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I have been having issues with my OER SK 40mm carbs. The problem is very elusive and I have not been able to find the source. I will do my best to describe the problem and I am willing to clarify if needed. I am going to refer to carb(#1) as cylinders 1 and 2. Carbs #1 and #3 are perfectly fine, it is carb #2 that is acting up. I started to sync the carbs, first by disconnecting all the linkages. I managed to balance #1 and #3 using an unisyn tool. However, #2 (cylinders 3 and 4) would be run leaner than the others. I unscrewed the idle mixture screws for #2 a few turns, and it leaned out even more resulting in popping out the carb and a nearly white spark plug. Also the #2 would draw a lot more vacuum than #1 and #3, and I unscrewed the idle speed screw for #2 and #2 still ended up drawing a lot more vacuum. #2 has enough vacuum that when I touch the unisync tool on #2 the engine will nearly die and then fuel spills out and then rpm picks up a lot. However, I got the car to idle smooth enough by tuning by ear and I would hold rpm at 2.5k and 3k and the #2 carb would cough and the exhaust would back fire, checking the plugs (cylinder 3,4) they were even whiter than at idle. Meanwhile the other plugs were a nice brown. This issue has bee driving me mad. Issues list: 1. carb #2 runs lean 2. (all linkages disconnected)(speed screw backed off 100% for #2) carb #2 still draws more vacuum that #1 and #3 3. Carb #2, touched unisycn tool the engine nearly dies, fuel drips out of throat, engine speed rises rapidly 4. #2 meaning cyl3,4 mixture screw(1 turn), nearly white plug, all others are a nice tan 5. #2 mixture screw(2 turns), completely white plug, all others are a nice tan 6. rpms held at 2.5k and 3k carb #2 cough and exhaust back fire 7. rpms held at 2.5k and 3k carb #2 (plugs 3,4) are bone white, all others are a nice brown 8. no binding in the linkages, linkages move very smoothly 9. Sprayed carb cleaner on all joints and seals, no increase in rpm 10. float levels all set to the same factory settings