912ULS stoppage during stalls

Bill Hertzel wrote:

The Summary...

Hot Engine, Nose UP.

Low Airspeed.

Engine faulters/Dies.

Lower Nose

Build Airspeed.

After a short time period the Engine recovers.

- - -

This has all the earmarks of a fuel vapor issue.

Are the Fuel lines covered in Fire-Sleeve?

Are the fuel lines above the Cylinders or Above the Block?  Heat rises!

Was the electric Pump ON before the stall entry or only after the engine faltered?

 

 

It is a myth that "heat rises." It is true that in undisturbed air, hot air will rise (this is referred to as "convection") but that is different from "heat rises." Heat moves to cold, regardless of direction. In this particular case we are talking about a space where the air is constantly flowing, and so the fact that hot air rises is, I think, irrelevant.

In this case the heat that is transferred from the hot engine to the cooler fuel hoses takes place through "radiation" not "convection." The hot engine is radiating heat, and anything nearby that is cooler will attract and absorb that heat energy. So distance, not position, is what matters most. If one cannot get enough distance, then a shield of some sort is in order. Firesleeve can act as a shield to radiated heat. So can a piece of metal, placed between the fuel hose and the hot surface.

 Fuel in the Hackman tubing indicates to me that you are over pressurizing the carburetor float bowls. 

Edit: or conversely, your Hacman is siphoning fuel. The pressure in the float bowls must be the same as the pressure in the air box. Or in my case, just using the K&N filters, under the cowl along side the carbs. When set to full rich, where is your carb vent source located when using the Hacman?

Mr. RTX I don’t understand why anyone would mess with the Bings on a Rotax 912 either, I never would, but this is experimental aviation. Yelling at him won’t help solve his problem. Heck, I might even learn something here.

Please tell me why you have a HACman kit installed?!?!?

Get rid of it ASAP!

The carburetors are self-adjusting and self altitude compensating and you are totally messing with that system.

You want a fix...Get rid of it and go back to stock configuration.

Jim Isaacs wrote:

 Fuel in the Hackman tubing indicates to me that you are over pressurizing the carburetor float bowls. 

Edit: or conversely, your Hacman is siphoning fuel. The pressure in the float bowls must be the same as the pressure in the air box. Or in my case, just using the K&N filters, under the cowl along side the carbs. When set to full rich, where is your carb vent source located when using the Hacman?

Mr. RTX I don’t understand why anyone would mess with the Bings on a Rotax 912 either, I never would, but this is experimental aviation. Yelling at him won’t help solve his problem. Heck, I might even learn something here.

I've pasted the schematic of how the HACman attaches.  The float bowl vent tubing is removed and a tube connected to it that runs to a manifold in the firewall.  That manifold has an air orifice in it with an air filter attached (the gray thing hanging downward off the bottom). The firewall manifold tubes attach to the control valve mounted on the cockpit panel.  A screw valve in the panel allows one to adjust the amount of air mixing between the engine manifold pressure and the pressure over the float bowls.  When the control valve is closed, no such exposure of the float chamber to manifold pressure exists, so the float bowl is exposed only to the ambient pressure inside the cowling on the engine side of the firewall via the manifold orifice.  

The diameter of that orifice varies among different engines and the placement location for the manifold itself.  The stock orifice for installation on a 912ULS may prove improper under some circumstances if the air pressure inside the cowling where the manifold is placed is not the pressure assumed to be present based on prior trials by the vendor.  The orifice size determines how much effect the air pressure inside the cowling has on modulating the results of exposing the float chamber to engine manifold pressure.  So, making the orifice smaller will increase the effect of leaning, and a larger one will decrease it.  In my use of it, the sensitivity appears to be just fine.

From what I understand, a complete obstruction of the orifice with the adjustment valve completely closed would result in a lack of venting of the carburetor bowl. Opening the valve with orifice obstruction will result in exposing the carb float chamber only to the engine manifold pressure, which, I assume, would result in much more leaning.  It can also lead to fuel leaking into the air lines.  

My first step was to confirm that the orifice was unobstructed. One cause of fuel in the air lines is an obstructed orifice.  My second was to weigh my carb floats again (just did it in September and had to replace two).  The individual float weights were such that 3 of the six combinations were overweight and 3 were not.  So, Rotax SB says to replace both floats if the pair's wait is over 7g, but I can't find info on whether mixing the floats to get two combinations under 7g is OK.  Local RV-12 owners have opposite opinions about that.  At this point, I don't know the original pairings of the 4 floats.

To RTX: I hear you.  You may be right.  My inclination is ti take it off and see if GSAtech.com will take it back (likely not).  But, I have had to stop to refuel on the way home from certain group fly-outs where we cruise above 5500 MSL while my RV-12-owning colleagues make it home easily with plenty of reserve.  Watching EGT carefully and adjusting the valve slowly while in stable cruise at 9500 MSL I can get around 3.5 gal/hr or a little less at full throttle vs. more like 4.5 - 5 gal/hr without the HACman

Essentially the floats need to weigh less than 3.5 Grams each.

I am very confident you should not use that HACman kit because I think it is the main source of your problems.