When you pressure tested the fuel system was it under load? If it wasn't, when you turned the key off did you have a very fast decline in pressure? I had a fuel pressure regulator go out, gave me symptoms of the fuel pump only so I change the pump. Problem not solved. I then pressure tested the other engine and the pressure held after pump stopped running, motor with issue would drop pressure very fast so I change regulator and bingo.

need to ck fuel pressure running when issue comes in..what pressure at that point..you could have plenty of pressure up to a point you have a flow issue that drops of the pressure..

Did u test it with a load on it past 4000rpm?

No I did not check it under load... However when I get it back in the water I will do that, knowing how easy it is to do, and that I can get the gauge for zero cost.

Ignition on no start...42-44
Running at idle ...36
Ignition off ...34

At wot your fp should be in the neighborhood of what it is ign on no start (or idle with regulator vacuum hose disconnected and plugged).

Noted!

check timing, I have had this problem. it wound up being distributor drive gear. look at fuel pressure at load to ensure it is not that. worn gear acts like stretched timing chain, no load snap throttle it will bounce timing a lot.

I called Mercury Tech support...before I read your post and they told me the same thing...Check timing, and that they were issuing a service bulletin that after a break in period the timing gear might wear slightly and that you should turn the distributor 1/4 inch left or right to fix the problem. When boat is back in the water I will check it out.

A little review... the engine would start popping after 4k rpms.
The trouble shooting was done in this order. Checked fuel pressure not under load and on trailer...OK
Changed plugs and then did load run, still popping...now at 3.8k.
PU FP gauge from the Zone and did a run under load every thing OK pressure wise.
Loosened up distributor and turned the cap approx. 1/2 inch counter clockwise (if looking at the transom) pulled hard all the way up to 4.5k.

The "Sorta" part is it use to pull hard all the way up to 5k. Now the bottom was a little dirty and maybe more fuel in the tank. So I will work on those variables after it get back in the water again.

Thanks for the update, I hate it when a thread stops with no results!!! = )