Using the “Gunson Colourtune”

by Andy Owler - June '95

     I'm convinced! Gunson's "gizmos" really work (or at least those I've tried). After reinstalling the engine of our first Bagheera 'S' restoration (the one we bought new in 1978 from Walchry Motors in Walton-on-Thames) all we had to do was turn the key and off we go. That was the theory at least, since the engine had been OK before the MoT failure and the beginning of the long saga of trying to find someone who would replace the rust with steel. After a timewasting twelve months waiting for a local body shop to give us the budget price they'd promised, we did the work ourselves (a friend and I, that is). Only after the work was nearly finished and I'd joined MEC was I told that the car was too far gone to be worth restoring! 450 hours of "learning curve" but the result is worth it!

     Anyway, back to the story: start he would not (this Baggy is a "he" unlike some of his more recent stablemates). After much cranking and bad language, not to mention singeing of eyebrows during a blowback when peering down the carburetor throats to see whether there was fuel getting through, we eventually got a very lumpy running motor. Oh joy! he ran, even if "only just". I'd checked the new electronic ignition system we'd fitted and found it all OK so it had to be carburetor trouble didn't it?

     My trusty Hometune man did his best but concluded that there was a leak in the exhaust manifold and I'd need to fix that before he could do anything more. I didn't understand why an exhaust manifold leak should cause lumpy running - banging on the over-run I could understand, but not at tickover surely? Anyway he's the expert isn't he? Off came the exhaust system and manifold but with no obvious evidence of leakage, inwards or outwards. I checked the flatness of the flanges and filed the manifold slightly to make it as near perfect as I could. The whole thing was refitted with new gaskets but with no improvement in the "lumps".

     I called him in again but the problem was still unresolved. He sprayed "Quickstart" around the manifold and the engine speeded up slightly. "That proves it mate, it's yer inlet manifold that's leaking". I groaned and set about removing the carbs to get at the inlet manifold. It crossed my mind to wonder about the carbs as I took them off. But Webers don't go wrong do they? That's one of their many benefits I had read about in my early rallying days when the SU on my Shorrock supercharged Viva needed constant attention.

     With the inlet manifold off (for the first time in the car's history) everything looked fine. With new gaskets and a little sealant for good measure the assembly was refitted with the same result - no improvement!

     Perhaps it's time to call in the real experts (and we're well supplied with Weber specialists in the South East). Let's not mess about any longer, we'll take it to the manufacturer. After two "experts" had spent time on it and charged about £80 for labour ("blowing out the carbs") and yet more new gaskets, the engine was a little better, but not much. "You need to replace the carbs mate - they're too worn to fix". After getting the price I retired to consider the options.

     Buying the new carbs seemed the easy way out. But I had this nagging feeling they were not the problem (like the manifolds had not been the problem and the experts were equally convinced that they were the cause). But I'd tried everything else hadn't I? (Wrong - I'd not tried everything else - someone else had!) And they hadn't convinced me. But it had to be mixture didn't it? Sparks OK; timing spot on; compressions not bad considering its age and guesstimated mileage. There wasn't anything else.

     Someone at work mentioned Gunson's "Colourtune" as a way of seeing what was happening in the cylinder rather than relying on mixture analysers as the "experts" had done. It seemed worth a try and my friend offered to lend me his Colourtune kit. Am I an Engineer or a Mouse? Have courage and read the instructions first!

     The main Colourtune component is a spark plug with a glass window in the threaded body where a normal plug has the ceramic insulator. By looking down the centreline of the plug you can see not only the spark between the electrode and the body but see the flame inside the cylinder. The correct flame should be blue. A yellow flame is too rich and no flame at all at least shows you've got a problem! With no flame and a good spark, it has to be the mixture (or lack of it!). The action of the accelerator pump can also be seen with a yellow rich flame during acceleration which quickly reverts to blue when the pump stops squirting neat fuel into the carb mouth. Great idea, let's give it a go!

     Here's the first snag - Baggy's have one carburetor "choke" for each cylinder. It's equivalent to having a carb for each cylinder. Merely checking one cylinder would not prove the other three cylinders and swapping the Colourtune from one to the other would be laborious and finger burning. Besides, the next MEC meeting was in seven days time, and I was GOING TO BE THERE!

