Choke Management

[kgwilson]

Chokes should all work like that IMO. Mine does & I have a Bing carb so the choke return spring must be too strong or there is insufficient friction in the choke cable, probably a bit of both.

[skippydiesel]

My Sonex Legacy(A) was built from plans. to an extraordinarily standards. Unfortunately for the origional builder, ill health prevented him finishing it. My Son & I have finished the project(12 months ) and in the process come to realise that most (but not all) of the original builders work is close to perfect. As for the panel lay out - it works & works well, considering the "real estate" available. The choke is a per most US certified aircraft, they do not stay closed and will not hold a position. The choke pegs will be my prefers way of using that system for the foreseeable future (my Son may not use them). As for the ATEC aircraft I sell - the paneal area is very much larger than the Sonex (I would guess x 3)  this and its shape allows for a more ergonomic lay out of controls & instruments - we have worked with what we have, others might do thins differently, it works well and at this stage we are happy with it.

 

I flew my Sonex for the first time this morning - bit rusty - didn't bend anything but will have to do a lot  better.

[IBob]

Skippy, congratulations!

As for the choke.......whether it is lockable or springs back, I am ambivalent.
Since I only use it at starting the 912 and almost immediately let it go, and since it does not work once you begin to open the throttle, I cannot see a reason to have it lock on. But quite probably others have different needs.

[Thruster88]

1 hour ago, skippydiesel said:

My Sonex Legacy(A) was built from plans. to an extraordinarily standards. Unfortunately for the origional builder, ill health prevented him finishing it. My Son & I have finished the project(12 months )

 

I flew my Sonex for the first time this morning - bit rusty - didn't bend anything but will have to do a lot  better.

Congratulations Skippy, I am sure we would all be interested in reading some pilot reports as you get to know your aircraft.  

[RFguy]

you only need the ENRICHMENT  (since it is not actually a choke) briefly.

Skippy, how about a magnetic solenoid   that just pulls against the spring in the engine bay. you either want it on or off.... since the throttle being closed is what activates it. (or allows it to activate, more truly) . certainly yes does need a spring. enrichment shafts and  the shaft o rings can get sticky due to fuel products that solidify.

[skippydiesel]

12 hours ago, RFguy said:

you only need the ENRICHMENT  (since it is not actually a choke) briefly.

Skippy, how about a magnetic solenoid   that just pulls against the spring in the engine bay. you either want it on or off.... since the throttle being closed is what activates it. (or allows it to activate, more truly) . certainly yes does need a spring. enrichment shafts and  the shaft o rings can get sticky due to fuel products that solidify.

I beg to differ - the Rotax system of temporary  fuel:air enrichment, to assist in cold starting,  is what I would call a choke. A choke,in this context, is a devise (butterfly valve) that can be opened/closed at the discretion of the pilot. Its action is to restrict/choke air flow, which in turn, lowers (increases) down steam vacuum (due to engine piston down movement) in the carburettor throat. The resulting" jet" of raw fuel into the carburettor throat/air steam (rich air:fuel)  enters the combustion chamber where it is ignited

 

The GA air craft, I have recently been flying, have an enricher- essentially a pump that is operated from the cockpit - set throttle position (fast idle), so many pumps (squirting fuel into the carburettor throat) followed by  engine cranking & start.

 

The effect of both systems is to enrich air:fuel ratio only the mechanism differs.

 

Further -

A choke is usually progressively opened as the engine warms - no such facility/action is required of an enricher system.

Most (but not all) choke systems incorporate a fast idle system ie while the choke is closed (full or partial) the engine will run at a higher than warm idle speed - this is one reason for having the throttle fully closed on a cold start Rotax 9 carburettor engine.

 

An enricher does not usually have a fast idle function - hence the setting of the throttle to fast idle ("cracking")

[Bruce Tuncks]

Once, I flew the jabiru with the choke inadvertently left on. It was not till I descended rapidly to stay clear of a cloud that the engine gave a strange response to the throttle and made me realize. Of course, I would be the only person here to have forgotten to close the choke.

