I just built a 355. It has flat top pistons, vortec heads, 262/268 xtreme marine flat cam, vortec heads, air gap manifold, and an Edelbrock 1409 carb. 28 degrees of timing all in. Runs awesome. My only issue is a cough through the carb if I gun it as if I was pulling a skiier. It will also do it at cruise if I lay into it fast. If I ease the throttle its fine. What could it be?

Timing.

It also very well be the accel squirter, I would make there that is adjusted right and your getting enough fuel.

there are 3 holes on the accelerator pump arm, i believe they come from the factory in the middle hole, move it to the hole closest to the shaft for more pump shot, I think I had to do this also (350 vortec, lt4 hot cam). Also you should be able to run 30 degrees timing.

X2. Bump that timing up some more.

I moved the accel pump to the closer hole and bumped the timing up to 30. I will let you know tomorrow.

Sounds like accelerator pump issues to me.

I had a 355 built once at it had the same problem, especially try to get on plane. I had a Holley and Holley told me with the crap gas in my area bump up the secondarys 1-2 sizes. Went 1 size and fixed the problem.

I'd lean more towards this being a fuel issue. I've seen 28 degree timed motors do just fine, granted it may not be optimum, but no coughing etc.

Yep. 100% fuel issue. I agree that timing should be around 32-34 but that will not bog it down on quick acceleration.

Think of a boat as a 5000# Lincoln trying to go up a 45 degree incline in 3rd gear from a dead stop. You are going to need some fuel with that kind of load.

Buy a carter/edelbrock tuning kit and fatten up the fuel curve right "off idle.