Carburettor Leakage

[skippydiesel]

Background; My Rotax 912ULS is new (21hrs). Fitted with a single carburettor (carby) float bowl breather/ overflow "catch can" (serves both carby's). Air filter & distribution via carby air box. Float bowl breathers attached to air box as per Rotax. Air box fitted with drain points, each end.

 

Engine starts/runs very well.  Carby mechanical/pneumatic synchronisation done within first hour of operation. Prop balanced by supplier, no indication of vibration. There is significant engine movement at engine start/shut down - may be due to design of Sonex/Rotax engine bed adapter.

 

There was no fuel leakage from the carby's during approximately 5 hrs of ground running and none noticed for the first few hrs of flight testing. From then on a small (about 10 ml /hr) amount of fuel was collected in the catch can.

 

Separate catch cans fitted to each drain in an attempt to determine if fuel from one or both carby's - from both.

 

Float chamber floats weighed and found to be marginally within specs - replaced with latest version.

 

Slight reduction in fuel collected.

 

I am advised that there should be no fuel leaking in normal operations (I question the logic of this)

 

The next step will be to remove carby's to measure brass armature clearance, that the floats act on, to open/close incoming fuel valve, thereby controlling fuel level, and adjust if necessary. In my installation this is a pain in the proverbial.

 

Given that this aircraft is undergoing test flying, including repeat stalls (in varying config/power settings) and is operating of a grass (rough) strip, it seems reasonable to me, that there may be some fuel "slop" in the float bowls, leading to a small fuel discharge/leak ie all normal. 

 

I will be interested to get your opinions, regarding small fuel leakage in Rotax 9 engine installations.

 

 

 

[IBob]

Skippy, if the airbox drains are after the filter (as they should be) surely air is coming in through them during engine operation?
My build has one drain only, small diameter, presumably to reduce the amount of air coming in that way.

 

And as a a separate suggestion: I wonder if what you are seeing is the result of (a small amount of) fuel vapourising in the float bowl then condensing in the air box? It seems very unlikely, but for the miniscule amount you mention (approx 3 drops per minute), whatever is causing it is a minimal.

I would be reluctant to be dismantling/swapping bits on a new Rotax engine unless absolutely necessary: they are manufactured to a high spec and usually just bolt in and go flying.

 

Question: are the carb vent pipes to the airbox sloped up, down, or level?

 

[rodgerc]

My brand new from the factory 912ULS leaked (poured) fuel out of both carbies the first time it was connected to fuel. The carbies were duly removed, stripped and ultrasonically cleaned by an attending L2. The cleaning process yielded about a teaspoon of rehydrated “gum” in the bottom of the cleaner’s bowl. I was informed by the L2 that it was not uncommon in brand new, out-of-the-box engines test run in Austria then shipped. 

Edited February 23, 2023 by rodgerc
Photos added

[skippydiesel]

10 hours ago, IBob said:

Skippy, if the airbox drains are after the filter (as they should be) surely air is coming in through them during engine operation?
My build has one drain only, small diameter, presumably to reduce the amount of air coming in that way.

 

And as a a separate suggestion: I wonder if what you are seeing is the result of (a small amount of) fuel vapourising in the float bowl then condensing in the air box? It seems very unlikely, but for the miniscule amount you mention (approx 3 drops per minute), whatever is causing it is a minimal.

I would be reluctant to be dismantling/swapping bits on a new Rotax engine unless absolutely necessary: they are manufactured to a high spec and usually just bolt in and go flying.

 

Question: are the carb vent pipes to the airbox sloped up, down, or level?

 

My airbox (a Rotax copy) has two small drain points, with push on tube spigots. Proportionally little air enters through the drain points. As the air filter gets progressively choked in service , I guess more air might be drawn in - could always put a small i line filter on the drain pipe.

 

I would doubt your vapour/condensing theory. Should petrol vapour enter the airbox, I would assume, it would then be sucked into the carby's.

 

In my search for an answer to this problem, I weighed the floats - they were just within Rotax specifications. I have replaced them with the latest variants - only small improvement to fuel "spill"

 

I will check tomorrow but fairly sure "vent pipes" go down from the float bowl, to push on spigots on the air box.