     The answer was simple, beg, borrow or scrounge three more Colourtunes so that we can see all four cylinders simultaneously! Neighbours and colleagues exhumed their Colourtunes from their junk boxes and I got a full set! I bought the Haynes book on these Weber carbs and did my homework. It sounded complicated but very little different from the text in the Haynes Chrysler Alpine book which every Bagheera owner buys with the car (don't they?)

     Gunson provide a black plastic tube that fits over the plug with an adjustable mirror at the outer end so that you can see down the plug without having to be directly in line with the window. It might work in an engine compartment less crowded than a Baggy's, but with the lumpy running the mirrors wouldn't stay where I put them. I then realised that if I got inside the car I could look directly down the spark plug hole through the rear window! Better still, if you get an old mirror (or two 150mm/6" mirror tiles from the DIY store) they can be placed inside the car resting against the inside of the rear window at the perfect angle to see all the Colourtune windows at the same time while sitting in the boot!

     It was immediately obvious that with the "choke" pulled out one cylinder was not firing at all. Yet if the accelerator were pumped a flame appeared (cold start jet blocked??). Pumping the accelerator showed an adjacent pair of cylinders not producing any yellow flame although there was a blue flame during constant speed choke operation (accelerator pump U/S??). And so on.

     Dismantling the carbs, one at a time with the book in hand and lots of paper towel to put the parts on, was easy. Thorough use of "Carb Cleaner" on the outside of the carb was well worth the effort if only because the position of all the joints can be seen. (It contains Toluene to remove the varnish and should be used in a very well ventilated area, preferably outdoors.) 

     Before undoing any of the adjustment screws, put a drop of Tippex or nail varnish on the screw head at the twelve-o'clock position and then close the screw counting how many turns from the initial position to fully closed. WRITE IT DOWN so that you can always put the carb back the way it was after cleaning. Remove all the jets and adjusting screws including the ones under the protective caps (if they are still there) not forgetting to count the number of turns.

     Clean everything inside and out. Allow carbcleaner to remain in all the little passages for at least 10 minutes to allow varnish to dissolve. Blow out the debris with a foot pump (or bicycle pump). The book says "Do not use metallic implements to clear blocked jets" although I confess that I gently used some very fine fuse wire on some stubborn dirt - the theory is that the tinned copper wire is softer than the brass jet - but do go easy, bristles from a stiff brush are better (but can also break off!)

     Follow the instructions in the book about checking wear on the butterfly spindles. I suspect that rear engined cars wear less since they are less exposed to road conditions than front engined cars with their open radiator grills. Nevertheless worn spindles can let in excess air which will make tickover mixtures harder to adjust.

     I found both float chambers with the float height set wrongly (and different from each other). Whether this was due to the "experts" attention or perhaps they had always been like that I cannot tell!

     The carbs were refitted to the engine and the recommended adjustments made. The Colourtunes were very easy to use and there was an immediate and obvious improvement in the smooth tickover and effective operation of the accelerator pumps. I did invest in a Weber airflow meter to enable each carb to flow the same amount of air (balancing the carbs). This is an expensive toy but much more accurate than listening to the "hiss" as recommended in the manual. A simple blanking plate can also be made, or bought, with a simple water "U-tube" to indicate flow rate in each choke of the carbs.

     The cost of the Colourtunes (I offered each of the lenders a small sum for their Colourtunes and only one declined to sell!) and the airflow meter was far less than new carbs would have been and the original carbs are back to full song. They are probably better than they have ever been but with the change to electronic ignition it is difficult to pinpoint where the dramatic improvement in light throttle running has been achieved. A steady 30mph through the Hampton Court radar traps is now a joy rather than the erratic and hesitant pain it has been since 1978!

     One final word of caution. Gunson say that the ColourTune plugs can be used while you test drive the car on the road, but they do also say that only light throttle openings should be used to limit the pressure in the cylinder. They mean it! So smooth was a test drive in our No.3 Bagheera (“Baggy Rouge”) after giving her the full tune up, that I forgot Gunson’s advice and gave her maximum throttle while pulling away from a junction. One of the plugs immediately blew out with spectacular and very noisy results! Now I need to buy another one! It is still a lot cheaper than the “experts” and a great deal more satisfying.

     Happy motoring!