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MG MGB Technical - SU carb setting and ColorTune

Hi MG-owners,

I don't have an MG, but an UK-classic Rover P4 car with a single
SU carburettor.

As this is a very responsive forum, I want to ask about the SU here.

I'm running the car about 200 miles a week, the half of this in town traffic, the other half on longer motorway runs.
I recently screwed out the (6) plugs and noticed they were covered with soot.
Some weeks ago, a person driving for some distance behind of me,,said my car was 'smelling'.
On cold starts I use the manual choke, but put this back gradually and off when the engine is warm.

Upon this found I decided to regulate the mixture with a gunson ColorTune set. I cleaned the plugs and I applied this set on the first cylinder, and saw the mixture was burning with a yellow flame (=too rich).
I regulated back the jet, until the yellowness was about to disappear and changing to blue ('bunsen blue').
I then regulated the low idle to it's former idle speed.

On a testdrive and later too, I found the car was less eager to pull again the low end of each gear (as after cornering without changing gear), an the engine takes longer to warm up.
Before this I was experiencing the engine to be stong in torque when accelerating from low speeds, this is noticebly less now.
(but: it was running 'on a bit of choke' continuously up to now..).

BTW: the plug (I checked one) was still a bit blackish!

A (P4) mechanic I asked about this all was suggesting to reinstall the old setting, saying the engine to be running lean now, with a danger of overheating and subsequent damage.

My own idea is, the plugs should be showing a pale brown face as a sign of healthy combustion; black sooty plugs are the result of a setting that is spilling fuel and polluting the engine (and environment).
BUT I don't want to destroy the engine..

What's your technical opinion and advice????


Thanks a lot,
rob
NL
R. Willemsen

Rob I am not an expert but am am an enthusiastic amateur with about four years experience of MGB ownership. The colourtune is quite difficult to interpret and I use it to get close to the right setting. The more accurate check is to lift the piston about 1/16th of an inch (there may or may not be a lifting pin to do this on your carb). The revs should briefly increase and then settle. If they drop it is too weak if they increase and stay increased then it is too rich.

You should definitely er on the side of being too rich. When my car is running to my satisfaction the plugs are black after local driving. Only on a long run do they clean up.If it runs well and the consumption is reasonable don't worry too much about the colour of the plugs.
Peter Scott

Until someone who really knows about these things can comment here's a few thoughts.

Are all the other settings OK (timing, plug gap, points, vacuum advance, valve clearances...)? The carb really is the last one you want to alter. It's taken me a lifetime to acknoledge that and I still have to remind myself each time I reach for the mixture screw.

Some people advise that the whole colour/plug reading job is much harder with modern fuels. Sooty plugs are pretty conclusive of rich even so. How was the fuel consumption before the tuning? Was the tailpipe black?

A wear point on the SU is the throttle shaft and its bushes. If you have any air leaks through the throttle shaft (spray some EasyStart or carb cleaner around the body where the shaft goes through and listen for a rise in engine note) tuning at idle becomes difficult. Using the Colourtune at idle will set things for rich above idle speed, as the air leak becomes insignificant to the mix at higher revs. I'd suggest trying to set the mix for blue at a constant 2500rpm, with yellow as you accelerate, and don't worry too much about the idling colour for the moment. Then see how it runs.
Steve Postins

There's still a piston lifting pin on the P4 (it's an HS4 carb, just like the B). Like Peter, I find that the plugs on my B tend to be black unless I've been for a good long run. Do be careful about overheating a 6 cylinder P4, hot spots can develop around cylinder 6 and expensive work will follow! I'd also echo Steve's advice about checking everything else - timing and valve clearances especially if you're finding it particularly sluggish. When you do the valve clearances use a new rocker cover gasket and just nip up the three cover bolts - it's very easy to tighten them unevenly on the long flat P4 cover and oil leaks will result. As I'm sure you know, the P4's hardly quick but the 6 cylinder should be massively torquey and pull from very low revs smoothly. I find the tailpipe on my B and on my father's P4 and Morris Minor to be black all the time even with perfect mixture set - blame it on modern unleaded fuel.

