Backfire after 45 minutes of steady running, 165 hp, inline 6, 1972 Grew

I have a 165 hp inline 6,1972 Grew that backfires and runs rough after about 45 mins of steady use pulling tubes or skiers. I have read many of the posts and believe I have a few things to check on my next use of the boat, but wanted to describe a few more specifics of my problem. Only happens when engine is thoroughly warm (operating at top end of safe green zone). Have had timing checked, new coil, plugs, wires, in last year. I am sightly embarassed to say it has been a constant problem for over 15 years and we just stop using the boat once it happens take a swim break for 1 hour and start up again. I can recreate the problem anytime given 40- 50 mins of steady use. Had 5 different mechanics review and each time implemented solutions at end of season that appeared to work, only to reappear next season. Have had stern drive replaced, rotor, distributor cap, fuel hose between pump and carb all replaced. I unfortunately was not present for any of the trouble shooting tests so although have implemented the proposed fixes, I have no definite observation of checking for water in fuel etc. I am looking for a fresh start and a new look at the best troubleshooting steps.

Ayuh,.... I'm guessin' ya got Points ignition,..??

If so, changin' the Points is an annual experience to keep it runnin' Great,...

First check your fuel tank vent, you can actually just take off the fuel tank filler cap temporarily, & run it when the issue starts, to see if that's it? It sounds like a gradually occuring lean running condition. Next, check your fuel tank pickup screen, the anti-siphon valve, then all your fuel filters, & lines. It doesn't take much gunk to cause this.

Thank you guys. I forgot to mention I had changed the points, so will work the fuel details. The last mechanic who worked on it quite a bit was zeroed in on the fuel but when he suggested replacing the fuel tank I wasn't quite ready to invest that much time and money. Will work my way through the system to make sure all is in good shape. Unfortunately can't make a trip to the boat for the next two weeks so excuse the time delay until I get back.