Mariner 135hp 2liter V6 outboard

I guess you mean the Optimax motor, if so they are good engines as always if maintained properly. The plugs on the port bank are a bit tricky to change so make sure the servicing has been done. The most common fault is the ignition coils failing which results in misfiring on that cylinder, not a big job to replace. Also only use proper DFI oil not cheap stuff.
 
The most common or familiar assumption from your description would to me, be the good old trusty 2.0 V6 carbs merc/mariner motor. Come in 135 and 150 variants.
Incredible engines and almost unbreakable...as long as you go pre mix with the oil...or at least keep a close eye on the injection fixtures and fittings.
Trim solonoids can be an issue as well as the poppet valve system.
Other than that they are brilliant things.
But to answer your interest in fuel consumption....well...they are very thirsty things and especially on a heavy boat.
I have one on a CVX16 and had one on a ring 18 years ago. Monstrous power and sound to them...but an equally monstrous thirst too.
 
Monstrous thirst ... roughly how many litres per hour are we talking here?
Not sure really. But significantly more than my 300hp mercruiser mpi in an 18ft boat. I know that will use about 70/80 litres per hour at full throttle from engine data. I’d imagine the merc 150 V6 2 stroke will be easily over 100 litres and hour at that. Even cruising speed will be getting on for about 50 litres an hour at a guess. Lots anyway.
 
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