Mixture Adjustment Techniques

[mallardstag]

Having previously had little success with the piston lifting/listen for rev change method of mixture adjustment which seems to be just too prone to errors and misinterpretation I am looking to source equipment to enable more precise adjustment to be made. The standard options that I know about seem to be plug colour inspection, colourtune or a CO meter (or a combination of these) all of which have advantages and disadvantages.

However it occurred to me that in 4-stroke piston engine aircraft which have pilot adjustable mixture (necessary as the air density changes with altitude) then optimum mixture is obtained at (or near to) peak exhaust gas temperature (EGT) which is measured with a small probe inserted in the exhaust manifold and a cockpit mounted temperature gauge. Some further research indicates that this method is frequently used on performance and racing engines and that EGT probes are readily available on eBay for about £40.

Has anyone here any experience with using EGT for setting the mixtures on stag engines? I guess two holes will be needed in the exhaust manifold one for a cylinder fed by LH carb and one for RH carb and suitable blanking screws when the EGT probe is removed.

What is the procedure for setting the mixture when using a CO meter as each of the two tail pipes are not fed by any one carb (due to exhaust cross-feed pipe and inlet manifold to carb arrangement) - how do you identify which carb to adjust to move the CO reading?

I would be interested to hear any comments about how to set the correct mixtures accurately and reliably.

Simon

[Goldstar]

Hi Simon,

I've no experience of this, the aircraft I flew in didn't need mixture control.

I was watching the Holley DVD the other day and they use peak manifold vacuum to determine when the mixturs is correct, would this work for Strommies? http://m.youtube.com/watch?v=UPGLUsW5ZeE

Paul

[dasadrew]

Interestig topic. Coincidentally, I was looking at fitting one of these maybe to my other car.

http://www.aerospacelogic.com/index.php?dispatch=products.view&product_id=220

The car is equipped with EGT measurement from the factory which shuts down one bank if the temperature gets too high. Unfortunately, the Italian ECU's which do this are unreliable and are constantly failing at €500 a piece.

The misunderstanding is often between the mixture at speed and the mixture at idle. Colortune I find is pretty good, but it only measures mixture at idle and assumes your jetting (or needling!) is correct to provide the right mixture over the whole range. Such an EGT would enable you to measure it as you drive, but you might need to gather data first to see what values are good and what are bad. Then you would need to try out different needles, jets and settings, I guess?

The "classic" way to set up fixed jets like Webers where it is very obvious that the idle mixture is taken care of by different jets from the fast running jets seems to be spending a day on a rolling road with a box of different Weber jets.

[alistair260661]

xx

sounds like a good idea, but....................... if you remember each carb feeds 4 cylinders - 2 on either side of the engine, and all four on each side of the engine exhaust through the respective exhaust mainfold............. so to get this to work you only need one probes in one downpipes... so monitoring one downpipe would allow you to monitor the mix in both carbs.... but I think it would be a bast**d to adjust because you would need to adjust both carbs at once, because you wouldn't know which carb was rich or weak................ unless you alternately stopped one carb feeding fuel............. nice idea though

[ChasR]

I guess that gives yet another reason to go to EFI!

[jakesmig]

i've gone EFI on a 3.9RV8 and am using an Emerald ECU with coil packs and no distributor. It is getting to the well sorted stage but it aint cheap and tuning the engine took a day on a rolling road, and will require a second visit soon to iron out a couple of issues. Value for money? Probably not but thats my choice.
Having said that, I could have opted for the Rover ECU which came with my Ebay sourced RV8 but I use an Emerald ECU on my MG Midget so I'm used to it. I also clear 30mpg in the Stag so you pays yer money you takes yer choice.
John.

[ChasR]

John, while the EFI system is aiding your MPG what gearbox/diff setup are you using?

[jakesmig]

John, while the EFI system is aiding your MPG what gearbox/diff setup are you using?

SDI five speed and as far as I know, standard Stag diff.
John.

[mallardstag]

sounds like a good idea, but....................... if you remember each carb feeds 4 cylinders - 2 on either side of the engine, and all four on each side of the engine exhaust through the respective exhaust mainfold............. so to get this to work you only need one probes in one downpipes... so monitoring one downpipe would allow you to monitor the mix in both carbs.... but I think it would be a bast**d to adjust because you would need to adjust both carbs at once, because you wouldn't know which carb was rich or weak................ unless you alternately stopped one carb feeding fuel............. nice idea though

Alistair,

Thinking about your comments I think that you are right in suggesting that you can get away with only one EGT probe position (I was thinking of two in the same manifold, one near each exhaust port) as the optimum mixture is always at peak EGT (this actual peak value is irrelevant and will vary from idle to high power and on different days due to air temp, pressure and humidity values). So adjusting each carb for the peak EGT obtainable by adjusting that carb alone should work - the highest peak EGT will then be when both carbs are optimally adjusted.

What is the technique when using CO meter - how do you adjust each carb?

Simon

[mallardstag]

Interestig topic. Coincidentally, I was looking at fitting one of these maybe to my other car.

http://www.aerospacelogic.com/index.php?dispatch=products.view&product_id=220

The car is equipped with EGT measurement from the factory which shuts down one bank if the temperature gets too high. Unfortunately, the Italian ECU's which do this are unreliable and are constantly failing at €500 a piece.

