Thanks for previous help on a similarily named thread - i got this engine fixed, but now new issues arose on the second engine. Since the situation is new, the problems new, I will start a new thread here.
So, the boat has two merc 260 engines, 1987, alpha one gen 1. Both the engines sound and work ok on dock, also idle running is fine.
The engines were mechanically rebuilt (new piston rings, bearings, heads done etc.) also new carb, plugs, wires, swapped coils, modules, cleaned contacts, distributor rebuilt. Everything I can think of new or rebuilt. Timing is fine, 8o and advances. Choke works as its supposed to. Fuel pressure OK 5psi, new filters. Fuel flow meter shows normal flow depending on rpm.

The engine was running and idling fine, but one day after starting died and became hard to start like it got flooded, started up finally when manually opening up secondary air flaps, after that started running higher idle rpm (about 150-200 rise). Perhaps this starting issue was one time failure from me, maybe I flooded it accidentally, but there is more going on, which is constant. When driving, it struggled to get over 1500rpm, backfired through carb once and after that I was limping back to port it idled fine, and then after about 20-30 seconds of idling, died and was hard to start. This can be repeated, after some driving, and idling afterwards it dies in about 20-30 seconds, and is hard to start like it´s flooded.
I once pulled accidentally the cranckase ventilation hose, and the engine did not die. There is a considerable amount of blowby going on. I know that this should not kill the engine, but somehow in my case it is related. Can it be that after hard driving the cranckase is so saturated with exhaust gasses, that when I take the rpm down, and the stuff gradually vents out, but the air suction from carb is not so hard it saturates the air to a point when it dies...
Since I the piston rings etc. are new I have hard time believing that the blowby is so bad, coming from the rings, that it kills the engine. But I know that the guy who did the mechanical rebuild on the engines, messed up the rocker adjustment on the other engine, and that engine was fixed when that got that done properly... could it be that there is an issue with the valves with this engine too? Could it be exhaust gasses leaking from the exhaust valves leaking to the crankcase so badly that it just floods the engine with exhaust, thereby depriving of air? Bad adjustment or missing seals? Or both?

Or... any other ideas? Maybe I am going too deep with this theory but it really seems that the engine is deprived of air, and somehow the crankcase ventilation, only after harder running, is part of it? Or its the ignition after all? I have swapped the modules, and I have three carbs that I have tried on it, one is new.

Can you post on that thread what it was that fixed that engine ? Thanks

The second engine had seriously loose rockers. I just thought that maybe if the guy messed up heads on one engine, maybe he messed up the heads on this engine in another way

Im guessing and I said guessing. It’s probably ignition (spark issue). Could be a vacuum leak but I’d start here.

assuming the plugs, gaps, etc are all ok. But since the guy did a questionable job on a simple thing rockers I’d not trust anything else.

double check your plugs, gaps, wires, coil, distributor, cap. Here’s what to look for. Good luck.

https://www.youtube.com/watch?v=L4bvnC7rz0c

now blow by is a whole other issue. Guessing something way wrong on the bottom end. Like the block wasn’t checked to spec. Was it overboard and right pistons and rings used? Something not right. Check your build sheet.