Bing carb reconditioning job

[M61A1]

I will be looking to find just what the pair should weigh.

I would think it unlikely that there would be a figure available for the dual float weight. Weighing the pairs of separate floats is a Rotax thing.

as long as they float about 50/50 submerged they should be fine. If they sinK more than that there may be a problem.

[Bruce Tuncks]

floating in water or petrol M6?

[facthunter]

Has to be petrol .It's around an SG of .78/.water is 1.00. IF ever there is bubbles in the fuel. (HEATSHIELD needed?) , Fuel vaporisation and the SG drops dramatically and even good floats will sink..Nev

[Yenn]

Wouldn't a small hole in the diaphragm mean that the pressure above the diaphragm would be less, resulting in the piston not lifting and less fuel being delivered than required.

[Bruce Tuncks]

That makes sense Yenn. What would you expect to see on a vacuum gauge? I reckon if the carby mounting rubber got holed then the vacuum would drop to a lower level than was usual for that power setting.

[Downunder]

What vacuum gauge are you describing? Manifold vacuum/pressure or diaphragm chanber vacuum?

[Bruce Tuncks]

Inlet manifold pressure. I imagine drilling and tapping a hole in the inlet plenum just after where the carby bolts on. This should give info about carby leaks i reckon. And it would be non-invasive to the fuel induction system.

[Downunder]

I have a manifold pressure gauge. There are so many variables to the figure, I think it would be impossible to diagnose anything before other more obvious symptoms become apparent.

[facthunter]

A small hole would make little difference as the DIFFERENTIAL pressure past the dashpot piston is what moves the piston up or lets it fall. That's why it's called a CV Carb. It's required to draw fuel out of the needle jet and substitutes for a normal venturi function over a wider range of variables. I'm not advocating having holes in the diaphragm though. Nev

[RFguy]

Can someone please point to a good drawing of the Bing used on the Jab ?

 

I am interested in learning. All my experience with carbs is with SU type on Datsuns.... I had sidedraught triple Solex but went back to SU 'cause keeping them balanced was painful . Then I made my own fuel injection. sort of worked. used 2nd hand BMW injector pullouts. But I think the SU was better than my fuel injection. Oh and I remember Strombergs.

[facthunter]

Unless you had an engine with siamesed ports I'm surprised you had trouble with the side draft Solex s which are pretty similar to the Weber side draft DCOE types. All CVs are a similar principle but jet centring was a trick bit with the SU and it had an oil damper to allow acceleration to be rich enough. I regarded the Stromberg as the best. More compact and better choke action. Same as the Bing. With a CV you can fit a bigger carb than you generally need and still get away with it. This does not apply when you"Tailor" the needle thickness as has been done on Jabirus... If you don't really have thing s like an exhaust gas analyser and really understand what you are doing, I really don't think you should be playing around with these alterations. Nev

[RFguy]

Hi Nev. Yeah the SU I remember had a pretty good asymmetrical damper action . Rebuilt a few .Other carbys I have never taken 'completely' apart. I had O2 sensors etc, rather fickle things v temperature. Used to get vapor lock problems in feedlines on hot days with one vehicle.

 

For the Jab, my interest is understanding faults that can occur and how to identify them. The forum topic has had a world of advice. I'm not going to go messing around with jetting. I'll try and get an old Bing and take it apart.

[facthunter]

They are basically pretty simple bordering, on crude. You'll be right if you're careful and not rushed. The choke only works fully with the throttle closed. (Like a solex). Your background helps with the basic principles. Nev

[Yenn]

the vacuum gauge I meant is a gauge measuring the drop in pressure in the carbie throat, which is really the same as a manifold pressure gauge, except that a higher reading is a lower pressure. In the good old days a competent mechanic could use a vacuum gauge as a diagnostic tool for engine tuning. I still have one although I don't have a petrol engine to use it on. My transport and tractor are compression ignition.

[Bruce Tuncks]

Fortunately for us, this Bing CV carby is also used on motorbikes. There are a few accounts of getting a hole in the diaphragm . It seems that your speed is limited to 60 mph if you have a BMW bike with a biggish hole in a diaphragm.

My take on this is that power is reduced if the carb piston doesn't lift properly. But I reckon a small leak, as it would begin, would not give a catastrophic failure. But it would reduce the top-end power a bit. You should notice this on the rate of climb and also an INCREASED vacuum gauge reading ( more vacuum ) for the full throttle setting. Has anybody done experiments?

