has anyone had this problem after fuel flow test all fuel emptied then after refueling fuel will not flow unless i blow back thru fuel lines then the flow starts. is this normal it almost seems like a air blockage    joe

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Thanks Joe. I see what you are saying. I'm also thinking that the lines would feed a header tank mounted on the firewall, which would continue to feed the engine in a climb situation. Then, when leveling out after the climb, the constantly down-sloping lines would refill the header tank, and there should be no need for any other header tank vent.

I would want a vent in the header tank to insure it will feed in the nosehigh situation I described in the post. It might feed w/o one, but there's a possibility(probability?) that the route to the tank vents will be blocked by the unusable(trapped there by the high spot) fuel in the lines to the tank. I would assume during the design that would be the case. There also might be a possibility that the header will refill only at the rate of flow to the engine w/o a vent

Others with more header tank experience may be able to say...but regardless, mine would have a vent to the top of one of the tanks

Joe

Some good points... but do be careful not to overpressure your carbs with an additional pump during take off-  get the ratings right... or you may end up with fuel spilling over the exhaust.... there has been more than one Aircraft BBQ that way!  (return lines are good as is a suitable pump that 'self regulates' pressure... :-)

Understand and thanks...my backup makes 3 psi, lower than the 4-5 that the Rotax makes, so it shouldn't be able to overpower the floats. At the parts house they had a 3 and a 7...I chose the 3 to stay under the OEM. A 4 would have been ideal. I haven't flow tested it with only the backup running and hadn't planned to, but believe I will after all this discussion

thx again

Joe

J. Aloof - If you look on my page you'll see my oversize gasculator. It is elevated in comparison to the Zenith plans. You should also see where my back-up pump and fuel filter (one of) is mounted. There is a pressure drain plug (1/8" NPT tapped hole) at the top of the gasculator; everytime I drain the gasculator (may never have to do it again) I must manually open that pressure drain to remove air in the system. If I had to do it over again I would run a vent tube to the side of the filler neck from that pressure drain plug. A boost from the electric back-up pump before startup to get flow going and it seems to work fine. Now there is a drop to the floor after the gasculator but it angles to the top of the firewall from there. I have a clear inspection filter in the cabin so I can visually monitor fuel flow. Never had a problem with delayed fuel feed in any attitude.

Chris

Joe Spencer - Joe it seems like what we all may be eluding to is some kind of a vent in the middle of the fuel system. Zenith plans does not provide for any kind of vent (thats strange considering the venting systems of other gasoline vehicles). I believe you can take that small gasculator in the factory system and vent it to one of the filler necks and solve a bunch of the problem. I also agree with the header tank concept that's why I made that oversize gasculator. I think we all realize that air ahead of the fuel is a major part of the problem. More than likely caused by low tanks coupled with a an attitude that takes gas away from the tank outlet allowing air to enter the system. However, at least I, have never heard of a 701 with fuel in the tanks have an engine shut down because of lack of fuel. What's your thoughts on that? - Chris

Hmmm...well, let me put it this way. There's a couple things about the system in my 701 that I didn't like, mainly the fact that it' may not feed in a steep climb if the Rotax pump fails and the other is the sorta puzzling very low gravity flow at the carbs with the line disconnected...so my solution was to just consider it a pump fed system and add an electric backup and fuel pressure gauge.  I'm ok with the Zenith design with the extra pump...simple, light, reliable history as far as I know, no air traps or complicated plumbing...I really don't see the need for a header tank or vent in the plans gascolator. Maybe I'm missing something there

I just get very wary when fooling around with unproven fuel system designs cause they tend to be full of surprises, often hard to foresee

 

my 2 bits worth

Joe

Low gravity flow is because of where they (carbs) are located (high.... not necessarly higher) in comparison to the fuel pump. Other aircraft carbs are usually on the bottom of the engine. The fuel pump giving out? Possible but probably unlikely. The reason I mentioned the vent is because we had discussions a few months ago about different fuel burn rates from the two tanks and what may be the probable causes..... possibly air in/entering the system. The vent from the gasculator to the tank can in no way can be a negative it could only help let air escape from the system. Air will migrate through fuel to a high point but air won't allow fuel to migrate to a high point. Well, I just find it strange that the system relies strictly on the vented gas caps given the possibly of air entering the system in a low fuel level in the tanks situation. Safe flying.  Chris

I've seen several 2 tank gravity flow planes that fed at different rates from one tank or the other and in every case that I've dealt with it was cause the rudder wasn't trimmed correctly. That's one thing that will definitely cause uneven feed and probably the most likely, especially condsidering the funky rudder control that we have

Worth what you paid

Joe.

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