1990 chevy ck 1500 loses power when wet out

I have a ck 1500 with a 4.3 litre engine 5 sp standard when it is raining or snowing out I lose power and I have to either step on the gas to get the revs up or let off the gas then step on it again. I have changed the plugs,wires,distributor cp and rotar button but still the same. the engine bogs down like I I have no gas until I step on the pedal

I have a ck 1500 with a 4.3 litre engine 5 sp standard when it is raining or snowing out I lose power and I have to either step on the gas to get the revs up or let off the gas then step on it again. I have changed the plugs,wires,distributor cp and rotar button but still the same. the engine bogs down like I I have no gas until I step on the pedal

Confirm good fuel pressure when it does it.

Does it have good hot blue spark to all plugs when it does it?

What is engine vacumn reading at idle and at 2K RPM?
Has base timing been checked?