Altitude and speed differences on alternate static

[Rob Simpson]

Hi All,

Recently back in the air 337G S/N 1807 after some major works over a few years and now doing the shakedown flights. I do have an wing heavy issue that I put in a separate post, but also have a question on pitot static systems and indications.
All new pitot staic hoses installed everywhere during the rebuild including new static bowls and alternate static valve and all tested and no leaks anywhere, but on check flights, the aircraft seems slow, indication 132 knots at 2000 feet with 24/24 set.
Just as a trial, i pulled the alternate static valve and the altitude indication increased by 250 feet and the airspeed went to 141 knots indicated.

I am surprised at the difference in indications and am wondering if others know of differences in indications with alternate static on versus off, or has anyone got any clues as to why I may actually have a higher static pressure with the valve in normal position.

Any clues would be greatly appreciated.

regards

Rob