OK, now totally sorted out. A good lesson --- the filter had looked fine to me, but on closer inspection the paper had gone to a sort of 'blotting paper' appearance. Thus it was partially blocking and only causing the AH to go funny at altitude. So ---- On Saturday flew with the new filter and what a difference ( but same vacuum gauge reading !!? ) I took the RV6 up to 3500ft and stood it on its ear each way, then pulled up and down in lots of swoops and plummets
--- totally perfect and accurate.
All for the price of a cheap filter, that I should have changed at the Annual
