I recently aquired used OS SF61 ABC with the PD-02 pump and the Type 86 carb. The engine starts and runs good. It will turn a 11-7 1/2 RevUp at 12,500 and a 12-10 APC at 10,400 on the stock mufler. I am having difficulty getting a reliable idle. The engine surges and eventually dies if I try to idle below 5,000 rpms. I have all the original manuals and have tried to adjust the Mixture Control Screw (very slight amounts at a time), but with no success.

All the fuel lines are hooked up correctly and there does not appear to be any leaks. The needle valve tunes normally at the top end, but it just won't idle. Any suggestions? I want to use this engine with a tuned pipe in a 70's pattern plane I am restoring (Phoenix 6), but I need to get a reliable idle before I start tuning the pipe.