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VW Polo MK2 Idling woes return (and Weber 32 TL setup)
After experiencing a rough idle on my Polo again I worked my way through the ignition system to ensure it was all in good health, and even replaced the fuel pump too. My spending an hour or so every evening for a good week definitely yielded some improvement. But something still wasn't right.
Ever since I changed the split rubber carb mount / gasket I had this idling issue. On pulling the spark plugs I found they were very sooted up so the air/fuel mixture was probably too rich. I turned my attention to tuning the carb.
Setting up the carburettor was over complicated by the fact that I mistakenly had assumed I had a Weber 32 TLA carb, with a manual choke conversion (a common modification). In fact I appear to have a 32 TL which always had a manual choke. It was used on some VWs and also Opels of the same era. For this carb the mixture screw and idle speed adjustment screw are in different places and there is a slightly different setup process:
The basic throttle angle is set by a small screw that touches the cam. This sets the initial idle (should be just touching the lever on the cam). The mixture screw is on the other side of the carb under the choke cable in a round recess which then needs to be set to the best lean idle position. The idle speed can then be reduced by backing off the screw on the throttle cam and fine tuned by an idle air bleed screw which is in a square recess between the throttle cam and idle cut off valve.
(in the image above the throttle position screw is behind the spring under a white ‘tamper proof’ cap)
When setting up the carb you need to purge the intake manifold of fuel vapours by revving the car up for a few seconds after making adjustments.
It’s hard to explain in text as tuning carbs is quite a dynamic process so maybe one day I'll do a video on it.
What really helped was having a vacuum gauge. Tuning using vacuum is a revelation to me. You simply need to attain the highest and smoothest vacuum reading. The sweet spot, for fuel efficiency, is where you still get a high and smooth vacuum reading while having the leanest possible mixture. It can also tell you about other problems that may be occurring in your engine.
So to cut a long story short after doing work on an engine affecting the air, fuel or ignition delivery it’s a good idea to properly tune the carburettor too! In my case I think fixing the vacuum leaks may have forced a richer condition causing a misfire when idling. Setting the mixture and idle speed up again compensated for this and removed the misfire causing the rough running at idle. The car runs much more smoothly now and I finally know how to tweak the Weber carb and get the mixture right.