New member of the 'Sinker Club' (carb floats)

[Mich48041]

The important characteristic of floats is buoyancy, not weight. If volume
remains constant, then buoyancy can be determined by measuring weight.
However, reducing volume will also reduce buoyancy. The result could be
increased fuel level, just the opposite of the desired outcome.
The volume of two different brands of floats may not be the same. Therefore
weight alone can not be used to measure buoyancy. The best way to determine
suitability of floats is to install them and measure the height of fuel.

[mwardle7]

I've been flying behind Marvel Schebler floats since the end of October. No fuel smell in the cockpit, finally. I had the good Rotax floats, the better Rotax "replacement floats", the Rotax "replacement floats" for the "replacement floats" and they all ended up sinking. The only difference I could find is a price hike from one part number to the next and longer and less reliability in processing warranty claims.

So glad to have floats that don't overflow the bowl. I wouldn't mess with them.

[Piper J3]

Joe

Imagine a boat floating in water where waterline is 2? below the deck. Now suppose you pump a lot of fuel onboard? the water line raises and the boat is lower in the water. Conversely, if you take weight out of the boat it floats higher and yet the square area displaced remains virtually unchanged.

I think if we take some volume off the top of the float it will float higher in the fuel bowl. Top half of float is above level of fuel if float is doing its job?

[Mich48041]

Jim, Your reasoning is correct. But just because the Marvel Schebler floats are
heavier does not mean that the fuel level is higher. The volume (density) and
shape of the floats must also be considered. Without knowing the fuel level
when using Marvel Schebler floats, why modify them? Maybe the fuel level is
correct as is. Other posters have stated that Marvel Schebler floats work fine.

[NinerBikes]

Originally Posted by Mich48041

Jim, Your reasoning is correct. But just because the Marvel Schebler floats are
heavier does not mean that the fuel level is higher. The volume (density) and
shape of the floats must also be considered. Without knowing the fuel level
when using Marvel Schebler floats, why modify them? Maybe the fuel level is
correct as is. Other posters have stated that Marvel Schebler floats work fine.

What is of concern is that:

1. There is enough fuel Quantity in the float bowl to prevent fuel starvation at full throttle, with given fuel pressure and fuel flow, under any circumstance.

2. That the float maintains a high enough level of buoyancy to activate the fuel float cutoff valve, so that the fuel level remains below the overflow vent level, and that the cutoff valve functions fully, limiting fuel level in the float bowl when the level reaches the cutoff point.

The relative density of the float material to the relative density of the fuel you are using at the time, controls how high the float sits in the fuel above the surface, with the pin activating the cutoff valve.

I'm sure there is a range of weight that is acceptable to the mass and size of the floats they build, and MS has taken that into account in their engineering of the floats for the Bing carbs, the proper mass and volume of space occupied.

All Rotax and / or Bing carbs has done is determine the amount of weight, for the amount of mass for their floats, that buoyancy is lost in their reference fuel, that it is out of spec, and the floats then need replacement.

It almost sounds like they've gone and measured and engineered a consumable useful life into their carb floats, with a spec in place to replace. Like carb floats are no different that spark plugs, design them so that they become a revenue stream.

[Piper J3]

Originally Posted by Mich48041

Jim, Your reasoning is correct. But just because the Marvel Schebler floats are heavier does not mean that the fuel level is higher. The volume (density) and shape of the floats must also be considered. Without knowing the fuel level when using Marvel Schebler floats, why modify them? Maybe the fuel level is correct as is. Other posters have stated that Marvel Schebler floats work fine.

It would be relatively easy to check buoyancy and float actuator pin levels by “floating” a set of <7-gram Bing Vs. stock Marvel Schebler in a bench setup using the Bing float bowl. Simply measure difference of pin height to top lip of float bowl. Perhaps then, the top of the MS could be shaved off to make pin float heights the same. This would be interesting project for sure.

I have emailed Marvel Schebler, Rotax, and Bing to explain the concept of shortened height for Bing 64 floats. Maybe everyone is stuck on the idea that the floats need to be symmetric geometry. By considering asymmetric geometry, and making the floats one-handed, maybe a more permanent solution can found to the dreaded sinking float fiasco…

[NinerBikes]

There's something inherently wrong about pulling floats from carbs and doing weight inspections on them, because the fuel you use is damaging them beyond functional use, and creating a fire hazard with leaking fuel.

A closed loop fuel injection system is the permanent solution. Far, far less silly routine maintenance, for those that would rather fly than still wrench on their planes doing inspections or finding hazardous fuel leaks. A float for a carb that absorbs fuel and sinks, failing to work properly is an engineering flaw. Plenty of Carburetor manufacturers for snow mobiles, jet ski's PWC and like type of engines like the Rotax that know how to make and manufacture carb floats that don't fail.

[Piper J3]

See my post #96 above where I explain how we removed the top cowling and ran the electric fuel pump to discover fuel pouring out of the right carb air filter. Luckily, no fire when fuel overflowed the drip pan and sizzled on the hot exhaust stack.

I have been thinking about how to mitigate the possibility of this happening again.

Here’s my plan:

If the decision is to fly the airplane, then upon arrival at the hanger, the first thing I will do is to raise the canopy and turn on master switch in order to power the electric fuel pump. Let the fuel pump run, and if a float is sunk, fuel will overflow the carb and most likely drip on the ground where it can be noticed prior to flight. I plan on letting the electric fuel pump run while I preflight and load the plane.

For what it’s worth…

[AirHound]

Originally Posted by Piper J3

See my post #96 above where I explain how we removed the top cowling and ran the electric fuel pump to discover fuel pouring out of the right carb air filter. Luckily, no fire when fuel overflowed the drip pan and sizzled on the hot exhaust stack.

I have been thinking about how to mitigate the possibility of this happening again.

Here?s my plan:

If the decision is to fly the airplane, then upon arrival at the hanger, the first thing I will do is to raise the canopy and turn on master switch in order to power the electric fuel pump. Let the fuel pump run, and if a float is sunk, fuel will overflow the carb and most likely drip on the ground where it can be noticed prior to flight. I plan on letting the electric fuel pump run while I preflight and load the plane.

For what it?s worth?

Thanks for that tip. Worthy of Checklist/POH IMHO. Just like emergency procedure/immediate action when a carb return spring fails. Worthy!

[bobg56]

After 53 hours on the 861-184 floats I had to replace them due to fuel leakage. I took the advice of one of the guys who posted "to leave fuel pump on during walkaround" that was good advice, I smelled fuel when I opened the oil service door and heard dripping, sure enough my Left carb was leaking. I called Lockwood and got the latest and greatest a 861-189C and rubber gaskets. The old floats in the LH carb weighed 10g...RH Carb 9g...I guess 10g is the magic number for a leak...new Floats weighed 6g...hopefully these will go more than 53 hours. I used to get a wiff of fuel smell on climb out every now and then just a few hours after the -184 were installed. It was not a constant issue and would clear up after level off. When I would remove the top cowl I never saw evidence of leakage until the other day. Also the engine seems smoother and EGTs better balanced on my test flt...Cost of $352.80