Carb flooding

[kenfox]

Good point Morph !

[V]

Back at the time when the Facet was installed, I definitely checked that it's the right pressure. Problem is, the carbs flood with the Facet off.

[German]

Make sure the thin brass plates inside the bowl which lifts the needle press on the two black floats. I am to be corrected but are the floats the right way inn? I think the hole in the middle is not exactly in the middle thus the floats might not be in the right position.

[Duck Rogers]

Make sure the thin brass plates inside the bowl which lifts the needle press on the two black floats. I am to be corrected but are the floats the right way inn? I think the hole in the middle is not exactly in the middle thus the floats might not be in the right position.

Nope, they're in the middle. Can go in any way.

[Duck Rogers]

V.....there are just too many unknowns in this scenario of yours to be able to diagnose and suggest a repair without being there physically.
Go look here for just another thing to consider: viewtopic.php?f=13&t=20355
Set those bowls on a flat surface and leave the floats in the bowls for a few minutes and see what happens.

[tigger_na]

This might not have something to do with your installation, since your carbs flood even if the electric fuel pump is switched off, but it is worth looking into regarding how your pumps are piped. The text was copied and pasted from a very good Hawk Arrow manual, written by Robert Metzler:

ELECTRIC FUEL PUMPS

The Mikuni pulse pumps are designed to suck fuel up to the engine and can easily handle the 3 or 4 feet of vertical lift needed with our engine and tank locations. Facet recommends mounting electric pumps below the fuel tank because they can NOT produce much suction. Many people think that an electric fuel pump will provide an extra margin of safety. That's okay but make sure that you understand what you are doing or you can wind up LESS safe. Dual pumps can be used safely but you need to do it correctly.

If you use pumps in series each pump must provide enough fuel pressure by itself in case the other pump fails. The second pump can boost the pressure of the first pump so high that the carb inlet needle valve can't close and then fuel will squirt out of the float chamber vent. To prevent that you need a pressure regulator ... but then you depend on that ONE regulator to stay in the air. Wasn't your original concern that you depended on ONE fuel pump? Pumps in series usually add more potential problems than they remove. I do NOT recommend series use.

For parallel use we usually use the Facet 40104 or 40105 4PSI pump when the pump is in the wings and the Facet 40106 6PSI pump when the pump is mounted low in the plane so that the carb sees the correct pressure. NONE of those models have a check valve built in so any time there is more pressure at the outlet port than is being created by the electric pump fuel will be forced backwards through the pump. If the electric pump gets turned off then a large part of the fuel flow from the Mikuni pump will flow backwards through the electric pump.

With engines mounted high and fuel pumps mounted low even with no pumps running there is one PSI pressure on the pump outlet port for every 37 inches in height just from the head pressure of the fuel in the line up to the carb. For this discussion call that 1 to 2 PSI head pressure at the electric pump outlet when all pumps are off. Fire up the engine and the Mikuni will add 4 to 7 PSI on top of that head pressure to give about 5 to 9 PSI total at the electric pump outlet. The electric pump can only pump 6 PSI so fuel from the Mikuni can flow backwards through the electric pump at high engine speeds even while the electric pump is turned on.

Electric pumps can force fuel backwards through the Mikuni pump too if it has grit in the internal flapper valves. Reverse flow can cause fuel starvation and other problems like unexpected fuel transfer to a different tank depending on how the pumps are connected.

McMaster-Carr sells a good check valve to prevent reverse flow. Part #7775K51 only takes 1/3 of a pound of pressure to open and it has a Viton seat which is rated for gasoline but you need two # 44555K131 barbed adapters to install one in our fuel line. I strongly recommend a check valve on each pump outlet anytime there is more than one pump in the fuel system. I also recommend a fuel filter on each pump inlet so if one filter stops up the other pump can still provide fuel. (In South Africa I found that Comet Aviation sells check valves (Check Valve In-Line 6.4mm ID Hose) for R170 excl VAT.)

I had my pumps (on a 912) connected in series and also had occasional carb flooding. Now that I connected them in parallel, with check valves and all, I have a mess of pipes and tees and valves and pumps, but it works!

[V]

Thanks for the suggestions.

1. Duck, yes, I understand that there are many unknowns. I've pretty much done - either myself or had it done by an AP - everything to the carbs as humanly possible: cleaned, checked, etc. Of course, there might be something that we overlooked and that's the sort of information I'm 'fishing' here for.

I did the carb float test myself several times. Placed the bowls on an even surface, put in the floats, filled with petrol so that the top of the floats came up flush with the bowl, waited 30 min. Floats did not sink.

One of the interesting points I found earlier was that when setting the float level by bending the 'tab', it's easy to bend the 'arms' 'inwards'. Since the float arms can move horizontally sligthly, the arms may catch on the center part of the carb that houses the jets. This has led to the valve not closing and the carb flooding. Now whenever I take off the carb bowls, I always check that the arms have free vertical movement even when I push or pull on them horizontally.

2. tigger_na: thanks, for the interesting article. My Facet is installed in series with the Mikuni. The idea was to have a back-up pump and I used it for priming the system. I usually empty the carb bowls when storing the aircraft and it takes a long time fill them again with the pull-start. I do not like filling them with unfiltered petrol from the drain at the bottom of the tank.

Electric pumps can force fuel backwards through the Mikuni pump too if it has grit in the internal flapper valves. Reverse flow can cause fuel starvation and other problems like unexpected fuel transfer to a different tank depending on how the pumps are connected.

This might not be the explanation for my problems, but it is interesting to note that I did not replace the internal flapper valves. Simple reason: the rebuild kit did not include them.

I'm going to try out a couple of things this weekend and will report back.

Cheers,

V.

[V]

Looks like the cause of the problem was that the two arms of the brass plate that is actuated by the floats were not level and only one float was pushing. One float wasn't enough to hold against the pressure of the fuel flow. Both carbs had the same issue, which resulted in the random pattern.

Cheers,

V.

[Duck Rogers]

Looks like the cause of the problem was that the two arms of the brass plate that is actuated by the floats were not level and only one float was pushing. One float wasn't enough to hold against the pressure of the fuel flow. Both carbs had the same issue, which resulted in the random pattern.

Cheers,

V.

And how did they get to be NOT level?
Glad to see its sorted..

[V]

That's the $1,000,000 question. Either they've never been level and somehow the worn valves compensated for it for a while or they got bent the wrong way by me or someone else while trying to set up the correct float level. Since the flooding started before any fiddling and continued after replacing the vitton tip valves, the first option might be the more feasible one. Will never find out, I guess, but from now on I know what to look for when a mysterious flooding happens.

Cheers,

V.

[Duck Rogers]