Following on from my post on SU HIF6 carbs (weak running at around 3000rpm and then possible a worn camshaft) as a test on the running engine I removed each spark plug lead individually, then replaced and noted on the first two cylinders I tried that this created a slight misfire but the engine revs remained the same. However when I removed the plug lead on the third cylinder (actually No 6) the engine revs shot up by 500 rpm. When the plug lead was replaced the engine revs remained high. Can anybody suggest what might be going here? Thanks.