OPEL KADETT CARB ADJUSTMENT

Q

I have attempted to adjust the idle speed on my 1979 Opel Kadett, but have not been successful. The idle speed appeared to be too high, and so I attempted to lower it using the spring-loaded screw at the base of the carburettor, which should (according to my workshop manual) be the bypass mixture screw.

This had no effect, and so I attempted to adjust it using the other recessed screw. This had an immediate effect on the speed of the engine – turning it clockwise increased the speed and anti- clockwise decreased the speed. I thought I had solved the problem, but now the car tends to overheat and run on when switched off. I have returned the screws to their original positions, but would like to know where I have gone wrong and what the correct procedure is.

The carburettor is the Solex PDSI. Alf Gainsborough A From your description of your adjustment process, you have initially attempted to adjust the carburettor using the wrong screws. The by-pass mixture screw will not have an effect on the running speed if the base idle/ throttle stop screw is holding the butterfly open too far. This is what I suspect may have happened, and at some point the base mixture screw has been turned in to increase the engine idle speed. If this is in too far, the by-pass screw will have no effect. Adjusting the mixture screw as you have discovered can have unwanted consequences. The procedure to follow is to first lower the engine idle speed by undoing the base idle screw. This should be turned out until the engine will only just maintain a tick-over speed. Then adjust the bypass mixture screw, which should now bring the tick-over speed back up to an acceptable level. Next, adjust the mixture screw until the engine idles at it smoothest point, (note that this is the smoothest, not the highest speed,) and then re-adjust the bypass mixture screw to achieve the desired tick-over speed.