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MG MGB Technical - SU mixture, spark plug color

My 74B has been running too lean - no plug deposits on RN9YCs - so I increased the fuel. Runs much better, but still no deposits. Also, even with the increased fuel, removing the new stock air filters causes the idle to become rough. Question, in your experience, if mixture is right on, should removing the air filter make the engine slow down, or is my mixture still too lean? Does a proper mixture ensure tan plug deposits using today's premium unleaded? I wish I had a Gastester.
Thanks in advance.
Willie L.

Sounds lean. Lean is bad and you're better off running slightly rich. I still get a tan plug.
Mike MaGee

Tune it by ear for maximum idle revs and with the lifting pin for the correct rise and drop in idle, and only weaken both by the same amount by reference to a Gastester if you have to. Plug colour seems to be the same with unleaded as leaded even though unleaded causes a black tailpipe on most mixtures rather than the grey with correct mixture for leaded. I've never tried putting the filters on and off while running to see if it makes a difference, I've seen it writ that the standard filter doesn't make that much difference, but I always do the mixture with them on.
Paul Hunt

Paul
seeing your comment about blacked tailpipes explanes alot to me. Both my B and The Westmisnter run with a blacked tailpipeso I have been trying for ages to get the cars to run with a greyish colour. I do not think I will bother now. Thanks for the explanation.
CK
Cecil Kimber

I do not think that we can gauge our fuel mixtures these days by the clues and standards of the past. I say this because I believe that whatever our current fuel is made up of, it does not leave quite the same deposits behind as the old fuel of pre-emissions standards times. I think that is partly due to a difference in the ingredients, their new ratios, and the type of burning process envisioned by petroleum engineers for newer, EFI, computer controlled engines with catalytic converters. It should not be surprising that such a concoction might burn just a bit differently in our relatively unsophisticated engine systems than it does in newer engines, or that its results might differ from those resulting from burning the fuels of days gone by. I can imagine that it even depends on whose concoction you use, these days.

Bob Muenchausen

Thanks for the input so far. I have made the mixture richer by moving the jets (new HIFs) from 0.060 in. below the bridge to 0.090 in.(!) below the bridge. Now the engine no longer slows when the stock air filters are removed. Car is easier to start and warms up quicker, too. Then I tried the piston lifting test (no lifting pins on new HIFs) and the carbs still show lean. In fact, the mixture screws were turned in another 1/2 turn and still lean on piston lift. What is your opinion of this piston lifting test for setting mixture? Thanks for your assistance.
Willie L.

I have always used the lifting pin method, but it is not easy to judge, I know others cannot get on with it. HSs need quite a sensitive ear, but HIFs are almost subliminal the difference is so slight, but it is there. But whilst it is difficult to hear the momentary rise and fall-back of the revs on correct mixture, it should be vey obvious if the engine dies when they are lifted the appropriate amount i.e. too weak, and quite easy to hear when the revs rise and stay risen when it is too rich. I was interested to see your measurements down from the bridge. Normally you turn the nut/screw two full turns from approximately flush with the bridge and that is the starting point. Turning the nut/screw one way or the other to get maximum idle rpms is the next stage, and using the lifting pins gives very fine tuning. The instructions for some years say when maximum idle is reached weaken until the idle is just about to fall, and do not use the lifting pins. If you have to meet tailpipe emissions levels then this is the final step, adjusting both nuts/screws by the same amount in the same direction until it just meets the limit. Adjusting purely by measurement is not the best way to go as it does not take account of individual differences in the carbs and the like, a bit like the difference between static timing and dynamic.
Paul Hunt

The grey deposits which helped you to adjust the strength of the mixture in the past were lead salts which change in colour with the temperature. No chance to observe the same colours nowadays.
Same thing for the exhaust pipe. The greyish colour was given by leaded salt which also helped to burn the carbon deposits.
Jean-Marc Thély

As others have said with unleaded I still get the brownish tint on the plugs with correct mixture and white crustiness when weak, it is only the tailpipe indication that has gone. Plugs from modern engines may well be different, running much weaker anyway.
Paul Hunt

The SU piston should only be lifted 1/16 inch - not all the way the pin will allow - not easy too judge - lifting it too much will make you think it is lean and encourage you to set it too rich.
Chris at Octarine Services

I am one of those "audio subtlety" challenged persons Paul mentions for whom the Lift-the-pin method is lost as a means for setting the mixture.

The one thing I always wished that SU had done when they provided the lift pin was to also limit its travel to the prescribed 1/16". Not only are the audible cues lost on some of us by competing engine noise, but some folks are also not good judges of what 1/16" actually amounts to, visually or by feel. And as Chris says, those folks can move the pin much further than necessary and will get the mixture wrong.

I use a vacuum guage to ball park the setting and over the years have come up with a procedure that works for me. I am not about to say that it is scientific or even right, but I do seem to get into the same league as those who use other methods and get respectable gas mileage and combustion deposits. I detailed this procedure sometime back and it resides, I am sure, in the archives somewhere in last year's stuff, I think.
Bob Muenchausen

How on earth can I measure piston lift exactly ??
Bas

Bas,
Maybe by using a combination of feeler gauges to make up 1/16" - 1/8" more than the measured clearance between the static air piston (while running at idle)and the bridge and sliding this combination into the air horn of the carb to lift the piston. That is about as accurate a way as I can think of. My comment about limiting the lift would not work because air flow lift would change the lift at any given air flow, therefore nothing mechanically to reference against.

