Start on two cylinders

I have an RV7A with a rebuilt (200 hrs SMOH) IO360 a1b6 200 hp. Been running great. Aircraft hangered all winter. Just switched over to ramp parking. After a few days of heavy rain I went to start and it kick over but ran for 15 sec or so on only two cylinders before the other two came on then ran smooth and I completed a good flight no issues. Parked the plane, went back a few days later again after heavy rain and the start was similar. Shut it down and went back 20 min later and it started fine. Servo and spider have been overhauled. Intake is in the nose by the spinner. Any thoughts?

Do you have a sniffle valve installed?

Carl

Not sure what a sniffle valve is. Can you help me out? I do have a filter bypass.

Yes, but it seems to be very small and simple. Looks like a bolt with a small hole in it. Are you thinking I could be accumulating water in the sump?