HTH,
Tim
Tim Jenner

Hi
Alas I am not one to profess knowledge in this area.
My colourtune experiences have been somewhat confusing too.
The best way I found was to take it for a drive along a known route. Make an adjustment then drive it again.
I guess that you have done this and it sounds like the performance has dropped off a lot. Just richen it a flat at a time and drive it again.
Also make sure you put in good quality fuel as this has had a huge effect on the performance of my B
Cheers
Steve
SW Tompson

Like others I tried the Colortune many years ago and found it difficult to get good results. The SU is actually a doddle as it comes with its own mixture checking devices - two in fact. The first is to adjust the mixture until you get the highest idle speed - practice turning it back and fore and hearing the revs rise as ir reaches optimum and falls away on the other side, both too weak and too rich will result in a lower idle than just right. The second is to use the lifting pin, which some find as difficult as the Colortune. I agree that the HIF is very slight, almost subliminal, but the HS is much clearer. Lift the pin until you can feel the weight of the piston, then lift a further 1/32" (guess it). If the mixture is too weak the engine will probably stall immediately. If it is too rich the revs will rise and stay risen while you hold the pin. What you are looking for is a monentary rise in idle then a falling back, even though you are still holding the pin up. Later cars that had to meet emissions requirements had to be adjusted just to the weak side of max idle, then weakened the minimum amount, if necessary, to meet a CO figure. But that shouldn't be of any concern to a P4. All that is only any good if the needle, jet, piston spring, damper, float valve and level, throttle spindle bushings, choke mechanism etc. are OK, and the carbs should only be done when the valves and ignition are correct.
Paul Hunt

Colortune should be used only as a base guide and then you take it from there. The problem is that the car isn't under load which changes things considerably. I'd also make sure your needles and nozzles are not worn. Take a nice longish drive and then pull over and pull a plug and see how the mixture is. Running too rich will kill your engine as the gas will run down the pistons and wash away any oil and screw up your bores.
Mike MaGee

I think Mike MaGee has hit the nail on the head about the Colortune. Like many others, I use it as a simple ball parking tool, particularly when dealing with SUs which have not been setup or setup quite improperly.

After that, I favor a Vacuum gauge to assist "visualizing" the ACTUAL effect of jet adjustment since I have never been one of those lucky people who notice any real difference in rpms by lifting the pin, no matter how carefully.

But even with that aid, on the road testing and final incremental adjustment by hand, using eye and ear as suggested, is still necessary.
Bob Muenchausen

Hi responders,
As the weather is getting cold over here, not good when you have to work on the car in the free air, I did reset the carb to it's approximate old setting (yellow flame in the ColorTune screen).
The engine is running much sweeter again (more torque when on low revolutions).

BTW: the carb is a HD6 with a lifting pin.

A slight 'flat spot' I noticed before in accelerating from standing seems to be away too.

The plugs got black in the former 'modified' setting too.

I tried the 'start pilot spraying' on the throttle shaft before, and, in spite of an amount of play in the bearing of that shaft I noticed when I disassembled the carb last year, this spraying test gave no rising in revs.

To the Colortune set: this time I found it easy to use with no tube/mirror connected to the plug: my engine allows to look down to the plug area and screwing at the carb at the same time.

To the theme of washing away oil of the cyl-bores by a rich mixture: is this happening of these rates of enrichment?
Could imagine condensing fuel rinsing away the oil film, but does the volatile mixture in the 'rich' setting of the carb this too?


Think I will renew needle and jet and let rebush the shaft, and than do some work again.
I will fiddle a bit with the Gunson to bring back the modification again and test in that situation with the lifting pin too.

thanks,
rob

R. Willemsen

IMO it's difficult to hear small changes in revs with head under bonnet. Perhaps a meter with built-in rev counter would help. I have one but not tried it for this purpose yet.

Anyone tried this?

Tony
Tony Bridgewater

This thread was discussed between 22/10/2003 and 29/10/2003

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