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FAB fuel drain hole question ?

L

Larry DeCamp

Well Known Member
I just completed a custom FAB for fuel injection. I am debating the merits of a drain hole inside the air filter. My thinking is fuel in the air box will get sucked in the engine or at least contained if ignited. Where as, fuel dripped in the cowl would be a much more serious problem. What say you ?
 
I have the stock fab. The instructions mention drilling a drain hole near where it attaches to the servo. I assumed this was for water drainage, rather than fuel.
 
Way safer to have fuel in the cowl area than in an enclosed area attached to the intake manifold. Your concern is a backfire, with flames extending from the intake valve and past the servo. Pooled gas will create a nice atmosphere with optimal air fuel ratio for quick ignition and the pooled gas will feed that fire for some time. Gas in the cowl is no issue, as any backfire flames will head outside the cowl area via the inlet snorkel.

Larry
 
Wunderon said:
I have the stock fab. The instructions mention drilling a drain hole near where it attaches to the servo. I assumed this was for water drainage, rather than fuel.
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Both. When you over prime an IO engine, all that excess fuel runs straight to the FAB. Even worse, most instructors improperly teach students how to prime a carb'ed engine and routinely flood the FAB with gas on every start. You want as much of that fuel as you can do drain out of the FAB in case of a backfire.

That said, the alt air door does a pretty good job of letting the excess fuel drain out without needing a hole. Fuel has a very low surface tension and easily goes through the small gap.

Larry
 
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I can't imagine this is necessary with a tailwheel though, as the fuel would just drain into the bottom of the fuel servo body and drain overboard via the sniffle valve? Unless my take on this is wrong?
Tom
RV-7
 
Good info, thanks.

lr172 comment makes a lot sense to me. A drain hole it will be for gas and water. A drain tube out of the cowl also has merit. Thanks to all for input 😊
 
I have 3 holes, one in front of the filter for water, one in the center of the air filter for fuel, one behind the filter for water. The air flow in the airbox will tend to push water to the back, ie flying in rain.
PS: vertical induction
 
tgmillso said:
I can't imagine this is necessary with a tailwheel though, as the fuel would just drain into the bottom of the fuel servo body and drain overboard via the sniffle valve? Unless my take on this is wrong?
Tom
RV-7
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Excess fuel will go into the FAB. I changed my start procedure and no longer ?prime? ahead of the start. I now just crack the throttle hit the boost pump wait until I hear the fuel pump load up (a second or two) and then crank. No chance of fuel puddling and causing an event.
 
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