I think that I might have an air leek on my inlet manifold. All bolts are tight but the carb on no. 4 cylinder is drawing in a lower amount of air compared to the rest of the carbs (I am running twin 45 webers on a 2lt Pinto). Also the idle mixture screw on no. 4 does not do anything. I have stripped the carb down and all jets and passageways are clean. When accelerating the car runs OK. Also if you pull the spark plug lead off no. 4 at tickover it makes no difference. There is a good healthy spark there.

Is there an easy way of checking for an air leek, or do have you any ideas what the problem might be?

spray wd40 or similar around the joints,when it finds a leak and gets sucked in the revs will rise

WD-40 or carb cleaner will show it but I prefer to take the nozzel of a butane burner and push a length of hose on. Then feed gas around the carbs and manifold to find the leak. The revs will change when you find it. A lot more controlable and less messy than WD-40.

adrian

Just bung a bit of rubber tube or washer tubing in your ear & listen around the suspect manifold area, when you find the leak it's really obvious - massive sucking/hissing noise.

Do a compresion test & check the valve cleances.

It is also possible you have twisted the spindle in the carb

Thanks for your advice, I found an airleak, replaced the gasket and it is running fine now