Building the SCHROEDER .09

Timer, Timing, and Running


These instructions were originally written by Roger Schroeder

There is good news and bad! The good news is that my Schroeder 9 runs great. It will turn an 8-4 Topflite plastic prop at 7,500 rpm, which is just about as well as my BB Arden .09. The bad news is that you still have to build the ignition timer assembly for your "9" before you can run yours. The end result will be well worth the effort, so let's get busy.

Timer Assembly

We really could have used a casting for the timer frame. However the method of construction outlined in the following paragraphs is satisfactory and the resulting timer works well.

Make the timer frame first. Note that the O.D. of the frame and the hole for the crankcase are not on the same centers. If you make the timer frame a snug fit on the crankcase, no locking device will be required to keep the timer in position while running. If the fit is loose the timer arm serves to lock the timer in position on the crankcase. After the correct spark advance is obtained, the timer arm is rotated clockwise and the end engages the crankcase to lock the timer in place.

Drill and tap the 4-40 threads in the timer frame for the spring post and the timer arm. The remaining 3-40 thread and the 3/32 diameter are to be located in assembly.

Fabricate the spring post, timer arm and fixed point support as shown on Plate 8. The 2-56 thread in the fixed point support is to be located later in assembly. Assemble the Super Cyclone fixed point and insulators into the fixed point support. The rod on the fixed point will screw into the insulator bushing and a lock nut is not needed on the top of the fixed point support.

Slip the timer frame on the crankcase and install the cam. Position the frame with the spring post up, as shown on Plate 1. Position the Super Cyke moving point on the timer frame so that the spring engages the spring post and the point arm is horizontal. Mark the position of the pivot pin on the frame, remove the frame and drill and tap the 3-48 thread at the position marked.

Check the tension on the timer spring. I found that mine had too much tension and caused a lot of drag on the cam. If the tension is excessive, bend the spring slightly until the tension is reduced.

To locate the fixed point assembly, reassemble the timer frame and moving point on the crankcase and install the cam so that the assembly looks as shown in Plate 1. Put a dab of epoxy on the back of the fixed point assembly and position as shown in Plate 1, leaving a point gap of .010" to .020". When the epoxy sets you can locate the approximate position of the 2-56 screw that secures the fixed point support to the frame.

The epoxy bond will usually break when drilling the pilot hole for the screw, but by then the hole is located. Tap the 2-56 threads in the fixed point support and drill the 3/32 dia clearance hole in the frame. It may be necessary to cut away some of the flange on the back of the frame to provide clearance for the head of the 2-56 screw. When attaching the fixed point assembly to the frame for the last time, I again used a dab of epoxy to keep the support from rotating around the screw.

This completes the ignition timer. Mine has been very reliable and the only adjustment required was the above mentioned reduction in spring tension.

Engine Timing

The cam must be positioned on the crankshaft and the induction hole drilled in the crankshaft to time the engine. A good cam position is where the points just open with the piston at the top of the stroke and the timer positioned on the crankcase as shown in Plate 1. A lot of trouble can be avoided if the 1/16" hole for the cam drive pin is drilled through the cam and crankshaft in one operation. The correct location for the hole is difficult to hit if the cam is drilled first.

A conservative induction timing will result if a 5/32" dia port is drilled in the crankshaft. The port is to be located so that the induction port just starts to open as the piston closes the exhaust ports. To accomplish this, assemble the engine and turn the crank in the direction of running until the piston just closes the exhaust port. Using a sharp scribe inserted into the intake venturi, scribe a circle on the crankshaft Now continue, turning the shaft in the direction of running until the edge of the scribed circle just disappears from view. Stop turning and scribe a second circle on the crankshaft. The center of this second circle is the center of the 5-32 dia. port in the crankshaft.


If you have fitted the individual parts of your engine as they were produced, no final fitting will be required to make your engine turn free. If your engine has a lot of friction, investigate the cause and correct it now. A tight engine will be very hard to start and may be damaged by forcing it to turn over.

From the experience of running my engine, I believe that the most promising method of increasing the power output is to increase the bypass area in the cylinder. The method of cutting the ports shown on Plate 2 is an easy way of providing the bypass, but the cross-section area is not large. If you wish to experiment with your engine, look at the bypass ports in the larger Cox engines and try to enlarge the bypass in the "9" accordingly.

Since the induction port timing in the "9"s crankshaft is conservative, the power output and top speed of the engine should respond favourably to enlarging the port in the crankshaft.