 

You haven't actually addressed my main question - is any level of spillage normal ? or as has been suggested there should be no spillage in a correctly functioning system?

 

[skippydiesel]

10 hours ago, rodgerc said:

My brand new from the factory 912ULS leaked (poured) fuel out of both carbies the first time it was connected to fuel. The carbies were duly removed, stripped and ultrasonically cleaned by an attending L2. The cleaning process yielded about a teaspoon of rehydrated “gum” in the bottom of the cleaner’s bowl. I was informed by the L2 that it was not uncommon in brand new, out-of-the-box engines test run in Austria then shipped. 

I have checked my float bowls for contamination, about 3 times - so far nothing. 

 

Do you have any spillage at all, from your float bowls?

Edited February 23, 2023 by skippydiesel

[skippydiesel]

I forgot one point that may contribute to float bowl fuel spillage - the Sonex is a tail "dragger" so carby's always sloped (not level) on ground .

[Old Koreelah]

Surely the only time a Bing carby will leaked is when the needle valve doesn’t seat properly?

[facthunter]

ALL float carburetters will have the floats sink if the fuel has enough bubbles in it. (Usually if the fuel gets too hot). Bubbles makes the fuel less dense. Injection is a  safety improvement because you don't have a manifold full of explosive mixture and you don't rely on FLOATS and gravity to regulate the fuel. Nev

[skippydiesel]

14 hours ago, Old Koreelah said:

Surely the only time a Bing carby will leaked is when the needle valve doesn’t seat properly?

Logic (?) would suggest that an opening (breather) in a vessel may allow a liquid therein to escape, if/when the bowl is subject to sharp movement and/or change of attitude. This train of thought has lead me to think that some fuel escape may be normal.

 

So far I have only had advise that no fuel should escape, in a correctly set up carburettor float bowl - counterintuitive?

 

To the best of my knowledge the causes of float bowl flooding (fuel overflow through breather) are -

• Saturated/ Heavy floats ie float that do not maintain the optimum fuel level solution replace floats (done!)
• Incorrectly located breather pipe - pressure difference causing fuel to be drawn into/out of breather (not in this case)
• Float/float valve armature incorrectly set - yet to check this out in brand new engine
• Float valve/needle not seating - foreign object keeping valve open (checked clear), fuel varnish on seat (not yet checked), valve tip damaged/warn (yet to check this out in brand new engine)
• Excessive fuel pressure, overcoming  inlet valve sealing, causing bowl flooding & fuel to be ejected through the breather pipe - fuel pressures (max 5.3 psi) are well within Rotax specifications.

Note: The only reason I know that, a very small amount of, fuel is being ejected, is because I have a catch can on each drain tube. My previous Rotax powered aircraft had no such accessory,  so I can not be sure a small amount of fuel may have been emitted.

 

[facthunter]

Why do you ignore the point I just made? It's happened on some cars requiring carburetter modification so why could you think it can't

happen with the Bing.  Nev

[skippydiesel]

10 minutes ago, facthunter said:

Why do you ignore the point I just made? It's happened on some cars requiring carburetter modification so why could you think it can't

happen with the Bing.  Nev

Is this for me Nev?

 

I didn't  ignore it - I don't have much to say - I take your point about bubbles/boiling fuel - other than redesigning the cowling (better air flow) I cant think of any easy way test for/ address this. I can see how it might happen and that it may occur during a particular (short) stage of flight (climb?) thus only producing a small fuel discharge (which is what I have). The new/lighter floats may also help in this situation, although the difference in weight between old/new would suggest marginally at best.

 

I should say my engine (Rotax 912ULS) is performing as expected - no odd behaviour. Fuel consumption is astonishingly low (10L/hr need more data) for indicated  performance  - last flight (from memory) 130 knots indicated (133 true) @ 4500 ft, 5200RPM/125MAP

 

After test flying completed, I intend to take the aircraft home for modifications to coolant & oil radiator location, which will also change the cowling (may be wholly replaced). This may also improve under cowl temperatures (better air flow) so by happy coincidence may also address your point.

