[jhirasak]

Jon,

I think that Rick might have identified the source of your problem. If cylinders 4 and 5 cannot be made to drop out by closing their mixture screws, the carbs feeding those cylinders are very suspect in having blockage in their idle passage and/or progression holes. You will have to dismount the carbs and remove the progression hole inspection cover to check for debris or incorrect position of the butterfly. If there were debris in this area, it cannot be blown out while the carb is mounted on the engine. You will have to blow from inside the throat at the progression holes to remove any debris that might have gotten trapped there.

While you have the carb off the engine, shine a light at the butterfly from the manifold side of the carb and look through the progression hole inspection port. There should not be any visible light until you start to open the butterfly. You can also check to make sure that both throats uncover the progression holes at the same time.

John

PS By the way, my carbs all have three progression holes.