life and death decisions.
The William Wynne Corvair fuel system for the 601XL/650 is about the simplest and lightest system around. It will work with non-Corvair engines just as well as with Corvairs. It is proven in thousands of hours of operation in numerous aircraft. Why re-invent the wheel (or the fuel system) when a good and proven one exists? I am not afraid to copy good and proven designs. I kind of prefer it to groping around in the figurative dark trying to come up with my own design, as a matter of fact.
I agree with Daryll, Dan. Your fuel system is way too complicated. Simplify. Add lightness. My two cents.
...unless, of course, one pump fails in such a way that it cannot pass fuel.
Although it was on a completely different aircraft, a friend had two pumps in series. When one failed, it failed in such a way that it blocked the flow of fuel. He was a low and slow flyer. He couldn't make it to a flat area, and died in the resulting crash.
The crash was used in the following year's IA renewal seminar as an example of poor design. The FAA was begging us to watch for this particular configuration on this particular aircraft. It was an accident just waiting to happen, and they wanted us to catch it before it did.
I'm not saying this will happen every time, but the idea of having redundancy implies having systems in parallel rather than series. That way, when one fails, the other can take over with less chance of a problem.
I'll freely admit I'm sensitive in this area, but losing a friend in a pointless accident does make an impression that lasts. I've met and respect Mr. Wynne, but we disagree on this point.
AH, even more to consider. I know I'm taking a beating for my over complicated system but it wasn't that many years ago that this was the system that William Wynne was recommending. IT was in that time frame that I was choosing and building my firewall forward package. His recommendations then included a fuel filter after the pumps like the Moroso with AN fittings, much like the Russel I'm using. He recommended such things as using the mechanical pump, and running the pumps in parallel with check valves to prevent back flow etc, the use a pressure gauge etc.
I see the drawback that Jim is talking about but I'll have to weigh that drawback compared various other choices. The parallel system obviously has it's own drawbacks. I wouldn't have thought that the pressure would drop to zero by having a check valve fail, but now i get it. It could be just a bit of dirt in on of these is all it takes.
One thing I'll do tomorrow is put a short stub of tubing on the input side of that check valve and attach a tigon tube, then run the pump. That should tell me if the check valve has failed. After that I'll have to make some decission.
I would think that if you have not run the engine driven pump the line to the pump is full of air and that is compressible.
Try opening the fitting at the inlet to the mechanical pump and then run the elec. pump to see if the check valve is holding.
Both pumps have a built in check valve as that is what makes them pump in only one direction, but that is a part that could fail. Yes those pumps do make a lot of noise all the time they are running, but I bet you wont hear it when the engine is running. Could be a great way to let you know you left it on though.
On another note I see about 20 places in your picture that could be fuel leaks at some point in there life ( all the connections). In my humble opinion I believe that the fuel system is a great place for the KISS system. Less is better, if I am flying and the engine is running then I know I have fuel flow and pressure. But I'm a simple kind of guy, that's why I chose a 750.
Have fun and keep building, soon you won't have to make airplane noise when your sitting in your plane it will make it's own noise.
Paul, there's a few things your mistaken about here. Neither the mechanical pump on the Corvair nor the Facet that I'm using have a built in check valve. I have 2 of the facet pumps, one without the check valve (the one I'm using) and one with a checkvalve. The one with the checkvalve takes up more room. If my problem proves to be a bad check valve I'm probably going to opt for the 2 inline Facet pumps. By that time I will have lost my confidence it the check valves. I'll know more tomorrow.
Blow back thru ether pump and you will see. How well can your heart pump without the valves?
I suspect there are check valves, and then there are check valves. Problem is, I don't know of a good cost/weight/size/failure rate tradeoff source on them. I think we put them in on the blind faith that they won't fail, which probably is not the case.
If a check valve is needed to keep the pumps from talking to themselves, it sounds like it is important for it to have very good reliability. I wonder how much fuel actually gets to the carby, if a check valve fails?
In my set up, the filters/pumps have their inputs in parallel, and the output of the pumps are paralleled, and go to the carby. If either check valve fails, it looks to me as though the fuel could just go round and round, and never really get anywhere. I hope I'm wrong on this.
Of course, we could always put two check valves in series each pump, so if one failed, the other would continue to work. :)
But thanks for confirming what I suspected on the Facet pumps: no internal check valve!
You've changed my mind. The pumps must act like check valves to work. I've been testing the electric pump with out running the engine. Which has me wondering. Like you said, why would I even need a check valve? I wonder if what was happening was that the line to the mechanical pump was full of air which was easily enough compress that the fuel at the check valve just flowed in and out to no effect with each pump of the mechanical pump. Now that it's working I'll keep considering what the best approach is.
The problem wound up being the check valve. I removed it and found that I could blow through it either direction. If I blew hard in the stop direction is would close. There must not have been enough flow to close it. I've put the 2 pumps inline (with the electric down stream) and removed the check valve. Good fuel pressure now.