colourtune

[oggy]

Hi all sorry I bet this has been covered before , I have bought a kit for my carbs and was just wondering when I have rebuilt them would I need to set them up again and is the colourtune any good for this job , if not any other surgestions , thanks again

[David Withers]

I'd say the carbs will definitely need setting up again once they've been disturbed, especially if fitted with new parts.

I'm not a fan of Gunson's Colourtune. My experience from long ago (and that reported by others) was that setting to their instructions gives a mixture that's too weak. The colour needs to have a splash of yellow in it, not be pure blue. You could try just setting to their recommended blue colour and see how the engine runs, but you'll probably find that the engine tries to stall when the throttle is opened, and/or the acceleration is flat.

The Colourtune is OK as a starting point but I prefer the original method: Having first made sure that valve clearances (aka tappets) and ignition timing are adjusted correctly, and the choke mechanism is clear, the carbs are set carefully to be in balance. The mixture is then adjusted equally on both carbs a very little at a time until the engine runs sweetly. I use the test pin on SU's to lift the pistons slightly to make sure the engine speed picks up just a little so as to confirm that the mixture is about right. I also listen with a length of hose between ear and carb intakes to check that there is an equal 'hiss' from each.

That's all from me tonight as I've had a long day and need to get to bed. Others can probably improve upon the above, though it's one of those operations that's far easier to demonstrate 'in the metal' than to try to describe in words!

[Mike Stevens]

I tend to agree with David. I've got a Colourtune and have used it in the past but these days I tend to do it by ear.

It is easy to set the mixture too weak which will run Ok, but with modern fuels I find the engine also tends to run-on so I set it a bit on the rich side to avoid that as it seems to help.

Apart from using too much fuel, it's better to run it rich than weak!

Cheers,
Mike.

[oggy]

Thank you for the info guys

[Clifford Pope]

I use one of those things like a vacuum cleaner "suckometer" to get the idling speed balance on each carb. Then tightening up the connecting clamps can be tricky because the spindle tends to turn a bit.
Obviously it's pointless getting the closing position right if the two spindles don't turn at exactly the same moment.

I've always found the piston-lifting method for mixture setting a bit vague. It worked well on Strombergs, but I can't get much response on SUs.
I get it roughly right and then have a look at the spark plugs after the first run. This seems pretty accurate, and I find over several tries of adjusting each carb up/down a flat ot two I can get the same degree of nice fudge colour in each set of 3 plugs.
Combined with actual pedal response of course.

[Alec]

Hello Cliff,

I'm glad that someone else has trouble with the lifting pin, as I have never been able to use it. I have used Colourtunes successful however, and always use a Colourtune for each carburettor to get a broader picture. Your point about getting both throttles to start simultaneously is also very valid, no point setting the idles if the throttle connecting rod picks up one before the other.

The S.U. jet setting reference is something I use as a starting point if a service has been carried out on the carburettor, and if it requires much variance from that point to get a good mixture, something else is wrong, e.g. needle incorrectly fitted in the piston.

Alec

[David Withers]

...I've always found the piston-lifting method for mixture setting a bit vague. It worked well on Strombergs, but I can't get much response on SUs...

I must admit that I had some reservation in mentioning the lifting pins. I agree that they are a bit vague in operation but I think this is due to the difficulty in knowing how far the piston is being lifted. It should be raised only about 1/32" (0.8 mm), and that's quite hard to achieve by feel alone.

[Clifford Pope]

May be it worked with the Strombergs I had on my earlier car because there were no pins, so I took the air cleaner off and lifted the piston a little using a small screwdriver levered on the side of the throttle body.

I just wonder whether perhaps there is more interplay between the inlet ducts with the SU manifold - ie a bit more leakage from one carb to the ports mainly fed from the other one? That would mean that moving only one piston has much less effect on engine speed.