Basic 2 Stroke Top & Bottom End Testing

A lot of riders are always asking how they can determine the condition of their 2-stroke engine and or help troubleshoot a current running issue. So I decided to put together a quick little article that addresses the basics around performing a compression test and crank case pressure testing. Enjoy and I hope this information helps with the basics to get you started and headed in the right direction.

Compression

Compression testers can be a very useful tool to help determine the condition of an engine. Most are readily available at auto parts stores. You’ll want the kind that threads into the spark plug hole.

Performing a compression test is very simple. Start by starting the engine and let it come up to operating temperature, once up to temperature shut engine off. Remove spark plug and thread in the compression tester adapter, hold the throttle wide open with the engine stop button pressed. Then kick (crank engine over elec start models) the engine over several times until the gauge needle peaks. Which ever value the needle peaks or stops on is your compression reading. It’s best to perform a compression test once you have rebuilt/serviced the top end and write it down. Check compression a few times through out the season. Once the compression changes 20 percent it’s time to check the condition of the top end. Inspect piston rings, piston, bore, and power valves for wear or carbon build up.

Crankcase Pressure Testing

With 2-stroke engines the crankcase is sealed off from the engines transmission. The crankcase halves are sealed with sealer instead of a gasket. There are two crankshaft seals and one is on the dry side (magneto) and the other is on the wet side (clutch side). These seals wear with engine hrs and can allow the engine to run lean or draw in transmission oil. A lean condition can cause overheating and can lead to piston failure. Burning transmission oil will wet foul spark plugs.
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Crankcase pressure testers consist of a hand held pressure pump, pressure gauge, and block off rubber plugs. The picture is showing you a Motion Pro tool. You can make your own once you understand the process and tool function. Before you start make sure the bike has been properly cleaned of any dirt from your last ride. You’ll need to remove the exhaust pipe from the cylinder and install a block off plug into the cylinder. The carburetor will need to be removed and a cylinder shaped (close to the same O.D of the carburetor) piece install in its place. This part will have a nipple on the end of it to connect a hose and pressure pump to. Make sure the piston is at BDC so the crankcase can be linked to the cylinder. Pull about 5 inhg of vacuum on the system and see if it holds vacuum pressure. Also apply 5 psi of pressure on the system and see if it holds. In most cases the engine should be able to hold vacuum and pressure for a few minutes without leaking down. If pressure leak is detected spray soapy water around all mating seams of the engine and check for bubbles. If needed remove the magneto cover and flywheel, clutch/crankcase cover, so you can apply the soapy water mix to the crankshaft seals, again checking for bubbles.
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Basic home made set up for pressure testing.
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