question Rotax 912 Carburettors

BTW that wasn't my aircraft. I've not been higher than 9500' so far, but still climbing well at that, and no sign of rough running.

BTW that wasn't my aircraft. I've not been higher than 9500' so far, but still climbing well at that, and no sign of rough running.

Like I mentioned earlier that's nose bleed territory and cold! I would loved to have seen the radar trace on that 40 kts or so??

Flightrite, at a guess, the TAS would have been more like 58kts....but they must still have wondered what they were dealing with!

As for nosebleeds, I've been on a few jump ships between 12000' & 15000' and never seen anyone get a blood nose.

I should add that the above pic was taken over uncontrolled airspace, and in an area with no significant altitude traffic......

Flightrite, at a guess, the TAS would have been more like 58kts....but they must still have wondered what they were dealing with!

As for nosebleeds, I've been on a few jump ships between 12000' & 15000' and never seen anyone get a blood nose.

 

I should add that the above pic was taken over uncontrolled airspace, and in an area with no significant altitude traffic......

Could have been exposed to a significant H/W component also making the GS zip!? The comment 'nose bleed' is in jest, commonly used with ref to excess Alt?

Flightrite, yep...we've weathered a few jokes about suffering birdstrikes from the rear.......)

It's surprising what height some "ordinary" planes can reach. Geoffery De Haviland got a gypsy moth to 23,000 ft in the twenties. That's pretty good without a blower. The plane does much better in winter but harder on the pilot when it's an open cockpit. Nev

That HACman is an interesting bit of kit.

It looks as though Full Rich has float bowls at atmospheric, and leaning then progressively lowers float bowl pressure by introducing some lower pressure from the inlet manifold.

It does say, repeatedly, to return to Full Rich before any adjustment of the throttle, or risk engine stoppage (or presumably excessive leaning), which also makes sense.

Still, if I were up for a lot of long haul altitude flying, I would be looking at it, I think.

What views do the more knowledgeable here have???

Every airframe/powerplant combination has an 'ideal' Alt, going higher doesn't mean better overall. As an EG and I know it's not light A/C but the principle applies my last jet I drove before I retired had a service ceiling 51K', only ever got to 49K' as the TAS dropped off way too much and the time to climb was a negative. Most of our general GA hacks seem to be best at around 8.5K' higher than that means too much time to worry about not crashing after an eng fail??

Reducing the pressure in the bowl makes the fuel more likely to get bubbles in it (like when you take the cork out of champagne) which together with heat will make the float sink and then it floods. THEN you do have issues. THIS carburetter has already had problems with floats sinking due saturation of the plastic. nev

WARNING.........UNSUPPORTED AND QUITE POSSIBLY INCORRECT ASSUMPTION (MINE):

That HACman is an interesting bit of kit.

It looks as though Full Rich has float bowls at atmospheric, and leaning then progressively lowers float bowl pressure by introducing some lower pressure from the inlet manifold.

It does say, repeatedly, to return to Full Rich before any adjustment of the throttle, or risk engine stoppage (or presumably excessive leaning), which also makes sense.

Still, if I were up for a lot of long haul altitude flying, I would be looking at it, I think.

What views do the more knowledgeable here have???

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Reducing the pressure in the bowl makes the fuel more likely to get bubbles in it (like when you take the cork out of champagne) which together with heat will make the float sink and then it floods. THEN you do have issues. THIS carburetter has already had problems with floats sinking due saturation of the plastic. nev

You raise a very good point, Nev.

Given that we are talking about very minor variations in pressure, I doubt that it's much of an issue.

Many carb installations already have a fuel temp issue That's why many have a return fuel bleed. Less pressure has an additive effect. to any already there. The float vent tube does affect mixture as people who locate it carelessly find out the hard way. Using manifold pressure is applying something significant. Vapour lock with some fuels is an issue. This involves the same cause/factors. IF your float action is already sensitive or marginal, how sure can you be that it's safe? Bubbles in fuel make it less dense so the float sinks. Only some parts of the fuel have to form bubbles, you are not boiling it and fuel varies a lot here. Nev

I SAY AGAIN: MY COMMENTS ABOVE ABOUT HOW THE HACMAN FUNCTIONS MAY BE ENTIRELY INCORRECT.

