I just got done with my rebuild a few days ago on my 304. I pulled it out of the garage and let it idle for a while then i put a few light miles on it. It ran fine all last night but this morning when i took off in it, it would constantly sputter and stall, it only happens when it is moving. Im open for any help any of you have to offer thanks.
Is the fuel filter properly installed? It should one with a dual outlet. The outlet closest to the side of the filter is for the return line and needs to be installed so that the return line outlet is on top of main outlet (not lower than than the main outlet) or it can make your motor do what you describe. If you do not have a return fuel line, you have found at least one of your problems.
Just a side note. You really should not let a fresh engine idle, turn the idle speed up to 1200 or so for the first few hours. I don't let mine get below 2000rpm when first starting, and for a few hours, Then I will drop it down to 1200 or so for driving.
If it comes with instructions, then it's not what the pros use.