[Bruce Tuncks]

These days, I close it as soon as possible, which is after only about 30 seconds.

[skippydiesel]

15 hours ago, IBob said:

Skippy, congratulations!

As for the choke.......whether it is lockable or springs back, I am ambivalent.
Since I only use it at starting the 912 and almost immediately let it go, and since it does not work once you begin to open the throttle, I cannot see a reason to have it lock on. But quite probably others have different needs.

I would humbly suggest you are not operating the choke as it is designed to be used (in a correctly adjusted carburettor) - the Rotax 9 choke should be progressively opened as the engine warms - I do this according to the sound of the engine. In the Rotax this will normally only take a few moments but is subject to ambient temperature - the colder it is the longer the partial choke is required.

 

This is why i go from two pegs at start, removing the first peg quite quickly, as the engine note smooths, leaving the second peg in place, to allow for fast idle and engine warm to continue. I remove the second peg again as the sound of the engine dictates  (to me this is an air restriction sound)

 

Note: I do not crack the throttle for a cold start - there is no need as the carburettor does this for me when I close the choke.

Edited December 6, 2022 by skippydiesel

[Thruster88]

The words open and closed are doing my head in regarding the Bing "choke". Choke on or off would be better. When the Bing choke is on the fuel enrichment valve is OPEN, it is closed when flying.

[skippydiesel]

10 minutes ago, Thruster88 said:

The words open and closed are doing my head in regarding the Bing "choke". Choke on or off would be better. When the Bing choke is on the fuel enrichment valve is OPEN, it is closed when flying.

Sorry Thruster - When I pull the choke button out on the panel this closes the throat of the carburettor by turning a butterfly valve - the converse is also true.

 

The method (pull/push/lever/button) of closing/opening the choke may differ with the aircraft type but the action within the carburettor remains the same - so for consistency & accuracy, I go with the  description of what is happening within the choke (not what is happening in the cockpit) choke closed/ choke open

[RFguy]

While Bing DO call it a choke, yes I will accept that, The Bing cold start assist is not a choke like the traditional restriction of old automotive carbs.

Actually the reference manual calls it a starting jet- but -

It's really a completely independent carburettor. 

Opening the enrichment path allows vaccuum on the engine side of the throttle  to pull fuel via  the enrichment jet into the engine. 

This is why Bing carbs enrichment does not work without the throttle closed- it needs vaccuum to suck fuel. 

 

[IBob]

Skippy, that's odd: when I pull the choke button on the panel, it turns a little disk in the internals, exposing one of two enrichment ports. I'm pretty sure it does nothing whatsoever to the main butterfly.

[Blueadventures]

4 minutes ago, IBob said:

Skippy, that's odd: when I pull the choke button on the panel, it turns a little disk in the internals, exposing one of two enrichment ports. I'm pretty sure it does nothing whatsoever to the main butterfly.

Your correct, skippy is 1,000% incorrect and needs to read up about them.  Therefore one peg will not be 1/2 choke; it'll be zero.

[IBob]

1 hour ago, skippydiesel said:

I would humbly suggest you are not operating the choke as it is designed to be used (in a correctly adjusted carburettor) - the Rotax 9 choke should be progressively opened as the engine warms - I do this according to the sound of the engine. In the Rotax this will normally only take a few moments but is subject to ambient temperature - the colder it is the longer the partial choke is required.

 

This is why i go from two pegs at start, removing the first peg quite quickly, as the engine note smooths, leaving the second peg in place, to allow for fast idle and engine warm to continue. I remove the second peg again as the sound of the engine dictates  (to me this is an air restriction sound)

 

Note: I do not crack the throttle for a cold start - there is no need as the carburettor does this for me when I close the choke.

Skippy, you may well be right. No doubt ambient temperatures have a bearing, and they are normally not low here. All I know is that I have evolved a routine that sees near instantaneous clean starts with a fairly prompt transition to an initial idle of 2200RPM, choke off, during which the engine neither labours nor overspeeds. I say fairly prompt as it is several seconds before the timed ignition goes to fully advanced (at which point the RPM rises).

