Inventive collector designs and builds his own low voltage, homemade igniter for his Maynard kerosene gas engine
Picture 1: The housing that resulted from a lot of cutting, welding, grinding and drilling.
In the summer of '88 I bought a 5 HP Maynard kerosene gasoline engine to be restored during the winter months. This engine was stuck, cracked, rusty and missing the carburetor and igniter. I decided to design and build a low voltage homemade igniter to use until I could find an authentic igniter. The first step was to purchase some Mica washers, Mica tubes and Tungsten igniter points from one of the GEM engine parts advertisers. Next came some studying of Fairbanks-Morse and Stover igniter specifications to obtain low voltage igniter construction ideas.
Pictures 1 and 2 show the housing that resulted from a lot of cutting, welding, grinding and drilling. The length of the housing was chosen to place the points approximately flush with the side of the cylinder bore. This length also serves to fill most of the entrance tunnel void, thereby keeping the compression ratio high.
The stationary point was brazed to a small steel piece that was welded to the head of a 1/4 x 28 machine bolt, as shown in picture 3. The other point was brazed to a 1/4" diameter rod that was bent 90° to form the movable arm and shaft, shown in picture 4. Picture 5 shows the bolt and shaft in their respective igniter positions. Note that a 1/4 x 20 hex nut was brazed to the shaft to form a bearing surface and gas seal at the point where the shaft enters the housing block.
Picture 6 shows the spring that holds the points open and the stop pin that determines the distance between the points when fully open. This distance should be no more than 1/16", otherwise there may be a problem obtaining sufficient dwell. Some igniter designs put the stop pin on the inside near the points, but I like to keep the number of components in the combustion area to a minimum so I located the stop pin on the outside.
Now we come to the more complicated part of the design. The length of the igniter trip finger is important to achieve proper igniter operation. If it is too long, the side bar may reach the end of its travel without the igniter reaching the trip point, or the square igniter push arm may not drop down past the trip finger during the igniter intake stroke reset operation. Conversely, if the trip finger is too short, the achievement of proper spark advance/retard range may be difficult. The cam on the half speed gear will impart a fixed amount of the side bar motion for each cycle of the engine. I measured the side bar motion with respect to the crankshaft rotation and plotted the travel distance vs. rotation on a graph. It isn't necessary to make a timing graph to build an igniter but for me it helped put various details in perspective. Side bar (and push arm) travel was very close to 5/8" total before the cam reached the power stroke plateau. In that 5/8" the following three events must occur:
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