[facthunter]

It's not just a minor discharge It can be a large amount causing rich mixture and power loss and fire risk. One car was the Mitsubishi Galant in Australia. Later versions of Mogas may be worse. Under bonnet temps are  cause of concern. Having the Carb's on top of a motor is more vulnerable. There is no fuel pressure in a float bowl unless with a blow through Turbo. Those tubes are primarily to balance  intake pressure with that exerted in the bowl on the fuel.  Nev

[rodgerc]

On 23/2/2023 at 9:07 PM, skippydiesel said:

I have checked my float bowls for contamination, about 3 times - so far nothing. 

 

Do you have any spillage at all, from your float bowls?

No leakage from my float bowls….During reassembly of the carbs post cleaning, the needle valves’ spring load were measured, floats weighed and needle valves pressure tested for leakage. 

[facthunter]

Even the fuels temp  (density) will affect the buoyancy of the floats. You don't need marginal buoyancy.  Nev

[spacesailor]

An Odd thought !.

Is the carby vent on the down-side of bowl when climbing.

And can that part be rotated to the up-side.

spacesailor

[Blueadventures]

Can get leakage at the float valve seat if corrosion is present.  Not seen with naked eye. Wal can replace the seats and first check for corrosion under a microscope.  Had it happen a year and half ago to an 14 hour engine that had sat around before first start.  Consider phoning and sending to Wal.

[skippydiesel]

45 minutes ago, Blueadventures said:

Can get leakage at the float valve seat if corrosion is present.  Not seen with naked eye. Wal can replace the seats and first check for corrosion under a microscope.  Had it happen a year and half ago to an 14 hour engine that had sat around before first start.  Consider phoning and sending to Wal.

I understand that the seats can also be polished - presumably to remove surface corrosion or fuel varnish.

[skippydiesel]

1 hour ago, spacesailor said:

An Odd thought !.

Is the carby vent on the down-side of bowl when climbing.

And can that part be rotated to the up-side.

spacesailor

Fairly sure that the vent spigot/nipple is about 1/2 way on one side. The attached short breather pipe is supposed to open to the same air pressure, being experienced by the carburettor inlet. In my case the the breather is connected to a spigot on the side of the air box, that the carby is attached too (in theory, same air pressure inside)

[skippydiesel]

Dwelling on Nevs suggestion (bubbles/boiling fuel) - next flight, will try to remember to record under cowl temperatures at variose stages. Could be instructive.

[Blueadventures]

8 hours ago, skippydiesel said:

I understand that the seats can also be polished - presumably to remove surface corrosion or fuel varnish.

What is the link to polishing.  If I had the issue confirmed I will replace the seats.  Wal does the change out; Rotax say replace the carb body, expensive.  I’ll buy replacement seats and pull old and insert new.

[skippydiesel]

1 hour ago, Blueadventures said:

What is the link to polishing.  If I had the issue confirmed I will replace the seats.  Wal does the change out; Rotax say replace the carb body, expensive.  I’ll buy replacement seats and pull old and insert new.

Polishing the float bowl valve seat, is mentioned quit often in the Rotax Owners Forum (I have no personal experience). In any case, I would hope replacement/polishing not needed in a new engine.

[facthunter]

Things like that seat are often cast in during manufacture and press fitting one in makes for unreliability. Nev

[Blueadventures]

1 hour ago, skippydiesel said:

Polishing the float bowl valve seat, is mentioned quit often in the Rotax Owners Forum (I have no personal experience). In any case, I would hope replacement/polishing not needed in a new engine.

If you reread my reply it happened on a new engine <14 hours.

[facthunter]

With new cars it was not uncommon to get a few machining "Burrs" blocking jets or making seats leak in new carburettors. Some parts are tumbled to prevent this. When you are cutting new fuel hose you can get rubber slivers too .  Nev

[skippydiesel]

1 hour ago, Blueadventures said:

If you reread my reply it happened on a new engine <14 hours.

Sorry I missed the 14 hrs - that's terrible! - Hope you got the fix under warranty.

 

Unless your "seats" were somehow miss aligned, I would have thought a polish/HD clean would have been sufficient - Wal's the man (in Au) so his word is law.