BTW, my archived comments I mentioned above detailing my unorthodox tuning procedure are as follows:

From: Bob Muenchausen Idaho USA on 27 May 2003 at 16:47:18 (UK time)

I have been using a Colortune since the 1960s, and with the changes to the nature of what is called gasoline here in the US over the last 30 yrs, I find it is not worth much as a tuning aid. It will still tell you if your mixture is rich or lean, but it is not the "spot on" tool it might have been years ago. At least that is my opinion.

I have a very different method of tuning for the final mixture, altho, as Paul says, following the factory shop manual procedure (an excellent version of which is outlined at Paul's site, http://www.mgb-stuff.org.uk/) is a must up to that point. The mechanical setup of float heights, linkages, ignition timing, etc, is often grossly underrated by many mechanics, and you really can't do a decent job unless you follow it.

Where my process varies is at the point of setting final mixture. I have not been blessed with an ability to discriminate by ear the drop in RPMs by using the lifting pin - never, not in 35 yrs of futzing with these carbs. But I have learned that a vacuum gauge can help me to visually get a similar indication of each carb's mixture adjustment.

I hook up the vacuum gauge to a tap in the intake manifold as close to the center of the balance pipe as possible. It probably makes no real difference, but it satisfies my sense of equality of vacuum influences. I then run the engine and set the mixture of each carb (they are mechanically independent of each other at this point) so that both together give a vacuum gauge reading of approx. 15.5" to 16.5" Hg at idle (about 1000 rpms for a 68, 18GF engine).

This means that I raise or lower each carb's jet in turn until I see the vacuum approach my target level. It then becomes a balancing act between them, but in truth, the amount of adjustment to what I consider correct from the initial start point of "X" flats down or .060" drop of the jet from the bridge, is really only a flat or two, and usually up (or leaner). I am able to see pretty easily a rise or depression of overall vacuum as I twiddle with each carb independently, and I just twiddle away until they both produce TOGETHER the vacuum level given above, always moving towards the higher end if possible.

Once this "balance" has been reached, I cross check with the colortune, and usually, it simply verifies that the color is the same in both, a light blue sometimes splashed with yellow or a yellowish white. That seems normal for this engine, and reflects the less precise control a float valve has on idle mixture than EFI and computers do.

As I have stated elsewhere, I get even burning, decent spark plug color (a light tan or gray) and mileage of about 30-34 mpg flying down the interstate in O/D at 70-75 mph and Normal operating temp shown on the temp gauge. I take that as meaning that this method at least gets me into the right ballpark if not exactly a homerun. FWIW.



Bob Muenchausen

Thanks for the info, Bob and everyone else.

I'd like to share my observations with using a recently acquired Colortune with mixture setting of HIFs. When the first indication of yellow to blue is achieved on the device (start of proper mix), all other diagnostic methods will suggest the mix is still too lean. That is, the very slightest lifting of piston pins will noticeably slow the revs and no plug deposits. Also, with the jets level with the bridge, two turns of the mix screw drops the jets approx. 0.080 in. - at this point way too rich according to Colortune. Beginning of blue color occurred at around 0.045 in. below. BTW, exhaust smells heavy at idle with all jet settings.
Willie L.

There are a lot of evidences that spark plugs can still take various colours, but I think that it is important to understand the phenomenon to draw the correct and general conclusions.

The gasoline is made of hydrogen, carbon and sometimes oxygen, all of them burning without ashes. The deposits observed on the walls of the combustion chamber and the spark plugs should be made only of incomplete combustion materials being mainly carbon. As everybody knows, carbon is black and there is no chance to make white at any temperature. Hence, if the colour of your plugs is not black, it must be produced by something else than the fuel itself, which was lead in the past and which is now most probably the additives of the engine oil.

This has two consequences. Firstly, the scale of colours produced by the oil additives is not the same as the scale produced by lead. What remains true only is that a sooty plug indicates a too rich mixture. A brown colour indicates a thin layer of carbon on the insulator, but, in the absence of impurities, weakening the mixture will not turn the colour of the carbon to white. If there are impurities which give a white shade, in principle you do not know the nature of the impurities and at which temperature the colour of this deposit was formed, and hence derive a conclusion on the strength of the mixture. Secondly, you particular experience is not reproducible to another motorist using a different engine oil, having a different oil consumption, and of course a different engine.

It is now recognized by the industry that the colour of the plugs do not give reliable information about the strength of the mixture with unleaded gasoline, and that it is necessary to use sophisticated equipment to tune precisely an engine. But our engines can accommodate more traditional techniques of tuning as describe above, and my recommendation is not to pay too much attention to the colour of your plugs if you are happy with the running of your engine, and particularly do not try to reproduce the same colours that you observed with leaded fuel.
JM Thély

This thread was discussed between 24/12/2003 and 31/12/2003

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