Before you spend any money on more parts, sounds like you need to dial this issue out first.

Since you have had this boat, has it always been like this???

I agree with the above checks,
- Timing
- Fuel pressure
- sure your cams aren't just worn out? pull valve cover and verify lift??? Maybe it has some low rpm cams in them, like RV/Truck cams or something? Which FYI are good to about 4k rpm's....

There is certainly no point in doing anything you described until you figure out the issue here.
- All this seems odd that it's on both motors

Something is making this thing lay-over at 4k, need to figure out why, my guess is valve-train or some timing issue.

Thunderbolt IV, I am going to look into the advance weights and see what I can dig up.

I only test drove it once the day I bought it, and I honestly did not pay attention to see if it was getting to wot or not. Right after I bought it I started tearing it apart to get rid of the totally 80's color themes. My guess is that it had the issue then too, but I just never paid attention as I was not running it flat out.

I am going to look into the timing advance, cause it is set at 8degrees at idle, right at mercruisers spec.

What is really puzzling is that originally I was thinking one motor was having an issue and it was causing the other to pick up the slack. But according to the engine synchronizer and the throttle levers position while cruising the motors seem to be sharing the load evenly. Which is why I was ruling out mechanical issues.

I will look into timing some more. If it is a fuel issue I just dont see the point paying to reb uild the q-jets when I can get a good set of 1409's for not much more then the cost of a rebuild.

Check ing. if tou have standard merc plug wires replace them with a good spiral core like the moroso's. also check your trigger inside tour dist cap if corroded replace with new one . make sure nto ground stud at bottom of dist if you dont have the three wire trigger. this should take care of your problem.

This suggestion seems to simple I know, but I did not see anyone on this thread ask it. Have you checked the throttle cable and linkage with the flame arrestors off (engine not running) to see if carburetors are getting full throttle opening?? Seen it before and it seems to simple, but its easy to check and eliminate. Hope you find your problem and I hope its simple.

I would also not recommend going into the engines for more power until you find this gremlin first.

Best Regards,
Ray @ Raylar

Haha yeah, that was the first thing I checked. The primaries open 90degrees. this has been a thorn in my side for a while.

Today I talked to an engine builder in the area that said the 1986 vintage mercruisers had probably the worst cam for power, basically a longevity grind. He said that swapping out the whole top end would more then likely get rid of the issue, but I would not be happy with the results as the cam would seriously limit HP/TQ gains. I was always under the impression you could not run an agressive cam on a marine engine because of reversion once you start changing overlap duration.

Ahh sooo many opinions! Maybe I will just get all new motors and be done, now where is the nearest ATM I can knock over??

how many hours on them, maybe weak valve springs/valve float.

Ray, check post #2 and #7. it's there:)

The engine builder I talked to today, said that the valve springs could be tired, and would certainly cause this issue. Not sure on the hours, no hour meter.

But what has killed me is that he said the cam in the motors is worthless and that swapping out the heads will not net a good power gain. I was thinking I should easily pick up 50 HP between the heads and intake and roller rockers, plus shedding a considerable amount of weight.

B.O.A.T.
Bust out another thousand.