5.7 vortec rough after start

Experiencing a strange problem. Engine is a crusader 5.7 vortec with 4bbl Holley, 315 hp, a replacement for same mercruiser engine (mercruiser exhausts and risers used so as to fit boat). Electronic ignition. Engine has 60 hours on it. Fresh water cooling, alpha I stern drive in a sea ray. In august it began to experience very difficult starts. When it did start, it would run very rough for about a minute or two at about 1500 rpm, then would smooth out. Belched a lot of black/blue/white smoke out the exhaust while this was happening. Thereafter ran and idled fine, 5200 WOT at >55mph. Same symptoms with muffs on on trailer. Noticed that even when shutting down a warm motor for few minutes, symptoms would reappear on restart... lots of thick smoke from exhaust (black/blue/white). Seems to flood easily (one pump of handle before start.. engine becomes flooded). So.. is it the carburetor (new Holly, electric choke, on E10 unfortunately), or is it a leaking gasket between fresh water/sea water systems in exhaust riser (spark plugs 1 & 2 clean... did not pull others)? Is something cracked? Or what?. Has me baffled.. .

Re: 5.7 vortec rough after start

Ayuh,........

Maybe you've got a Couple of Issues going on...............

The Clean Sparkplugs suggest it's getting Water,...........
And,.....
It seems your Holley is being alittle too Generous with the fuel.........

Has the oil Level risen at all,..??....
Pulling the Manifolds,+ looking for rust tracks might give you some ideas......

Re: 5.7 vortec rough after start

Thanks.. what I meant by clean plugs is that they looked normal.. tan insulator, bit of black soot on electrodes.. but not clean like washed...

I'm concerned over the possibility of water leak.. is this the symptom if a bit of water collects in the exhaust manifold (say leading back from the seal between manifold and riser? A local mechanic when I described the situation thought it was the carburetor.. but i cannot understand why it would cause all the trouble even on a warm engine on restart...?? IF it is antifreeze leaking, it cannot be very much, since the level has not gone down. No water in Oil.

Re: 5.7 vortec rough after start

If an exhaust manifold/riser gasket leaks, it's after the engine is shut off. Not while it's running. When it's running, the exhaust will blow the water right out the back of the boat.
When you shut it off, what water is in the riser leaks into the manifold and into any cylinder with an open valve.
If you suspect water, pull the plugs and look for water or rust. Then crank the engine over and see if any water comes out the sparkplug holes.

Re: 5.7 vortec rough after start

Ayuh,.....
When you said "Clean" plugs, I assumed you meant "Clean" as in New Looking......
'cause that's what Water does,..... It Steam Cleans the plugs.....

If there's Any carbon,+ normal running crud,......
I'd have to agree with your Mechanic,.......... Fix the Carb........

Re: 5.7 vortec rough after start

Belated followup.... problem was apparently leaking gasket between manifold and riser... leaked antifreeze (fresh water cooled) while sitting, caused hard starting. This became apparent after winter storage, when enough antifreeze had leaked to hydrolock the engine...

Thanks for everyone for their help.

Re: 5.7 vortec rough after start

Ayuh,...

And a very sincere Thank You for coming back to tell Us,....