Choke Management

[skippydiesel]

Thanks IBob - will get myself "up to speed"

[IBob]

Skippy, I think part of the problem is that we label this particular control as a choke, because that's the sort of function it performs, and that's how we are used to thinking of it. And I am no different: mine is labelled 'choke'.
But as others have stated here:
1. It is actually a starting carburettor.

2. The throttle also needs to be fully closed for it to function (because it is this full closing of the throttle that provides the necessary 'choking'.)

 

Here is mine.....labelled choke:

 

[RFguy]

Vso 26-30 kts ? and Vne 124 kts wow ! you can get in and out of short strips AND chase the Germans in a vertical dive

[IBob]

RFGuy them's the numbers........however with that fat wing and various other gear hanging out in the breeze, the reality is you'd need to be doing something well past radical to reach 124kts.

As most owners will agree, the Sav is happy at about 85kts. Anything much above that and there is very much the sense that you are pushing it..........)

[facthunter]

They're good in a roll with sauce.   Nev

[spacesailor]

The HummelBird is VSO 40 & VNE 140 MPH .

BUT

Someone pushed their,s to 170 + MPH.

Before the elivator ' blew away ' .

Creativity or ' crazy ' .

spacesailor

[bushcaddy105]

Back to chokes - (I am late to this thread). I use the choke cable assembly from a 1980's Subaru, and my 1990 Suzuki Sierra has the same setup. The pull knob can be set anywhere in its travel and is held there by an electromagnet. When a coolant temperature sensor decides that no choke is needed (and this is quite soon after starting), the magnet releases and a spring returns the cable to the off position. You still have manual control but if distracted it looks after you.

[RFguy]

"When a coolant temperature sensor decides that no choke is needed , the magnet releases and a spring returns the cable to the off position "

ha ha like when just airborne ?

[facthunter]

Haven't we agreed that opening the throttle puts the bypass bit out of action?.   This thread is like "Blue Hills" or Dame Nellie Melba's swansong.  Nev

[RFguy]

depends if we are talking about Bing carbs, or something else with a genuine restriction. 

[facthunter]

If we are talking about a PROPER choke like a Holden carburetter had but so far we haven't as far as I can see. and I wouldn't recommend that  sort of thing anyhow. IF it had a mixture control you keep full rich on for take off. The Bing has a "BYPASS fuel enricher".  (My words). like so many motorbikes have. Until they mostly went injected. Nev

[IBob]

Before moving onto Holden........I would like to say how much I like the Rainbow Aviation approach to troubleshooting Rotax engines, as set out in their excellent carburettor functional descriptions in the links posted above. They take a 2 step approach:
1. Find out what has changed since the engine came from the factory.

2. Put it back to how it was when it left the factory.

Which might sound glib or simplistic, but not so long ago we had someone in another thread here experiencing erratic or rough running problems. And when last reported, they were off to alter the carb needle settings...........(

[kgwilson]

I have a problem with rough running that has developed recently between 2800 & 3000 rpm. Perfect below that & perfect above to full power (3300rpm). I am only 5 hours away from the next service so will check all the settings then to make sure they are all at the recommended settings and of course there are no leaks or crud there.

[IBob]

All the best with that Kgwilson.

And I wasn't referring to any post you may have made.

Edited December 8, 2022 by IBob

[RFguy]

Bob, all over rich Bings powered engines  I have seen are all problems with float bowl vent location OR enrichment not getting fully off, or accidental carb heat.

 

However there is another interesting one I am partially troubleshooting right now, Jabiru engine that was about right, now rich after LCH. Heads instead of 150C , now 100 C, which would have an effect on vapourization/ atomization  and combustion. I have bought a Oxygen mixture sensor so I can get a benchmark to understand what's going on- (IE to figure out if its really rich or just low EGT, or a combination thereof)  since there would be an effect of both perhaps lower head temps (change to behaviour in the chamber) and also there would be some cooling of the EGTs due to the much cooler heads and headers. -glen

Edited December 8, 2022 by RFguy

[IBob]

Glen, I know nothing about Jabiru engines, and just a bit about the Rotax 912 since I own and maintain one, so have a particular interest there.

