I don't know if this will help. Back in the day there was a product called Dampstart and it was a waterproofing spray mainly for use on the High Tension side of the ignition. Usually bad running in the rain is HT related(but not always) so try waterproofing around the sparkplug and coil etc. Although I did have a cagiva that cutout when going over speed bumps that turned out to be water on the tipover switch ECU connection. Good luck