I'd want a 8x3 minimum but ya a quick on and off run tell the tale in a matter of minutes.

My flame arrestor flowed enough and it actually ran worse without it as it seemed to straiten and settle the air going in. What was eye opening was cracking the hatch and gaining some solid rpm. The afr didn’t change at all, but the engine was thirsty for better air and more of it.

Not to say that various boat manufactures are light on ventilation but then we increase the power by 30 to 50% and in some cases double from stock and then only compounds the problem. More vents in the right places the better. Rob the squirrel cage from your furnace and mount it under your rear seat and with a little rigging and converter... whalaa... lol, get that hot air out of there. I use to be know for leaving my blowers on intentionally!