Carb flooding

[V]

Rotax 503 DCDI and 2x Bing 54 Carbs plus in-line Facet pump (generally off). Carb started flooding on idle. Cleaned carb, replaced vitton tip valves (they were worn), reassembled. Carbs still flooded or run dry intermittently the one or the other. Inspected floats (all ok), set fuel level in the bowl. Still intermittent flooding or running dry. Got somebody else to set the fuel levels and the intermittent flooding continued (after a while). Engine starts ok, idles well, keeps high rpm, but after a few minutes of running the flooding starts randomly in one of the carbs (it isn't always the same carb!).

The first sign of the flooding on ilde is an rpm drop and eventually the fuel starts coming out.

When switching on the Facet auxiliary pump the flooding starts earlier. Fuel pressure too high? Cleaned fuel pump (Mikuni dual) and installed rebuild kit. Flooding continues. Inspected the seat of the vitton tip valve: no dirt/damage.

Double checked that the Facet can not be switched on accidentally.

What else could it be?

Cheers,

V.

[Duck Rogers]

The possibilities are endless because we don't know what all you did prior to this and/or how you did it...
Just one idea: - when you re-assembled the carbs, did you make sure the needle and clip is BELOW the white retaining cup that holds the big spring in the piston?

[V]

Hi Duck, thanks, yes, needle and clip are underneath the retaining cup. Needle is 11K2 and clip is 2 notches from the top. The flooding problem started before any work on the carb and while the vitton tip valves did need replacement, there must be another problem that we're clearly missing. Had the carb checked out and the float levels set by an AP, engine tested ok, but at the next flight the carb was flooding again by the time I got to the holding point.

Any other ideas?

Cheers,

V.

[Dobbs]

Hi V, if you increase the rpm to around 2300 - 2500, above the range where the engine runs rough, and hold it there - does the flooding continue?

I have seen on a number of trikes, that when running slowly, read rough, the carbs overflow because the floats are being knocked around in the bowl and cannot do their job properly.

[V]

Hi Dobbs, thanks, good idea, I'm going to re-check those engine mounts. I put in new rubbers about 40hrs ago, but the engine has been taken off since then. Perhaps they are loose and the engine vibrates more than what I think. Or maybe the rubbers have deteriorated in the hot hangar.

Otherwise my idle is set to about 2300 rpm with warm engine. It runs smooth for a few minutes, then the rpm starts dropping gradually and around 2100 the fuel starts coming out. The engine starts shaking noticeably under 2000 rpm.

I pretty much taxi on idle and it takes a couple minutes to get to the holding point. After the usual checks, once I'm lined up for take off, I always look back one more time (Windlass trike) and confirm that there is no fuel coming out of the carbs. This has been the point where I turned back to the hangars at least a dozen times now.

Cheers,

V.

[nickjaxe]

You done any work on the carb vent tube lately...and do you fit them in the U shape as recommended...an are both carbs fully upright so the floats wont stick.

Nick.

[V]

Hi Nick,

thanks, yes, as part of the carb cleaning process I took off the vent tubes. Washed them with carb cleaner and rinsed with petrol. Applied carb cleaner from the spray can into all passages of the carb, making sure to blow in the opposite of the usual fuel flow direction, so that dirt comes out and doesn't just go further in.

The vent tube goes around underneath the bowl in a nice U-shape. The breathing holes at the bottom of the U are there and free of any blockage. That's actually where the fuel comes out when the carb floods.

The carbs are as upright and level as humanly possible.

Cheers,

V.

[Morph]

Your float needle valve is not closing properly.

With the engine off, run the electric pump and it will start overflowing through the overflow tubes. Stop the pump, open the float, and bend the little tag on the float lever up a small amount. Close up and switch the pump on again. It should stop when the fuel is about 12mm from the top of the bowl

If it does not the the needle, or seat or both are worn.

[V]

Hi Morph, thanks! It's a much more convenient way of setting the float levers. Will try it, the carbs do flood when I switch on the electric pump.

The needle (vitton tip valve) is new. I figured that the valve seat might be worn or damaged, but it doesn't seem to be a replacement part in the Bing 54 - at least I can't not find a part number for it. Is it serviceable?

Cheers,

V.

[Morph]

Actually I don't see it as a part in the manual either but it is mentioned as item 59 in this image

Check this website, they mention the availability of seats

http://www.bingcarburetor.com/offroad/5484kits.html

[kenfox]

take carb off remove float invert carb insert needle suck on fuel pipe u should get a vacuum check this by closing pipe with ure tounge, if u have vacuum move on needle and seat is fine. remove needle and block hole with finger check for vacuum again. some carbs have removable seats and some dont if seat is held in with a plate and a screw and u dont get vacuum (good vacuum) the oring around the seat is worn, the other type of removable seat has a nut around the seat and is sealed by a sealing washer if there is no plate with screw and the top of the seat is round it is not removable dont try remove it by the sounds of it ure seats are not removable ,and yes they are servicable if ure seats are worn pm me and i will give u a trick of the trade to reseat them

[Dobbs]

So V, is your flooding problem solved

[V]

Hi Dobbs, not solved yet , didn't manage to make it to the airfield this past weekend. Will keep this thread up to date with the progress.

Hi kenfox, thanks for the advice. Will try out the vacuum test. The valve seat in the Bing 54 does not seem to be removable. It is a brass insert, but it looks like it's pressed into the casing.

Cheers,

V.

[V]

So one of the things I can't stop pondering about is that the flooding is not consistent to the one carb. It is randomly the one or the other. Could the flooding be caused by a factor external to the carbs. Ie. fuel pressure being too high? I know the Mikuni vacuum pumps very seldom give problems and it is unlikely for them to overpressurise the fuel system. Is it possible, though?

The fuel pump has been serviced: cleaned, membrane and gaskets replaced, ensured that the weep-hole is not blocked. However, the rebuild kit did not contain the valve replacement disks and pins so that we had to re-use the old ones. Is it possible that they could be causing an over-pressure?

Further points:

1. I have an inline Facet pump in the fuel system. If I switch it on when the engine is idling, the carbs start flooding pretty much immediately. It clearly creates a fuel pressure that is too high for the valves to handle.

2. The flooding is more prominent after a period of idle (taxi to the holding point). On high rpm the carbs probably suck in fuel as fast as the fuel pump can deliver it. I do need to do more ground runs to test this out properly, though.

Cheers,

V.

[Morph]

Which facet pump do you have installed? The correct one is 40104 with a pump pressure is 1.5 to 4 Psi or 40105 3-4.5psi. The moment that pressure is reached it should stop pumping

You get models with pressures up to 9psi which is too high

http://www.facet-purolator.com/cube-fuel-pumps.php