Possible choke problem

[DWW]

Only the fast idle screws….

[caveman]

Reference the rpm drop, did you check you have oil in top of the carb dash pots? No oil can cause your drop in rpm prior to picking back up. If that’s ok and the problem persists, your mixture is lean. I check mine before every trip.

[DWW]

Thanks, yes topped up the oil in dashpots with the correct fluid.

[bitsobrits]

Reference the rpm drop, did you check you have oil in top of the carb dash pots? No oil can cause your drop in rpm prior to picking back up. If that’s ok and the problem persists, your mixture is lean. I check mine before every trip.

I check mine once a year. Top them up every 3rd year or so. I drive 1500-2000 miles per year. I can't imagine why you would check damper oil level more often, as there is no need to do so, and frequent checking can lead to oil loss if you are wiping the damper piston down each time.

The oil level should be maintained to about 1/4" below the top of the air piston tube, NOT to the top or near the top of the dashpot outer housing. As long as the oil level is above the top of the damper piston (the bit connected to the screw cap) when the engine is off, it will work as designed. More oil than that makes no difference, and can only lead to an oily air piston, which adds weight and complicates the delicate calibration of the assembly.

[DWW]

The best SU Carb tutoring video in my opinion.

Once you understand how it works the rest is down to logic.

[caveman]

No harm in checking but I top up in line with SU guide. I do 200-400 miles a year so no top up required for a while.

[DWW]

Continuing with carbs, so I have done the idle and the balancing and that is now all fine. Have not yet got to doing the mixtures but still getting first cold start of the day problem. When I sprayed some brake cleaner into the carbs it started immediately but shortly died as the fluid depleted. I had the spark plugs out (brand new plugs) since the engine started firing on five only and they were all very black and sooty, the car has only had a few starts and a short test drive. It all may point to running too rich. After cleaning the plugs it now runs on all six again. Any ideas, can a too rich mixture setting impede cold starting? Will as usual appreciate any contributory input.

[caveman]

I think you’d be hard pushed to find a Jag that fires instantly on initial cold start, like it does when you spray brake cleaner, and particularly if it’s been stood for a few days.

[mgcjag]

Hi Danny...please see my previous posts above for suggestions.....you need to get back to basics.....check your ignition and timming.....then set up carbs....you cant really say your idle and balance is correct if you havnt checked your mixture...they inter play with each....pull the pistons and check the needles. .....are they the correct ones and fitted correctly.......then check the jet position. ......check the fuel level in the jets.....are the float chamber levels correct.....is the fuel filter clean....just get all these items ticked off one at a time .....Steve

[DWW]

hi steve, I appreciate the sentiment, nothing has changed except the weather. The only mistake I have done was to treat the fast running idle screw as the slow idle adjustment and that has been put right now, it idles at 800rpm quite happily and doesn't stall on throttle blips any longer. I always felt the running was on the rich side and the proof was in the spark plugs. During the summer months where I drove the car more extensively there was no problem starting from cold.

[DWW]

Steve thanks, I will certainly check the fuel filter (I have already thought of that earlier today) and ignition timing next.

[caveman]

Danny, whilst on the subject of filters you may as well check the thimble filters on each carb to be 100% sure.

[DWW]

Thanks, good idea and easy to carry out.

[DWW]

Update: Firstly thank to all who contributed and helped me overcome my Carb Phobia including the video I posted earlier, sure helped me understand the ins and outs of these relatively simple SU carbs.....All the carbs have now been setup properly and she starts from cold almost on the button and purrs like a kitten. The main issue was that who ever setup the carbs previously had the mixtures and the fast idle set incorrectly.

[mgcjag]

Good news....we did try to tell you but you thought that Guy Broad had done them correctly. ......if you want something done properly then find out how to and diy.......Steve

[DWW]

[Series1 Stu]

Although it's a shame your can't rely on the so-called experts.

Regards

[DWW]

Yes indeed, don't know what happened there. I now prefer to learn more about my car and carry out work myself where possible and know its are done right.

[DWW]

Update: First longish drive after setting up the carbs, car starts easy from cold on choke, idle perfect and smooth 850 rpm after temp reached, mixture setting satisfactory and generally drives very well. Thanks for everyone’s help here who contributed, you’ve been a great help that’s for sure.

[DWW]

Happy to report that the carb mixtures are now set to 5% CO2 on idle on both pipes. Even though I previously set the mixtures to what I perceived satisfactory, I felt she was running slightly rich which was confirmed by the small amount of black soot on the exhaust pipes. Out came the gastester which confirmed around 8-9% reading. I then leaned out the carbs quarter turn at a time until things settled around the 5%. Also gave the spark plugs a thorough clean. Happy days.