I have a similar problem also but mine seems to be at all engine temps, Power is reduced when the primary circuit in the carby is in use but when you open up the secondaries, power returns to normal. And also when you try to adjust the idle, the mixture screw can be screwed all the way in and the engine will not stall. I thought this was an inlet manifold vacuum leak but I can't find one!! . I still run the standard A12 and Hitachi with extractors.

Any ideas????????