The misunderstanding is often between the mixture at speed and the mixture at idle. Colortune I find is pretty good, but it only measures mixture at idle and assumes your jetting (or needling!) is correct to provide the right mixture over the whole range. Such an EGT would enable you to measure it as you drive, but you might need to gather data first to see what values are good and what are bad. Then you would need to try out different needles, jets and settings, I guess?

The "classic" way to set up fixed jets like Webers where it is very obvious that the idle mixture is taken care of by different jets from the fast running jets seems to be spending a day on a rolling road with a box of different Weber jets.

Hi Drew,

I suspect that monitoring EGT whilst driving has little use as the EGT will only remain constant when under fixed throttle position, manifold vacuum (load), RPM and mixture setting. This is easily achieved in an aircraft when cruising but impractical on the road. A rolling road would allow this but is a little beyond the average budget! The EGT probe/meter itself also takes a few seconds to stabilise to any given reading due to thermal lag in the probe.

Any 'static' tuning method (CO meter, colourtune, piston-lifting etc.) necessarily assumes that the needles and jets are correct throughout the power range as you suggest and would be of little value if these are incorrect, I guess we all have to start with checking these before tuning or assume that they are correct.

The Ferrari kit you link to looks to be more about protecting a rather thermally critical engine from overheating than anything else but does add a bit of 'bling'.

The big advantage of colourtune that I can see is that you can look at one cylinder at a time and thus isolate the effects of each carb mixture adjustment from the other (if you ignore the small cross-flow inlet manifold passage effects).

Have you tried a CO meter to set the mixture?

Simon

[mallardstag]

Hi Simon,

I've no experience of this, the aircraft I flew in didn't need mixture control.

I was watching the Holley DVD the other day and they use peak manifold vacuum to determine when the mixturs is correct, would this work for Strommies? http://m.youtube.com/watch?v=UPGLUsW5ZeE

Paul

Hi Paul,

I like this idea of setting to obtain peak manifold vacuum, I'll do a bit more research to determine if this corresponds to ideal mixture with the Stag Stromberg set-up. Where would be the best place to attach the vacuum gauge? This sounds much easier than EGT measurement.

Simon

Edit:
I've just found this article which suggests that under a given fixed set of conditions peak manifold vacuum does indeed coincide with ideal mixture (but like EGT will be altered by RPM, timing, air pressure etc.).

[dasadrew]

Hi Drew,

I suspect that monitoring EGT whilst driving has little use .............

Any 'static' tuning method (CO meter, colourtune, piston-lifting etc.) necessarily assumes that the needles and jets are correct throughout the power range as you suggest and would be of little value if these are incorrect, I guess we all have to start with checking these before tuning or assume that they are correct.

The Ferrari kit you link to looks to be more about protecting a rather thermally critical engine from overheating than anything else but does add a bit of 'bling'.

Have you tried a CO meter to set the mixture?

Simon

Hi Simon,

I guess I'm a bit lost as to what you are striving for. For me, a weak mixture is a danger when running at constant speed (got that T-shirt a long time ago with a holed RS2000 piston!) so I would want to know what the mixture is doing at speed.

The Ferrari kit is useful in that it provides a historical read out as well, so you could see the EGT development on a longer run. The actual very specific advantage in a 348 is knowing when the Motronic is about to shut down 4 cylinders as being alongside a long vehicle when overtaking and losing 4 cylinders is no joke! Still, it's very expensive.

I used to have a Gunson's Gas Tester but it wasn't stable enough to set up carbs, I found. An electronic whizzkid at work took it apart and found that the components used were horrendously cheap and had a wide scatter on values. He replaced them with more accurate circuitry but the meter needs re-calibrating and since that time (about 18 years ago) it's been sat in its box!

If it's just mixture at idle you want to set up I'd personally plump for Colortune to get all cylinders the same, then followed up by spark plug checking after longish runs to see whether the overall level is too rich or too weak.

[Goldstar]

Hi Paul,

I like this idea of setting to obtain peak manifold vacuum, I'll do a bit more research to determine if this corresponds to ideal mixture with the Stag Stromberg set-up. Where would be the best place to attach the vacuum gauge? This sounds much easier than EGT measurement.

Simon


Edit:
I've just found this article which suggests that under a given fixed set of conditions peak manifold vacuum does indeed coincide with ideal mixture (but like EGT will be altered by RPM, timing, air pressure etc.).

Hi Simon,

That's an interesting read. I guess short of using the servo outlet you would have to retap the manifold and then blank it after you have used it? I assume you would have to start with the Stroms at the same state and then adjust both by the same amount. That's what you do with the Holley idle mixture screws. As the Stroms have fixed jets (I assume) and no ECU (!!) I think you'll have to live with the changes under different conditions although shouldn't the advance curve on the dizzy alter the timing and the combustion?

Paul

[KOY 23]

How about using an AFR gauge? I have one fitted which clearly shows the mixture under all conditions.

John.

[Mike J]

I like the idea of this. But the Triumph V8 is pretty crude. I adjust mixture by checking the colour of the relevent plugs. My MOT readings are always on the good side.