The Mercury 1979 20hp gets straight up then down to the plane within 1.5 seconds from idle to WOT in the condition it is currently in I would rate it the same as a 15hp 2 stroke late 1990's - 2000's model Yamaha, with the Yamaha having a slight edge against the Mercury in the middle throttle range but the Mercury having the edge at pull off and at top end speed, I think with brand new coils, new HT leads, new CDI box and new crank seals the 1979 Mercury would match if not beat the later 2 stroke 20hp makes including 18hp Tohatsu for a third of the Tohatsu and Yamaha price
Their is debate and info on the web that sais that the 1970's Mercurys were sold and marketed with a lower power rating than they actually had, which some say was a cleaver marketing move by Mercury, not until 1982 if I recall correctly were mercurys actually sold with the actual HP being correct, and in that period just after true HP ratings they then sold a 18hp for a very short while then a 25hp, I feel this is probably a 25hp as later rated as such (and the 18hp was never really a correct rating in the new official HP testing was new and had not been fully established) if brought back to new condition, may also be able to get up 30hp with some jetting and minor tweaking and porting
If you have medium level outboard mechanics knowledge I recommend a Mercury model 200 20hp fir SIBing, used parts are widely available, new old stock coils are available, and unlike later 1990's to current models these old engines can take bring pulled apart and put back together over and over again, working on them is as easy as working on 1970's pushbikes but you may need imperial scanners (5/16 and 3/8 only really needed) for some work you can get by if you have a selection of different makes of metric tools (some makes seem to fit imperial just!) mole grips and plyers will help
the understanding and at least the medium knowledge of electrics and rewiring is needed, all worth it, the sound of a Mercury 200 20hp and other boaters comments will make you smile
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