Obsolete Outboards
by Max Wawrzyniak

Start to Finish, Book II:
(Part 1 - Part 3 - Part 4)

Reviving a Vintage Big Twin - Part 2

Today was a Saturday and so I got a lot done on the
'53 25 hp Evnirude.

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click to enlarge images

After a 2 hour morning "paddle" aboard the Larsboat on a local river, I comenced to working on the 25; first order of business was to remove the carburetor for cleaning and rebuilding. The carb on this 50-year-old engine is about as simple as these things get; removing it required the disconnecting of the linkage that synchronizes the carb butterfly valve and the magneto "spark advance" and the removal of just two nuts. I always suggest disconnecting linkages in such a manner so as not to disturb adjustment settings, and so I merely removed a tiny cotter pin on a bellcrank and unhooked a little tie rod.

click to enlargeAfter removing the two nuts and also the fuel feed hose, the carb was removed from the engine and dissasembled. All in all, it did not look too dirty, but I went ahead and dropped it into a gallon container of carb. cleaner to let it soak while I worked on other things.

click to enlargeUsually I just spray-out carbs with an aerisol carb cleaner, which is the way that most engine manufacturers recommend cleaning late-model carbs; in the old days, one allowed a carb to soak in cleaner and then blew-it out with compressed air. A carb allowed to soak for a day or two in the "old" carb cleaners came out looking like brand-new, but it is difficult to find the "old" cleaners now, as they have been replaced by 'environmentally-friendly" carb cleaners that do not clean as well.

click to enlargeWhile waiting for the carb to soak, I decided to replace the water pump impeller. The six bolts that mount the lower unit were removed, and the lower unit allowed to drop down to the extent that the shift rod would allow. The shift rod is what connects the shift lever on the Starboard side of the motor to the lower unit, which is where the actual "transmission" is located.

click to enlargeThis is where later OMC engines have an slight advantage; they usually have a small access door in the side of the "tower" or exhaust housing which allows one easy access to the coupler that needs to be disconnected in order to remove the lower unit. The Big Twin does not have such an access door. The coupler is disconnected by allowing the lower unit to drop as far as it can, and then by reaching into the open void between the towerclick to enlarge and lower unit with a screwdriver and disconnecting the coupler. Really no big deal when removing the lower unit, but a farely major problem when re-installing it.

It is bad practice to carry a lower unit by holding the driveshaft; in the case of this particular engine it, the driveshaft is not secured into the lower unit by anything other than the tiny key that drives the impeller.

click to enlargeWith the lower unit off the engine, the four screws securing the pump housing were removed, and the pump housing and impeller slid-up off the driveshaft. Most of these old OMC's have an "O" ring sitting in a groove milled into the splines at the click to enlargetop of the driveshaft. the purpose of this "O" ring is to prevent water from rising up the driveshaft and getting into the crankshaft bearings in the powerhead. The "O" ring needs to be removed in order to remove and reinstall the pump housing and impeller, but be sure to replace it once you are finished with your waterpump work.

click to enlargeThe impeller removed from the engine was obviously "toast" with the blades retaining a curled shape, and one blade retaining a backwards-curled shape. Obviously, this impeller had seen better days. With new impellers readily available from both Johnson and Evinrude dealers, and also through aftermarket suppliers, it just doesclick to enlarge not make any sense to chance using an old impeller, even one that looks in better shape than this one did. I installed a new Sierra-brand impeller and reinstalled the pump housing and also the "O" ring on top of the driveshaft. A SMALL dab of waterproof grease was rubbed onto the driveshaft splines; don't over-do it.

Getting the lower unit back onto the engine took a bit of "engineering." One has to hold the lower unit just below the level of it's installed position while one inserts the screw into the shift rod coupler and tightens it, while also halding a flashlight so as to see inside the engine. I hunted up a bolt of the same diameter and thread as the lower unit's mounting bolts, but which was longer, and used this long bolt to hold the lower unit onto the engine but allowing about a 3/4 inch gap between the mating surfaces which then allowed me to focus a flashlight on the couple while I inserted the screw using a magnetic screwdriver.

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Once the coupler was fastened, I secured the lower unit into place with it's proper mounting bolts and removed the one long bolt. Although dealing with the coupler is easier on MOST later OMC's, one advantage of this older engine is that it did not utilize a water tube for conducting water from the pump up into the powerhead, and dealing with the water tube can sometimes be difficult on the later engnes. Onc must simply take one's time and not allow frustration to take-over when reinstalling lower units.

click to enlargeAt this point I went back to the carb, which had been soaking for about an hour or so. The passages in the carb were blown-out with aerisol carb cleaner (BE SURE TO WEAR EYE PROTECTION !!!) and I went looking for a carb kit. The Sierra catalog does not list a carb kit for a '53 Big Twin, but does list a kit for '54s. I used a "54 kit which had many extra parts not needed for the '53, and I needed to trim the bowl gasket a bit with a razor blade in order to make it fit, but I did replace the parts that needed replacing; click to enlargeprimarily the bowl needle and seat, the afore-mentioned bowl gasket, and the packing that seals the high - and low speed needle valves. The existing float looked OK, as most do; keep in mind that these old cork carb floats are coated with varnish, and that carb cleaner is intended to remove varnish, so keep the cleaner away from the float..

click to enlargeThe reassembled carb was bolted back onto the engine and the linkage reconnected. On these old OMC engines, the twist-grip throttle is connected to the magneto, and advancing the twist grip advances the magneto timing. The linkage connecting the carb butterfly (throttle) valve needs to be synchronized to the magneto in order to obtain the best performance. On this engine, as on so many old OMc's, that is a simple matter of lining-up a mark on a sheet-metal "cam" screwed to the underside of the magneto, with the roller or follower on the linkage for the butterfly valve. One retaining hole in the sheet metal cam is slotted so that the cam may be moved in and out, and the cam is moved so that, when the butterfly roller is next to the mark, any slack or "looseness" in the linkage to the butterfly has been "taken up" and the butterfly shaft itself just begins to move. Making this adjustment takes all of 5 minutes and requires no tools other than a small wrench to fit the cam's retaining screws.

Now it was time to finally reinstall the magneto, a procedure made considerably more complicated by this early engine's tension-cable linkage to the twist grip. As previously mentioned, later engines use a shaft and bellcrank arrangement which is much easier to deal with. Other than getting the cable back onto it's sheaves, mounting the magneto involved no more than placing it down onto the crankshaft, being mindful of not allowing the "followers" on the new ignition points to "hang-up" on the cam which actuates them, and refastening the four screws which secure the magneto. The flywheel was set in place and lightly secured by it's nut, then the flywheel was slowly rotated by hand to insure that the coil "heels" did not make contact with the flywheel magnets (they did, and the flywheel was removed and the coils moved back away from the flywheel a merest thousand of an inch; see magnetos column for more details)

click to enlargeWith the flywheel again set on the crankshaft and the nut hand-tight, "flipping:" the flywheel by hand produced sparks from both sparkplugs. A magneto that sparks this readily makes engine starting less of a chore.

With about 3 1/2 hours expended on the engine today, the replacement of old fuel lines will wait until the next session, amd at that point the engine will be about ready for the initial starting attempts. Once the engine has (or has not) proven that it will run, I will then have a look at the recoil starter to see what, if anything, it needs.


Continue on to Part 3....