When OMC was working on a replacement for there 22 horse power apposed twins, Johnson was upgrading there 22 cubic inch 15 horse while Evinrude was upgrading there 27 cubic inch 17 horse Sportfour. The Evinrude four cylinder 25 horse was heaver and more expensive to build and actually did not run any smoother than Johnsons twin so the bean counters decided to repackage Johnson’s design as the Evinrude Big Twin. This design had entirely separate exhaust and driveshaft housings. Minor improvements were made until the 1954 aqua sonic development proved quiet motors sold well. The 1955 Big Twins (& johnson RDs) were the epitome of quiet. However average power output dropped 5%. Testing found air was being restricted from the carburetor and a service bulletin was issued to correct the problem.
Unfortunately I no longer have any service bulletins prier to 1958. The bulletin instructed you to open the quarter inch hole in the front lower motor cover to two inches, and remove and discard the rubber elbow and fiberglass accustical liner from the intake air silencer. Also long term use found the new one piece Exhaust & driveshaft housing was, despite the baffles, allowing moisture into the lower crankshaft seal area and rusting some crankshafts. The sound restricting tubes were removed from the top of the idle relief chamber, leaving it with a flat cover. The lower bearing seal was changed and a bulletin was issued about greasing the spline at the top of the driveshaft.
In 1956 the exhaust baffle was added and the tubes came back on the exhaust relief chamber. In ’57 the exhaust baffle plate was removed. Again grease on the driveshaft spline was emphasized. We still see some motors with rusty crankshafts. Some with driveshafts impossible to remove, others where the splines are completely rusted away and the crankshaft turns and the drive shaft doesn’t. . . 😉
