Originally Posted by 22clipper
I'm sorry I'll try that again;
When I pull carby heat the intake air is thinner so the engine slows (or the govenor opens the throttle) causing a rise in MP.
If you are talking about during the carb heat checks then only the engine slowing. As below 80% RRPM the govenor on a robo is not working (if on primed but not engauged).
Originally Posted by 22clipper
When I climb the intake air is also thinner which should tend to raise the MP. But other factors (less atmospheric pressure on the exhaust, blades easier to turn) overcome this trend leading to a net fall in MP.
Yes all these things affect the MP but also the govenor and the pilot. To name but a few.