I have a Zenith carburator off a Farmall 100 that has defied everything I have tried to make it work. Tractor will only run about 3/4 choke. I know its the carb as I have swapped it with one off my A and when put on the A id doesn't run correctly there either. I've had a part several times, soaked in acetone, run small wire through the passages, sprayed, inspected, blown out with air, nothing helps. One area concerns me - it can be seen in the middle of the carb throat as a brass circle, looks like it ties into the idle screw and best I can tell there are some very small holes in it. Can't tell how many and if there are some plugged up. I can get some liquid through it. There aren't any small passages anywhere else that I haven't been able to get out, clean and inspect. Any thoughts before I give up.