Two-Stroke Top End Rebuilding

By Eric Gorr

Thoroughly wash your bike because dirt stuck to the underside of the top frame tube could break loose when servicing and fall into the engine! Use a stiff plastic brush and hot soapy water to clean off the grit and grime around the base of the cylinder, on the carburetor and intake boot, and especially underneath the top frame rail. Degreaser can be used on metal surfaces, but take care not to leave it on rubber or gasket surfaces.

You'll need at least some 3/8-inch-drive metric sockets and box wrenches (open-end wrenches will round off the edges on the cylinder or head nuts, and shouldn't be used for top-end rebuilding), a needle-nose pliers for removing circlips, and a gasket tool to scrape the old gaskets away. For soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad, a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of soapy water. Regarding measuring tools, you'll need a compression tester, a feeler gauge, and a digital vernier caliper.

Compression Testing

A compression tester is a useful diagnostic tool, and readily available from Sears or auto parts stores. Buy the threaded type and make sure the kit comes with an adapter that matches the spark plug threads of your engine. Performing a compression test is simple. Start by removing the spark plug, thread in the adapter, and hold the throttle wide open and the kill button on. This will prevent any spark and enable the engine to draw in maximum airflow. Then kick-start the engine several times until the needle on the pressure gauge peaks. The pressure reading depends on two main factors; the compression ratio and the altitude at which the engine is being tested. The compression ratio will also depend on if the engine is equipped with exhaust valves and their condition. When the exhaust valves are in the closed position the compression ratio will be greater than if the valves are carbon-seized in the open position. The difference may yield a pressure reading 25 psi. The quality of compression testers varies greatly. The main thing that a compression tester can identify is a change in condition. Whenever you rebuild the top end, take a compression pressure reading and mark it down. When the pressure changes 20% check the condition of the piston and rings. Pistons usually last twice as long as rings.

Crankcase Pressure Testing

The crankcase of a two-stroke engine is sealed from the tranny. It's important that the two crankshaft seals be in optimum condition. One side of the crankshaft uses a dry seal and the other a wet seal. The dry seal runs on the magneto side and the wet seal runs in oil on the tranny side. When the dry seal wears, the crankcase sucks in hot air, causing the mixture to run lean and overheat the engine. When the wet seal wears, the crankcase sucks in tranny oil, causing the engine run rich and eventually wet-foul the spark plug.

A crankcase pressure test involves the use of a vacuum pump with spark plug adapter, and rubber plugs to block off the intake and exhaust manifolds of the cylinder. The piston must be positioned at BDC to allow the transfer ports to be wide open linking the bore and the crankcase. The hand-pump produces vacuum pressure up to a standard setting of 5 psi. The normal bleed-down pressure loss is 1 psi per minute. Cylinders with complicated exhaust valve systems can be difficult to block-off air leaks, and harder to test. Crankcase pressure testing kits are available from Motion Pro.

If I suspect that an engine has an air leak in the crankcases, I do a visual test. Start by power washing the engine clean. Then remove the magneto cover. Spray the magneto clean with an aerosol can of brake cleaner. Make sure to use a non-chlorinated type of cleaner (green colored can). Now spray baby powder to all the suspect areas of the engine. Spray the powder on the crankcase around the magneto, at the crankcase seam line, the cylinder base, and the reed valve. Run the engine for a while, the white baby powder will highlight any fluid or air leaks on the engine. The baby powder test is much better than the alternative test of blowing raw propane gas at different areas of a running engine and listening for a change in the idle rpm. That is dangerous because it involves flammable gas and a hot engine with random electrical shorts.