Lee Dutton Posted October 3, 2011 Report Share Posted October 3, 2011 My car, a 66 4a fitted with SU carbs has been running with a very rich mixture i.e. sooted plugs and sooty smoke from the exhaust when starting. I decided to do some adjustments myself with the help of a Gunsons mixture tuning device (the glass plug type) When I fitted it and started the engine the spark which was clearly visible, was yellow, I adjusted the needle nut up and down which made little or no change,then I thought I should have loosened the locking nut which has given me a big problem, is there a special tool i.e. a thin spanner or can anyone advise please. The jet adjusting nut moved easily and was only finger tight however the locking nut is very tight and with the fuel line between the float chambers in the way has proved to be difficult to loosen. Best regards Lee Quote Link to post Share on other sites
Ashley James Posted October 3, 2011 Report Share Posted October 3, 2011 Www.Burlen.co.uk will sell you the correct tools for adjusting the mixture and don't bother with the Gunsons, just make sure the jets are 35-38 though below the jet bridge using a vernier. This is where the manufacturers set them and it will be correct of the float needles aren't leaking and the float heights are correct. Ash Quote Link to post Share on other sites
Brian-Northampton Posted October 3, 2011 Report Share Posted October 3, 2011 Hi Lee, Your HS6 mixture nut should only feel finger tight, there is no locking nut to move the jet up/down. After each turn of the mixture nut, always blip the throttle. Does the jet move OK when you pull the choke out... and back in again. It's not stuck in the choke position is it? And stick to the Gunsons, it's a great piece of kit and takes the guesswork out ( and other assumptions.) If you're jets are not stuck, then as Ash suggests, the next place to look is in the float chamber. The SU is sensitive to the level of the fuel in the chamber - if you think about it, the height of the fuel in the float chamber will be the same height of the fuel in the jet. The float in the fuel chamber will activate a needle valve to shut off the fuel supply when the correct height is reached. If the float is incorrectly set (or punctured) then it will not shut off the fuel, similarly if the needle valve is faulty, it also may not be shutting off the fuel. Do you see any fuel seeping out of the overflow from the fuel chambers? Oh and you haven't said if this is happening to one carb or both carbs? which cylinder is your colourtune on .... or do you have two of them? Cheers, Bri. Quote Link to post Share on other sites
Peter Cobbold Posted October 3, 2011 Report Share Posted October 3, 2011 Sunken float? Quote Link to post Share on other sites
Lee Dutton Posted October 4, 2011 Author Report Share Posted October 4, 2011 Thanks to all for the info especially Bri very clear advice.Will be in the garage today playing, and will let you know how I get on regards Lee Quote Link to post Share on other sites
RogerH Posted October 4, 2011 Report Share Posted October 4, 2011 Hi Pete, is a sunken float an oxymoron. I tried Oxymoron when welding some PTFE but I couldn't get it to stick. Roger Quote Link to post Share on other sites
Ashley James Posted October 4, 2011 Report Share Posted October 4, 2011 A huge amount of time can be saved if the carbs are set as all the manufacturers including Rolls-Royce did it. The HS6s have plastic floats, so no adjustment, but all cars with SU carbs were sent out with the needle shoulder flush with the face of the dashpot and the main jets between 35 and 38 thousandths of an inch down from the jet bridge. If your engine and carburettors are standard, this will give you the correct mixture as it has me for forty odd years now. My Bentley and Healey 3000 work perfectly with this setting, but my TR3A is slightly weak because it turns out to have a sports cam, so I'll have to use a richer needle. Burlen sell an SU workshop manual and parts book, which is very useful, although it doesn't include this vital information. Ash Quote Link to post Share on other sites
Fireman049 Posted May 14, 2012 Report Share Posted May 14, 2012 I must admit that the setting of the jets to 0.035" to 0.038" is new to me? That setting makes for a very weak mixture. Has anybody experience of this setting? I've always started with 12 flats down as per the SU instruction manual and that's a far cry from the setting above. Tom. Quote Link to post Share on other sites
Ashley James Posted May 14, 2012 Report Share Posted May 14, 2012 Tom It is how all manufacturers set their cars before despatch and it's usually as accurate as you get it. Even Rolls-Royce who designed their own SUs and had SU make them did it. However the float heights must also be correct and the shoulder of the needles flush with the face of the dashpot. If your car appears weak, maybe it's a tad retarded. Have you set the ignition timing exactly to the book as well. Ash Quote Link to post Share on other sites
Fireman049 Posted May 14, 2012 Report Share Posted May 14, 2012 Hi Ash ~ Yes, the strobe timing is spot on and the needles are set as you describe. Fuel level OK. I've just reset my jets to 0.038" and will get the car up to running temperature and see if she'll idle properly. I've re-adjusted my tappets. Tom. Quote Link to post Share on other sites
Fireman049 Posted May 14, 2012 Report Share Posted May 14, 2012 Ash ~ I've just tried the new set-up but she doesn't want to know! She refuses to idle at normal idling speed and when I press the accelerator she back-fires through the carbs. and exhaust. The engine is up to normal running temperature. I've now provisionally set the jets at 8 flats down and she runs perfectly. I'll put the Colortunes on her when I can get the chance. Thank you for your help ~ Tom. Quote Link to post Share on other sites
gwr Posted May 14, 2012 Report Share Posted May 14, 2012 Tom, I struggled for years on various cars and bikes to get the right mixture and balance on twin carbs. Then I invested in a colourtune and a flowmeter. Job done in ten minutes. I'm sold on these pieces of kit. I'm sure Ash's method works well, but surely if you can actually see the colour of the ignition flame it takes out all the guesswork. Good luck Quote Link to post Share on other sites
Fireman049 Posted May 14, 2012 Report Share Posted May 14, 2012 Hi Gordon ~ Yes, I have the flowmeter and Colortune and they are my preferred method of setting up my engine. Tom. Quote Link to post Share on other sites
Ashley James Posted May 14, 2012 Report Share Posted May 14, 2012 (edited) Tom, I struggled for years on various cars and bikes to get the right mixture and balance on twin carbs. Then I invested in a colourtune and a flowmeter. Job done in ten minutes. I'm sold on these pieces of kit. I'm sure Ash's method works well, but surely if you can actually see the colour of the ignition flame it takes out all the guesswork. Good luck I've been doing it this way for decades with every car that uses SUs and it works perfectly every time. It is how the cars were sent out new, so I'm confused. At the moment I have a Healey 3000, a Bentley MKVI and the TR3A, all running perfectly and in the factory (my son rents a third for his Classic Car business) are a 4.9L MKVI with HD8s and a 4.25L one with H6s and both are set the same way and run as mine does. Tom Please check the static timing with a test meter or bulb on the points and let me know if it's actually retarded right now? My son jokes that at Alpine Eagle they fitted a tuned MGB engine to a customer's car and set the mixture with colourtune! It wouldn't run properly and it turned out that only one carb was working! I think my method is the correct one and if it doesn't work, then something else is wrong. Ash Edited May 14, 2012 by Ashley James Quote Link to post Share on other sites
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