Many carb installations already have a fuel temp issue That's why many have a return fuel bleed. Less pressure has an additive effect. to any already there. The float vent tube does affect mixture as people who locate it carelessly find out the hard way. Using manifold pressure is applying something significant. Vapour lock with some fuels is an issue. This involves the same cause/factors. IF your float action is already sensitive or marginal, how sure can you be that it's safe? Bubbles in fuel make it less dense so the float sinks. Only some parts of the fuel have to form bubbles, you are not boiling it and fuel varies a lot here. Nev

From what I’ve read many float type aircraft carbs use a virtually identical system, it’s just built into to carb.

IF you are talking about the"venting" it's best to the duct from the aircleaner just as it enters the carb where if the pressure drops by any restriction in the aircleaner it then doesn't enrichen the fuel as a result.. The Mitsubishi Colt had problems where it would blow clouds of black soot then stall. The excess fuel exited the bowl through the balance pipe I've just described straight into the engine intake. They are balance tubes, not overflow ones. Nev

IF you are talking about the"venting" it's best to the duct from the aircleaner just as it enters the carb where if the pressure drops by any restriction in the aircleaner it then doesn't enrichen the fuel as a result.. The Mitsubishi Colt had problems where it would blow clouds of black soot then stall. The excess fuel exited the bowl through the balance pipe I've just described straight into the engine intake. They are balance tubes, not overflow ones. Nev

No, I’m talking about the process of using a low pressure source to lower the float bowl pressure to lean the mixture.

The HACman type is similar to what they refer to as the Back-Suction-Type in the description in the link below:

https://www.flight-mechanic.com/float-type-carburetors-mixture-control-system/

You still have the issue of low pressure on the fuel especially with mogas. which is blended of a range of hydrocarbons and prone to vapourising problems .The BING is also a CV type carb which is constant pressure drop and variable gas flow by the dashpot rising.. The fuel flow will still respond to a pressure change acting on the fuel but you are introducing another complexity. The needle profile is matched to the dashpot position and regulates fuel flow that way. At higher altitudes the dashpot will be only part way open and a lot ft's profile won't apply nor will the mainjet regulate much as you aren't at a flow rate where it will come into effect.

We have a lot of engines acting up that later show no faults when test run. That could easily be vapour related problems or icing. I've NEVER been at altitude on anything other than avgas with Piston Engines.. Once without any leaning capability it ran pretty rough (obviously rich) and ran out of puff at FL150. or thereabouts. Nev

Ok i have listened to some outlandish statements without any evidence to backup these statements. Nev tell me more about these problematic Bing floats, is there a sevice bulletin from Rotax ? (how often should I change my floats) because I am not aware of one from jabiru. Have you flown a Rotax (or any Bing equiped aircraft) that has exhibited any of these symptoms , at what altitudes and weather conditions and have you been able to attribute WITH certainty the failure caused by them, In fact have you flown a Bing equipped aircraft with a leaning device ( no hearsay evidence or comparison with Anzani Merlin or any other red herring). I have a Hackman like device on my aircraft and it works very well and I can tell you from personal experience a Jab at 12500 ft runs very rich without leaning. Experience based observations from real world flying on the components we are talking about. We do this and any other subject a disservice when we speak without the evidence and hands on experience to back it up

Hi Paul -I am complexly unqualified to answer the above but I I do know that Rotax require that you weigh the floats found in the 912/914 carburettors. This is a check for petrol absorption/water logging if you will. If they are too heavy they mist be replaced. I know this because I went out and purchased the jewlers digital scale to be able to do the weighing.

Floats must be weighed on a precision scale (+- tolerance of 0.1 grams), and if the two floats together weigh over 7 grams then they both need to be replaced. (Floats are supplied in pairs.)

