Merc 230 MIE w/ Thunderbolt IV - Chevy 305

[Diverted Income]

Have been chasing down a problem for a while now. Port engine runs fun, starts fine, but usually won't pull a full load. Runs out of steam around 2500 rpm, but occasionally it runs up to speed for no obvious reason. It may start out that way or just happen in the middle of a cruise.

Here is a list of what has been done or tried:
Carb rebuilt (stuck needle and seat)
New plugs
Cap and rotor (who knows when they had last been changed)
Fuel/Water separator/filter (fuel looks good coming out of filter - I cut my filters open, everyone of them)
Tried both port tanks and even opened the cross over. (thinking blocked pick up or anti-siphon there)
Had a timing light on it. (looked like all the advance was coming in 32* if I remember right)

This week I am replacing the original looking plug wires.
If that doesn't do it I am swapping ignition boxes from one to the other to see if the problem transfers. Think that is a long shot but cheap to try. Bad plug wire makes sense since it sporadically runs perfect, but we will see. Any other ideas?

[gitchisum]

I have the same thing happening on my Stb crusader 270. Ran through most of the same stuff you are checking and had the carbs rebuit. Rebuilt my anti-siphons and it was still happening, I can get 2800/2900 out of it, at WOT, then it may stay there or lose rpm's for no apperant reason.

I did most of my driving from the fly bridge, so I wen down to my lower controls one day when it was cold out, and it runs perfectly. Throttle levers line up at all rpms and I can get WOT of 4000 out of each engine. Lower controls work fine, and upper controls don't.

Not sure this is what you are facing, but symptoms sound similar

[Diverted Income]

Same up or down. Verified air doors on Q-jet secondaries are free too (while running) Cable issue would be fine with me, but that's not it in this case. Thanks for the suggestion!

[captainmaniac]

You mentioned Fuel/Water separator/filter... silly question - did you get all of them? I have a fuel/water separator for each tank on the forward bulkhead, plus an inline filter near the carbs, and I believe there is a filter inside the carb fuel line fitting as well. I have Chryslers so may be set up differently than your Mercs. Other thought would be - are any of your fuel valves (in the fuel line) not fully open? If a valve handle has loosened off over time, it may look like it is wide open but may be only partial...

If you have the typical fuel cross over system, have you tried running both engines off the same tank? Might help isolate to tank/fuel/lines vs engine.

[Diverted Income]

https://youtu.be/VZaPRwUrteY

Plug wires fixed it. Appeared to be original Merc wires on the Port engine. STB had been done already at some point. Sorry about the crappy video, real windy yesterday.
Thanks for the suggestions guys!