 

I do understand the importance of the float bowl vent, as providing appropriate float bowl pressure (in the Sav the vent/s are plumbed into the airbox downstream of the filter, which makes sense). I am fortunate that my enrichment (aka choke) has a free action, and shuts positively as soon as released. I hadn't thought about carb heat, but try not to make that a set and forget thing: I can set various degrees of that, so tend to return and adjust it when in use.
Having said all that, it is perhaps 800m of taxiing from touchdown to my hangar and any time I have pulled the plugs they look very rich. However, my fuel consumption is certainly not excessive and the thing has run like a dream from new, so I have just assumed that the Bing setup runs rich near idle.

 

You say the Jabiru was about right, now rich after LCH? I'm not quite sure what you mean there???

[RFguy]

Hi Bob ..Jab mixture was about right before LCH, now rich. 

I am led to beleive that all 912 engines have problems with the intake manifold at low gas velocities (idle etc) and that they all tend to foul / rich  the front two cylinders  with extended low idle  , due to intake manifold arrangement. But I dunno, that's what I have heard and read 
 

[IBob]

Glen, I've just come to accept that my plugs look awful after a home taxi (and to that I should add most of the finals of the landing, since the Sav uses very little power if you're wanting it to land). But must be working fine at other power settings, since it runs so smoothly and never misses a beat (well, okay, one cough once, travelling just under cloud base without carb heat on).
Add to that I ran the first set of plugs for almost 200hrs (as per the LM Manual, though some change them every 100).
If it works, don't fix it???

 

With the Jab, I can see where LCHs could result in a number of changes...as you point out.
Is there history from other users you can draw on?

My 912 has no EGTs. It has the standard coolant temperatures on 2 and 3 (LHfront and RHrear), Rotax used to do CHT but moved to the coolant. I would think temperatures are very location specific, so would expect comparisons between different engine types to be worthless.

[IBob]

Earlier in this thread, I made a reference to troubleshooting Rotax problems by finding what had changed since the factory, then putting things back as per the factory build.
I need to qualify that:

It has been pointed out to me that variations in third party exhaust systems etc on the Rotax engine may affect carburettor performance, and that adjustments to the jetting, properly carried out, may offer a workable solution to that.

[facthunter]

I'd use the term "after market" The original manifolding is badly "out of phase". (Pulses uneven), so you're a bit up the creek already with even mixture hopes. Nev

[IBob]

52 minutes ago, facthunter said:

I'd use the term "after market" The original manifolding is badly "out of phase". (Pulses uneven), so you're a bit up the creek already with even mixture hopes. Nev

I'm sure you're right Nev. But I'm left with the thought that, for an engine with so many design shortcomings.....it works remarkably well....)

[facthunter]

IBob IF you read all of what's written here One would NOT think so.. It's commonly known that the mixtures are NOT even.  Nor are the Jabiru's for that ,matter.. IF you look at the Continental 0-200 updraught set up it's much better and goes back 80 years.. The "X" distribution is quite well done. But people KNOCK those motors. How many fail? Nev

[IBob]

Facthunter, I don't disagree with your theory, in any respect.

But I do read much of what is written here, and what I don't see is any undue pattern of weakness or failure for the 912.

It is what it is: not the 'perfect' engine, not ideal for everything and everyone. But for the slot it does fill it seems to me to function remarkably well, and especially so since the designers grasped the nettle of liquid cooled heads and high engine speed, securing many benefits, but also complicating the design.

[facthunter]

It's only a Little aircraft engine and should not be so complex to get right. I'm not going to list the things that are complex. You (and I ) know them where with most other little engines you just open the throttle and use carb heat and the recommended Prop.. . You're supposed to be flying your aeroplane not fiddling with it all the time.. Service is of course.   Nev

[IBob]

15 minutes ago, facthunter said:

It's only a Little aircraft engine and should not be so complex to get right. I'm not going to list the things that are complex. You (and I ) know them where with most other little engines you just open the throttle and use carb heat and the recommended Prop.. . You're supposed to be flying your aeroplane not fiddling with it all the time.. Service is of course.   Nev

I'm not following you there, Facthunter.

The 912 as I know it doesn't require any fiddling at all. I just open the throttle, the prop is pitched about right and I put the carb heat on as required. Otherwise, it takes care of itself in all modes of flight and requires no fiddling at all, in flight or on the ground.
I do blank off a bit of radiator in winter, to get the oil temp up.
And I take an interest in how it all works.