Fantastic, first hand practical information . What led you to replacing the floats? What were your symptoms, are the new floats any better ?

Ok i have listened to some outlandish statements without any evidence to backup these statements. Nev tell me more about these problematic Bing floats, is there a sevice bulletin from Rotax ? (how often should I change my floats) because I am not aware of one from jabiru. Have you flown a Rotax (or any Bing equiped aircraft) that has exhibited any of these symptoms , at what altitudes and weather conditions and have you been able to attribute WITH certainty the failure caused by them, In fact have you flown a Bing equipped aircraft with a leaning device ( no hearsay evidence or comparison with Anzani Merlin or any other red herring). I have a Hackman like device on my aircraft and it works very well and I can tell you from personal experience a Jab at 12500 ft runs very rich without leaning. Experience based observations from real world flying on the components we are talking about. We do this and any other subject a disservice when we speak without the evidence and hands on experience to back it up

https://www.rotax-owner.com/en/rotax-blog/item/34-912-914-float-inspection

I have recent first hand experience of Rotax 912 carb float problems on my first flight post Covid-19 lockdown. After half an hour of flight, my engine progressively started running roughly. The engine vibration was enough that it caused me to seriously consider an outlanding, and the pucker factor increased exponentially. After landing back at home base, my checks included prop bolt torque, engine mount bolts, etc. All good. I slept on it then recalled that there was an historic (but still current and applicable) service bulletin regarding the carb floats.

After removing each float bowl, the cause of my rough running engine was staring back at me. On the RH carb, one of the two floats had sunk. After spending an eye watering $220 for one pair of floats, all fixed. As mentioned, the pair of floats have to weigh less than 7.0grams. My sunk float by itself weighed 6.9grams. (New floats weigh about 3.2grams each, dry) Essentially, one “good” float alone doesn’t have enough buoyancy to shut off the needle valve hence, fuel keeps being supplied. The floats are not a hollow structure (as I was expecting) but rather a solid “block” of some sort of foam/resin. Unfortunately, some of them absorb fuel and become heavy, and subsequently sink. Using a magnifying glass, the surface of my sunk float looked like a kitchen sponge, whilst the good float was smooth on the surface. There have been about 9 different part numbers for the floats since 2012, so this issue is not sorted. Very expensive items for what they are too. My engine has only 125hrs/22months from new.

In the pic below, you can clearly see that the RH “good” float sits about at the top of the locating pin. My sunk float (LH) had slid down until it sat on the bottom of the float bowl. As it turns out, IAW the service bulletin, the floats are supposed to be weighed frequently. (25hrs from memory)

As an aside, if you have a 912, the inspection procedure is very quick and easy. Not so quick in a 914, as the bowl removal is much more complicated.

Perry

Thanks for relating your experiences and the symptoms relating to this issue , this is precisely what I have spoken about when asking for real world experience. Are all Bing carburettors subject to this float problem or is it just Rotax equiped motors. Are two stroke motors effected. From memory my Jab has a different float set up.my BMW 1976 R 750 and so far no problems, is it a fuel related problem or is the material they make the floats from the issue

It's a well known common issue with the Bing carby's on the Rotax I know of a few (inc the one I fly reg) that have been problematic and the Net is full of info about these problem child's!

Hi Paul. My googling and reading between the lines, there seems to be a link between using fuel with alcohol (ethanol) in it, (which is probably most mogas to be fair) and that fuel breaking down the resin coating. That’s definitely not conclusive and is only from what I have read. The carbs that Rotax use on the 9..’s are manufactured by Bing, and by no means are all floats failing. A very similar Bing carb is used on BMW (older) motorcycles and the same floats are used in them. Reading through some of the BMW vintage bike blog sites indicates that people have experienced similar issues there too.

One would think that a hollow brass or plastic float, as seen on most motorcycles, would have done the job nicely.

I now carry a spare set of floats in my aircraft just in case, and very quick to replace.

Perry