I have two wonderful freshly rebuilt engines when at dock, but when going out... the strangeness begins. This is getting really serious, I'm running out of things to rule out

Backfires through carb sometimes at about 1200+rpm, but ONLY under load (driving) and after about 5min driving, but then keeps happening, when "its that day".
Also, the base timing jumps permanently to another setting sometimes, one day its 8, the other day 12 degrees? The distributor is not disturbed (really tight).
Also the engine runs hot (heats quickly to about 80C-90C degrees, the thermostat works and is lower, it opens before that as It should - I can tell from through hull exhaust vapour and the riser hoses), and heats up to working temp much more quicker than the other engine. This happens regardless if the timing has "jumped" or not.
Also the engine has constantly minimum double - maximum 5 times idle fuel consumption than the other engine, about 10-20l/h vs stable 5l/h on the other engine (I have flow meters, and have interchanged to rule out broken meter). The consumption evens out at about 1500rpm, so only at idle.

I am battling this strange engine for the second year now, I have also started threads here (thanks again to everyone thinking along), but since the situation evolves (I have fixed one thing etc.) I am making again a new thread. The boat is a twin engine 1987 CC Stinger, with Alpha one gen1 drives and Merc 260 engines. One of them works fine, but the second one acts strange. I have gone over everything I can think of and am seriously seriously lost by now.
I have new coil, wires, cap, plugs, fuel filters. Between engines I have interchaged: Carbs, pick-up pipes, thunderbolt modules. The fuel vent is not clogged. To rule out things, I have installed a new camshaft, new harmonic balancer (to rule out slip), valves are freshly adjusted as they should (+1 turn as in Merc manual). All the wire connections are cleaned, checked repeatedly. The distributor is rebuilt, so no play and new gears. Fuel pressure verified OK. Carb does not have nozzle drip, choke is fine and carb itself is rebuilt (so are the other carbs I have tried).
I have also interchanged drive units to rule out water flow problem from water pump and checked for obstructions in the oil cooler, and tightened the ball springs on the riser Tee pipe.

So... I am thinking more and more... can it be that I have something really wrong with the heads themselves (valves bent, a crack etc?). Could this explain the hot running and high consumption? The heads were actually rebuilt before this started happening, so maybe something went unnoticed or happened?
Also, is it possible that the thunderbolt IV unit changes its base timing somehow sometimes, for an example when it gets hotter when driving (mounted on riser)? It literally is 8 one day... and 12-15 suddenly the other day. I have had this repeatedly, and the distributor is marked, tight, not moving.

Thanks again everyone!

Hi,
Assuming this is a mechanical advance distributor. Have you removed the rotor and checked the ignition advance weights and springs. They can rust and get stuck. It is possible that at higher RPM there is enough inertia to get some advance and then not enough spring pressure to return the weights to base timing.

Does the engine ever crank slowly? Thats i sign of to much base ignition timing?

A pop or backfire out of the carb is normally a lean condition. Extra heat will be caused by a lean condition also. But if you say the fuel consumption is higher that doesnt make sense and you have also swapped carburators.

Does it idle fine? There could be a vacuum leak of some sort. maybe try spraying something flammable(carb cleaner) around the intake, carb etc while its running and see if it ever changes RPM(would idle up)?

Jadento

Agree with above.

My MSD distributors rusted internally and ate up the advance springs. Weights also got very stiff. Cleaned up OK and new springs fixed it. Sprayed weights/springs with corrosion preventative and still good after 10+ years.

I would swap dist and try again. Retarded timing will make eng run hot. Maybe double check valve timing if dist swap doesn't help.

If balancer has more than 1 timing line, put some white-out or crayon marks on the correct line to prevent confusion. Same with timing tab if it's not obvious where Zero is.

4 degrees timing is not going to fix the issues hes describing

Pretty sure he said tbolt iv and swapped modules.

but i agree , sound mech adv weights being stuck .

op - you have a pic of your dist’s ?
if really tbiv’s - have you ever changed the dist sensors (pick ups)

Hi all, Thanks for thinking along!

The boat does not have a mechanical advance. It has Mercruiser Thunderbolt IV electronic advance. In that system the distributor is simple, no moving mechanisms inside. Just a pick-up sensor and then there is the black box (thunderbolt), that handles the advance. When checked with a timing light, it advances fine and works, so that creates the mystery - how can the initial timing change so its different on different days.
On the other questions - it cranks nicely.
The pickup sensor is a couple of years old - since its a simple on-off device, and works, I could not fathom it being the cause of this changing behaviour?
Balancer has one line, and before assembly we checked that it is in the right place.
I have checked carb/manifold for vacuum leaks, did not find any.

I agree with "boostbros" that 4 degrees will not help with the heating. It is possible that these are separate problems. The thunderbolt can be the culprit for the changing timing (its just I cannot google up a thread with similar symptoms - everyone says either it works or it doesnt). Also it heats up quickly whatever the timing.
I know that when redoing the heads, they took a considerable amount of material off from the contact surface (to get more material at the cooling passages), meaning that the combustion chambers became smaller. Could this be the problem?

Not true about the tb1v. Have seen several with sensor issues but still ran. Sporadic intake backfires.

How much was removed from cyl heads ? Surely they measured ….right ?

Well, they did not say how much they removed, and they dont remember how much they removed. So I´m left with that unknown unfortunately.
The heads came back in the autumn, boat testing resumed in spring, so lot of shelf time inbetween. Since they looked so different when I got them back, I would guess a millimetre+ maybe, so quite a lot.

Ok, I will try to replace the sensor too, if you say that sporadic intake bakcfires because of that may be a possibility.

Thanks

I've seen distributor shafts have the rotor head on the shaft come loose. This allows the shaft to be pulled down from the gearing, retarding the timing. It often got locked into position from getting hung up on the bottom of the distributor body. It was an intermittent issue...hard to find.

Will inspect that, thanks!