SPRE Follow-Up – 65 Tiger Cub Motorcycle

Introduction

In the weeks following the end of FRS 106, I (Diego Fierros) came to the E-Quad to work with Professor Littman on the History of Engineering project as part of the Summer Practical Research Experience (SPRE). As part of my work this summer, I took a look back at the ’65 Tiger Cub motorcycle to fix a couple of lingering issues. These problems were:

Blue-gray smoke in exhaust, suggesting oil leakage into the engine.
A dim, flickering tail lamp, suggesting inadequate power supply to the electrical system

It was my objective to identify the source of these issues and to address them.

Oil Leakage Issue

In the investigation of the grey exhaust smoke, shopmaster Glen Northey and I took a look at several parts of the Tiger Cub’s engine. First, we removed the carburetor and examined the “jets” inside. These small valves control how much air is mixed with fuel when the engine is running, and we theorized that a damaged jet could be allowing the engine pressure to drop, which in turn was allowing oil to seep into the engine. A Tiger Cub carburetor is shown below:

Our carburetor turned out to be functioning normally, so we turned to the engine’s top end, shown on a tiger cub engine below:

We removed the engine “head” (the top) and inspected the pistons. Sure enough, there was a small gap between the piston rings and the piston shaft. This gap was allowing small amounts of oil to leak up from below the piston into the ignition chamber, burning it along with the gasoline. Having identified the cause of the problem, it was decided that the piston head would need to be replaced with a larger one, and the piston shaft would need to be bored out slightly to allow for this new piston to move. We removed the entire top end of the engine and mounted it on a lathe, as shown below: