The 1.6 litre is just under 100 cube's. 115 hp on a V-20 should get about 30 mph. A crossflow V-4 will drink the fuel if cruised at over 4000 rpm. I've had 2 OMC crossflow motors and found 3800 to be optimum cruise for mileage.
Have you checked compression, with a 15P prop it should turn well over 5000 if it's making the rated HP.
I've seen an early 70's V-20, lightly loaded, with an 85 Johnson make 28 at WOT.
My old V with a 135 hp crossflow Johnson ran mid 30's and got at least 2.5 mpg. The older V-20's were lightly equipped compared to the boats from the 80's and later.....how loaded down is your boat?
Your boat needs a 17P to get cruise speed in the proper rpm range, but doesn't have enough HP to turn it.
I tried a popular brand aftermarket prop on an 83' crossflow and it ran awful. We had to run an OMC prop, might have been an exhaust backpressure issue. What mfg prop are you using?
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