Well today I started to pull the carby to bits and so far have found that the diaphragm looks and feels perfect. So the worry is just a theoretical one. Also I found 2 separate floats in the bowl, they rise and fall on vertical brass posts. They feel ok, so tomorrow I will weigh them and then do Nev's idea of seeing how well they float in petrol.

And the cork bowl gasket and the choke gasket came out real easy. They clearly had been put in with no sealant. I will have to decide about sealant for the new gaskets.

[Downunder]

I would not put sealant on the choke gasket....

And the sealant used on the float bowl is just there to retain it when you drop the bowl on the aircraft to check for contaminents or remove main jet. It's not there to actually seal.

 

I would remove the needle and inspect it for wear while you're at it.

A broad screwdriver down the hole to take out the retaining "plug". The rotax ones are loctited in but I've not had a problem getting it out. You will also be able to see what groove the circlip (e-clip) is on.

Rotax had a problem with vibration and needle movement around the clip, so they fitted an o-ring to the botton of the retaining plug.

[Bruce Tuncks]

Thanks guys. The 2 floats were between 3 and 4 grams ( 7grams for the pair) and had a calculated volume of 10.8cc each. This gave a density of .324 gm/cc which means they should float half-submerged in avgas at .718 gm/cc. I checked this out and they did float well, with the midpoint pin half way out of the avgas.

Nice tip about a drop of sealant just to stop the cork gasket falling out.

The old diaphragm looks and feels as good as the new one, but I won't try and sell it. The old "O" rings were good too except that one was broken . And the needle had no wear that I could notice.

I still can't work out just what the engine would do with a small hole in the diaphragm. A big hole would prevent the piston from being sucked up, so the fuel needle would also be down and stopping the fuel. This would cause the power to go right down. Mind you, a light cable attached to the piston to pull it up mechanically would enable the power to be restored. I wonder if this has ever been done.

[M61A1]

Thanks guys. The 2 floats were between 3 and 4 grams ( 7grams for the pair) and had a calculated volume of 10.8cc each. This gave a density of .324 gm/cc which means they should float half-submerged in avgas at .718 gm/cc. I checked this out and they did float well, with the midpoint pin half way out of the avgas.

That sounds about right. Mine always float with the pin on the fuel level.

[Downunder]

Mind you, a light cable attached to the piston to pull it up mechanically would enable the power to be restored. I wonder if this has ever been done.

And with a cable attached, you might be able to go the other way and lean it as well....

[Thruster88]

I still can't work out just what the engine would do with a small hole in the diaphragm. A big hole would prevent the piston from being sucked up, so the fuel needle would also be down and stopping the fuel. This would cause the power to go right down. Mind you, a light cable attached to the piston to pull it up mechanically would enable the power to be restored. I wonder if this has ever been done.

Yes it has been done, the Bing 54 is a good and simple carburetor operated by cable on the piston. They use the same crappy fail to floats, my five year old 582 needs a new set, $400 is high level extortion.

[facthunter]

Slide carbs go back well over a hundred years and they work fairly well if designed properly. They have to be fitted to a very precise size for the engine. A CV has a bit more latitude there but not when it's been fine tuned on the needle taper as in a jabiru. One carb per cylinder is easy to get even mixtures. Manifolds make problems of varying amounts and mixtures and serious engine damage when a backfire happens is possible/likely. the more cylinders one carb. feeds .Nev

[Bruce Tuncks]

Nice idea about a carb for each cylinder. In my case, tuning them would have been easier than all the work I did with deflectors downstream of the carb plus a pipe from a vacuum cleaner between the lhs and ram-air ducts to try and even out the EGT's and CHT's for all 4 cylinders.

[Downunder]

As the owner of a multi carb engine, if I was going the route of supplying and metering fuel to individual cylinders, I think it would have to be electronic fuel injection to reap many benefits without alot of the problems of multi carbs.

 

http://www.sdsefi.com/jabiru.htm

[RFguy]

Bruce, I gather you are aware of the SB for the Jab engine mech pump, 2017?

 

https://jabiru.net.au/wp-content/uploads/2019/01/JSB040-2-Jabiru_-Mechanical-Fuel-Pump.pdf

 

All engines up to S/No. 22A3811 & 33A2768

[Bruce Tuncks]

Thanks RF guy. Yes I did do that check. I reckon the pipe is an interference fit and somehow one or 2 outside the specs got through. This is a population thing to happen. And thanks for the attention to detail. I predict you will have a good and safe career as a pilot if you continue like this.