Ville123

Hello,
I have a twin engine 1987 boat with alpha one gen 1 drives and two merc260 5.7 sbc´s. One engine runs fine, but I am having serious trouble getting the other one running nicely. Maybe someone has a tip about something I have overlooked.
The engines are freshly completely rebuilt, compression and leak tests are good, also new camshaft. The problems were present with this engine also before rebuild, and were one of the reasons for the rebuild. After rebuild, but before new cam it also produced a lot of soot from the exhaust, so I also put a new cam in. The soot and foul smell stopped after that.
Symptoms: Running is laborious and does not sound right when compared to the other one that purrs effortlessly, also starting/turning over is "hard" by ear. This engine runs much hotter (does not overheat but noticeably hotter than the other). Fuel consumption is at idle about 7times more than on the second engine (35l/h vs 5l/h), but evens out at higher revs (I have flow meters, new). At stable 2000-2500rpm the engine makes a random "popcorn" noise from exhaust.
Carbs have been rebuilt and interchanged, also the ignition modules but the problem persists. The timing on both engines is 8o, as it should be. The distributor bushings are rebuilt, new bottom gear, cap, rotor, wires, plugs. Fuel pump produces 5-6psi. I cannot check flow but the pressure is good. I do not know if this means anything but I installed a small seethrough fuel filter on both engines, before the pump because of the flow meters. I noticed that when revving this engine sucks the small filter not dry... but almost, the level fluctuates a lot. The second engine filter level does not change whatever I do. The boat has electric carter low-pressure high-flow pumps retrofitted, and the water separating filters are after the pump, before the carb.
The boat has no anti-siphon valves on the tank.

It feels like the timing is just wrong, but how can that be when the both modules produce the same result on one engine, and the cam/crank physical position has been checked twice (I suspected that the chain had skipped a notch). Also installed a new harmonic balancer to rule out a slip on the rings, (the mark was also physically checked).

I am completely lost for now, I have messed with this for two seasons. Any ideas would be very very appreciated.

Thanks

SB

Modules or ignition sensors ?
these are the sensors (pick ups) for the tbolt iv distributors. If you never replaced these, they are suspect.
Also, your backfiring could akso be worn cam lobe but i’d do this first if never replaced. Use genuine merc parts fir these.

carnutsx2

Just spitballing…….Timing ring on balancer slipped and the timing is off ?

resurrected

Sounds like you have chased everything, the only thing I can think of that may be left is a vacuum leak.

Ville123

Hi, the balancer ring is new, and the camshaft is new. The sensor is a couple of years old... do you believe that the sensor could give somewhat "off" readings, although it works?

Thanks,