Stock 502 at about 580 hrs. Never given us problems till a few weeks ago. After it has been sitting in storage for a few days, it looses its prime and won't start. The other engine stars fine. We swapped out the fuel filters and that wasn't the issue. Ran down the lines and tightened everything up, that wasn't the issue. Once we manually prime (fill the fuel filter and sprinkle the carb) it starts right up. It will run fine all day after that but when it sits for a few days, it has the same problem. We replaced the fuel pumps last summer so they only have about 15 hours on them.

I'm perplexed- any suggestions?

check and see if they have anti siphon valves,you may have one with trash in it.

Does it have Weber carbs? If so, that is the problem. They are well known to evaporate the fuel out of the float bowls while setting for a week or more. When I had one I learned to crank it for 15 seconds, stop and stroke the throttle once to shoot some gas through the accelerator pump, bring the throttle just off idle and hit the key again. It would usually start right up then. Kind of a pain. Your only alternatives are electric fuel pumps that will refill the bowls quickly or change carbs. That's what I did. A Holley 830 did the trick. Noticed a significant performance increase as well. Picked up about 3 mph just from the carb change. Took some time to get it jetted properly but it was worth it. Good luck.

@ mike - dumb question, but where does one look for the anti siphon valves?

@ Mac - it appears to have something resembling a rochester style carb. Does the weber have an identifying feature? We are looking at switching both over to the Holley's but that was going to be something we did this winter. Oh well- might not be able to wait.

It will say Weber on it. Might have to remove the spark arrester/air cleaner to see.

it screws into the tank,it is a 1 way check valve,i had a repair job with the exact same issue as you,the valve had aluminum chips stuck in it and the boat was 6 years old.it had a 415 hp 502 efi engine in it.