Re: need more power. 1954 johnson QD-15
Ryan - here are a few things you can do to make it a better motor for a pond rocket.
If rebuilding the powerhead is within your skill set, find yourself a set of pistons and rods out of a 1962 Johnson or Evinrude 10hp. It'll have heavier duty wrist pins with roller bearings to replace the bronze ones in your existing motor.
Find a flywheel off a 1958 or later 10hp. It's a bit smaller in diameter and a tad lighter. Will allow quicker acceleration and put less stress on the crankshaft. You're existing recoil won't work any longer, but you've removed it so that's not an issue.
Find an old style 2-blade bronze prop from Michigan. Might allow you to jack the motor up higher on the transom. The folks at Michigan Wheel might be able to point you in the right direction as far as prop number. Likely something in the AJC series.
Don't bother with the 15hp carb. Won't do anything but make the motor run like crap. No jets to change, either, as the carb is metered by the high and low speed needles.
Same with the pipes. They'll just make a lot of noise and annoy the neighbors. Throwing pipes on a motor without doing the other things to increase flow through the motor can actually make it run worse. Then you're getting into porting (an art form in itself) and crankcase volumes and flow rates, etc. All of which takes someone who REALLY knows what they're doing to make it work. And the little 10hp really isn't the best platform for a lot of machine work as it would very likely come unglued at anything approaching the revs of the Merc you saw.
You'll find there's more speed in getting your set-up optimized than there is in trying to modify the motor. Making sure the bottom is clean and straight, getting the motor as high as you can and still plane off and pump water, trying as many different props as you can get your hands on, playing with motor angles, getting the motor to run as good as possible by dialing in the carb correctly and setting the timing bang-on.
There's a lot of drag in those lower units so cleaning up the surface could get you a little speed. Might try thinning the skeg a little and sharpening the leading edge. You could also fill the gearcase cap screw recesses with some sort of compound that would be easily removable later when you need to service the unit. Maybe some kind of semi-hard putty or something that dissolves with a little heat?
Most speed comes from easy stuff. In the end, if you want to go faster, get a bigger motor. A 1953-54 Evinrude 15hp powerhead will bolt right up to your 10hp leg...
