Factory Parts/Service/Sales Bulletins
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Transcript of a Chris-Craft
undated Service Bulletin attached to Sales
Submitted by John Kwast
INCREASING THE HORSEPOWER
OUTPUT OF THE 10 H.P. MODEL K
CHRIS-CRAFT OUTBOARD MOTOR
It must be well understood before an individual embarks upon the following procedures that his warranty is no longer good and that he makes these changes at his own risk and expense.
Following is a list of modifications, which may be done in a home workshop or at any machine shop, that have been found by users of this motor to increase its power and to improve its racing characteristics:
INTAKE MANIFOLD -
a. Bore or drill open the inlet opening adjacent to the carburetor to one-inch diameter.
b. Remove, by rotary or hand file, stock from inside the manifold along a straight line from the ends of the outlet ports to the one-inch diameter inlet. Open well and blend contours. Then, polish smooth with fine emery cloth.
c. Opening the length of reed opening toward the reed fasteners by as much as 1/4-inch has been claimed to improve performance. However, the reeds are more apt to break with the lessened support.
CYLINDER HEAD -
a. A new, special high compression head which produces a gain in power at higher engine speeds has been developed. This special head is interchangeable with the head now on your Model K and it may be purchased through your local dealer at a price of $12.00, plus postage and taxes, if any.
b. If the proper care is exercised to establish the clearance between the cylinder head and the piston dome, this new cylinder head may be milled off .025 to .040 inches, increasing the compression pressures further.
SPARK PLUGS -
When the power is increased in an engine, a need for a colder spark plug is generally encountered. J-6-J or K-3 Champion spark plugs seem to do the best job. The K-3 plug is more subject to fouling, and so the J-6-J should be used if possible.
The contact points of the magneto should be accurately set and should contact alternately as closely as possible to 180 degree crankshaft travel.
Gains in performance have been claimed for opening the venturi to the throat size of the carburetor. This is accomplished by removing all the jets, shafts, and butterflies from the carburetor body, by locating off the throat (or the engine side of the carburetor), and boring the venturi to the same size as the throat. The resulting edge left by the boring is smoothed with emery cloth.
Running the exhaust gases into air or water should be checked by performance tests made to see where the gain lies.
a. The water line on the motor should be run at or close to the anti-cavitation plate of the gearcase.
b. The gearcase may be streamlined to effect better underwater performance by filing careful amounts off the leading edge and off the sides.
This item is the most important single item for obtaining speed. Whereas it does not produce power, the extent to which it converts power to speed is the measure of the propeller's worth.
Michigan Wheel Company propellers are the most widely distributed and, therefore, easiest to procure. Their recommendation of propellers for this motor are their AJ 430 and AJ 431 - the latter propeller having slightly more pitch than the former. The AJ 430, over all, has proved the better of the two Michigan Propellers.
A very excellent propeller is marketed by the Johnson Wheel Company of California. But, its range of distribution is very limited making it hard to procure.
CHRIS-CRAFT OUTBOARD MOTORS