To give you a little back ground on what I have done. Over the winter I installed a new roller cam setup from RMBUILDER (thanks again Bob) for my 496 N/A engine.
I took it out a few weeks ago and everything worked fine. I only gained around 100 RPM. I expected a minimum of 300 to get me around 5,000 RPMs
So the story begins. I enjoy throwing $$$ at this money pit so I changed my Barry Grant 750 DP to an 850 DP. The 750 was jetted at 80-88 and the new 850 is 82-88. I thought the extra jet size would make up for the bigger 850. In neutral if I hit the throttle fast I get a stumble(dead spot) then a backfire through the carb . If I go past 1,500 rpms it will not do it. Unfortunately I have not had it out on the water yet (flooding ) to see how it performs under a load.

So what do I check first the accelerator pump linkage? Then jet up, if so how much? Or just put the 750 back on and enjoy the summer

Sounds like you need more squirt. Check your manual, you should be able to adjust the shot size and when it starts.

I'm guessing you'll have to jet-up more than one size, and also on the secondaries as well. I'd start with the squirters, though.

Try 3 -4 sizes on the acc. pump nozzle. See if it helps and go from there.