Re: Getting the most from my 1957 35hp
There are a variety of props available for the motors, up to 14" in pitch. They are getting harder to find, though. Your best bet for matching rpm is simply to do so by sound, which is more accurate than you might think (out of sync motors will make a "rolling vibration" sound), or to use Tiny Tachs.
These are inductive tachs that work on the same principal as an inductive timing light, except that you don't need a 12V electrical source. Just wrap the red wire around a spark plug lead and ground the black wire and you have a digital tach with an hour meter.
If you order them, be sure to get units that are made for 2-stroke motors and have Tiny Tach assemble them with leads that are long enough to go from the dash to the motors. Tiny Tach claims that you can't lengthen the wires yourself, but I've actually done that. If I were to do it over again, though, I would probably spec the longer wires to begin with.
http://www.tinytach.com/
Frankly, I wouldn't spend the time fooling with carbs, special pistons, etc. These old motors were rated at the crank and you aren't even going to get 35hp out of them. Lots of power out of old motors like the Bigtwins, really isn't the point, anyway. I saw the pics of your boat in your other thread and believe that it will run at least in the high 30s with a pair of Bigtwins on it. I am basing this on having once had my '58s on a 15.5 foot Glassmaster trihull, which is probably heavier than your boat.
My advice is to compression test the motors and leave the powerheads alone, if you get good numbers. If you seem a little low, run a can of BRP engine tuner (removes carbon) through each and recheck. If the numbers that you get are even (+ or - 10%) and 100 psi to 120 psi, you've got pretty decent motors. If you are still a little low, my next step would be to replace the head gaskets.
I'm not saying not to crack the powerheads if you have a reason to, but short of that, you are making a lot of work for very little, if any gain. Put the time in carb rebuilds with stock parts, water pump rebuilds, new ignition coils, points, condensors wires, etc. If you want them to look really spiffy, paint them too.
BTW, your motors later evolved into the OMC 28 hp motors of the early 60s and the 33hp motors of the middle/late 60s. Because of this, there are many parts that are interchangable. For example, if you need a new starter and find one on EBay for a 28hp motor, it will fit.
One particular issue that I am aware of with the '57s, is that the clutch dogs are pretty hard to find. That said, you might not have '57 gearcases on them anyway. The older motors are pretty well known for haing a variety of parts on them, after years of fixing stuff to keep them going.
PS: The one mod that I would do to the motors, is to put fuel pumps on them. Its really not hard to do and saves you having to deal with pressure tanks. If you think you would want to do this, post the info here and I'll explain how.
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