Engine backfires shuts off when first running, then perfect after 30 minutes low speed both motors

[Neptune02]

Hello
So I seem to have this reoccurring problem. Similar with both motors. When I first go out and start powering up. My port engine starts backfiring. My Starboard will not stay running when powering up. It shuts off. Once Im running under low power both engines around 2,000 rpms for about 30+ minutes I can then power both motors to full power with no problem. Can I have water in fuel? How do I check? I have a 2002 Chaparral Signature 35' volvo penta 5.7 engines. other forums say that possibly water coming in through vent? Any help or advise appreciated.
Thanks,
Fred

 

[Donald0039]

Pull the fuel filters and pour into a clean mayonnaise jar. What does it look like?

What is the history of the fuel in the tank?

 

[Neptune02]

Fuel is clean. was drained a few years ago. The problem has been going on and off for several years. Once both motors run for 30-40 minutes low rpm. It then will run strong, Both fuel pumps replaced. Throttlebody carbs rebuilt. I do notice my port carburator has a lot of moisture on the outside of the carb where it bolts onto the intake manifold. I dont think that is normal or is it? Will check the fuel filters. Thanks for your input Donald. Any other thoughts appreciated.