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

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.

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

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

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,

Yup, can still work but give erratic spark at times. I’ve been in a few boats that would backfire once in a while and after chasing many things, the sensor fixed it.

Did you use an OEM merc sensor or sone cheap China Weird named one ?

Again, your symptoms sound more like a cam going flat, but since issue was with old engine, something else is probably up.

Speaking of which….any battery/charging/starting issues with this engine ?

The "pop corn noise" under load could potentially be a drive issue. A drive issue could also explain your extra fuel consumption at idle, and hard engine rotation when trying to start.

Ok... I will try and change the sensor. I believe everything else has been interchanged between engines without results. I do not know what brand the sensor was, it was just replaced before I bought the boat, which is 3 years ago. The engine has been out most of this time, so not much work hours after that. I somehow got the idle consumption down today, messed around with the idle mixture screws. The point was, the carb had drip, and to compensate that the screws were out... i tried to find a sweet spot and something happened, the flow meter now reads 6-9l/h vs 5-7l/h on the second engine. But the main issue still persists, the engine runs "hard". I actually tried adjusting the timing by hearing, got it to about 5o, or 12o... not much difference, still popping and running "hard", altough it was a bit happier at 12o... but I did not manage to drive it under WOT, a bit stormy today, so dont know what the upper situation on the advance would have been with these settings. I suspected the markings were wrong on the casing... i have an impression that it is a bit frankenstein engine, we found a drag camshaft in it previously.

Its sounds like a locomotive, or a truck and that is what bothers the most. If I wouldnt have the other engine next to it, that purrs tamely I would probably believe that it is what it is and go with it...
https://www.dropbox.com/scl/fi/067x0...=cmp8rtt1&dl=0 a video here. Dont mind the pulley wobble, the harmonic does not wobble.

The drive... everything is possible. When we put the drive on this year, we aligned properly. The drive itself was rebuilt a year ago, no leaks or apparent problems before installing.

Does it pop enough to read the plugs, can you narrow it down to one cylinder? I’m assuming new plugs and wires and when you swapped distributors you moved the coils . When it pops does it take the power valve out of the carb? Is the poor running engine making correct voltage to run the ignition ? Have you put a vacuum gauge on both motors? When you say rebuild, what did they do to the heads during the rebuild? Could it be a valve that is a hair tight and occasionally hangs ? Something stuck in the exhaust? And you swapped the flame arrestors.

Right now I haven't figured out how to narrow it down to one cylinder. I have new plugs and wires, have not swapped the coils, also have not checked the voltage... the general voltage is fine in general (on the gauge) and charging etc, but I haven't checked with a voltmeter specifically. Will do that. Regarding the vacuum gauge, I have a vacuum gauge that I used for measuring fuel pressure, it can also be used to check the engine vacuum, but how do you do that on that type of an engine? So I haven't done that yet.
Also, what do you mean by popping taking the power valve out? The power valve is inside the carb... I have not opened it up to check. But there has not been a change in performance... but could that be an issue? It does not backfire through the carb, but it did a couple of times when trying to start it up, we found out that the distributor was mistakenly put on 180 degrees off. Could these backfires have messed the carb?

The exhaust is not plugged. I have a silent choice, the problem persists with through hull or below water exhaust being used. The risers and manifolds were clean when installed.
I have ran it without the flame arrestor when testing, so that cannot be it also. Although they were both cleaned.

Something with the heads could be possible. The heads were dismantled, bench worked on the seats and the gasket surface also, also new valve seals etc, and put together again. There could be something mechanical going on.
This is very frustrating . The engine has a history, was rubbish originally when I got it, 20-30% cylinder leaks, one connecting rod bent (probable waterlock because of a rotten manifold), rings installed with locks on top of each other... so the engine and carb were rebuilt 2022-23 winter, after that ran, but badly, soot, truck noises and wrong oil pressure. Lost one season. So it was rebuilt again 2023-2024 winter, we found a sloppy distributor bushings, wrong oil pump relief spring, and a street performance camshaft inside it. So all the hopes were up for this year, but still does not work. We got rid of most of the issues, now the truck noise remains .

Sorry if my terminology is not spot in, i´m not a native english speaker. Thanks for thinking along!