As a side issue, it is worth checking the idle speed (throttle stop) and the idle mixture screw settings, as set out in the line maintenance manual under basic throttle adjustment. These are very quick simple adjustments. My new engine had the throttle stops set correctly, but the idle mixture screw settings were only approximate, and not identical. I have also seen a new 912UL with various idle and running problems, where the problem was incorrect setting of one carb needle. Which is to say that whoever does the Rotax carb settings is not bullet proof.

[IBob]

Bleuadventures

11 minutes ago, Blueadventures said:

Your correct, skippy is 1,000% incorrect and needs to read up about them.  Therefore one peg will not be 1/2 choke; it'll be zero.

Blueadventures, I don't think that's quite right either: pulling the choke rotates a disk that exposes one of two different sized ports. A full pull is a big port, a partial pull is a smaller port. So a partial pull is still giving some enrichment, I would think (always assuming the main throttle is fully closed).

[IBob]

I suppose we are all talking about the same carburettor here: the Bing 32???

[Blueadventures]

4 minutes ago, IBob said:

I suppose we are all talking about the same carburettor here: the Bing 32???

64 and yep there is a midpoint opening first but normal is to have pull full; it's not a progressive butterfly like most carb setups.

[RFguy]

skippy are you sure that pulling choke closes the throttle? I have never seen any aircraft do that.

I learned quite a but abotu them after doing the Dunstone Bing Masterclass.
read about it here :

http://www.bingpower.de/download/datenblaetter/type94e.pdf

 

[RFguy]

The starting carburettor is quite clever ,Bob. the sytem permits a big gob of fuel first breath and then uses the starting jet. 

It is an impressive and ingenious piece of engineering.

[facthunter]

It is by no means unique to that carburettor. It's mostly compromised by people NOT keeping the throttle closed . IF everything is calibrated properly the air passing through the extra system is adequate for a good idle speed . Normal idle speed is adjusted for when the engine is warmed. The motor needs some throttle afters tarting to get the motor smooth for warm up..  Nev

[IBob]

32 minutes ago, RFguy said:

The starting carburettor is quite clever ,Bob. the sytem permits a big gob of fuel first breath and then uses the starting jet. 

It is an impressive and ingenious piece of engineering.

It certainly works very well for me, RFguy: in 230+hrs the only time I have not had near instantaneous and smooth starts, was due to stale fuel after lengthy Covid lockdown. So much so that if I ever start to get less clean starts, I will be looking for the problem.

 

 

Further to all this: a while back, someone posted a link to a very good video on this carburettor. I did watch it, but cannot now recall where it was. Anyone???

Edited December 6, 2022 by IBob

[skippydiesel]

Okay - you got me BSing (just a bit) - what I described was a simple choke found on most lawn mowers, very old motorbikes and the like. I need to read up on the Rotax, view a few diagrams, etc. The principal will, I assume,  be much the same with  some variation on how its achieved. 

 

FYI I stand by my two peg system - it works very well.

[kgwilson]

It is not a Rotax issue, it is how the Bing carburettor and its enrichment function for cold starting works. My J3300 has a type 94. The 912 uses type 64. Both are Constant velocity or sometimes described as Constant depression and both have a starting carburettor on the side of the main housing requiring the main carburettor to be set at idle to allow the appropriate rich mixture from the starting carburettors own jet. Once the engine is running it will continue to idle using the starting carburettor fuel supply until the main throttle is opened allowing fuel mixture in and the starting carburettor is shut off when the choke is closed and fuel is prevented from entering the engine via the starting carburettor jet.

 

[IBob]

Skippy, here are two excellent detailed writeups, with illustrations, on the Bing64. The first covers the operation of the main carburettor. The second covers the operation of the starting carburettor.

 

https://electricmotorglider.com/2017/02/01/bing-64-cv-carburetor-part-1/

 

https://electricmotorglider.com/2017/03/01/bing-64-cv-carburetor-starting-carb-part-2/