1939 ford flat head wont fore after a rain

[rockfla]

Since your complaint is "only" when it "rains", its not a compression issue. Now your down to spark or gas delivery issue. Have you checked at the point when this happens to see IF you're getting spark to the spark plugs??? Quickest and easiest test!! IF no spark then you have your problem area to search......... IF you have spark, then you have your problem area to search!!! Hard to answer without more information on what exactly is or is not happening and when it is or is not happening. IF it is a "spark" issue the initial point after it rains you "might" look at your distributor cap and see IF you have condensation building up from the humidity/rain.