Tools used in this video:
Compression Tester: https://scooter911.com/shop/langmoto...
Leak Down Tester: https://scooter911.com/shop/performan...

Parts shown in this video:
Spark Plug: https://scooter911.com/shop/ngkcr7hs...
Big Bore Kits:
https://scooter911.com/shop/blueline...
https://scooter911.com/shop/ncycylin...
https://scooter911.com/shop/ncycylin...
Engine oil: https://scooter911.com/shop/genuine4...

There can be many reasons your scooter wont start. There can be an issue with the carburetor, or maybe your ignition system is malfunctioning. But if your engine has a mechanical failure none of those factors will matter.

In this video a Wolf RX50 scooter with a 49cc 139QMB engine is examined to discover which internal component has failed resulting in low compression. The only way to know if your scooter has low compression is to use a compression tester. In general, good compression on a 139QMB is anywhere from 140180 psi. Low compression would be from 0110 psi. Typically an engine with less than 100 psi will not run at all. An engine with over 100 psi might run but it might exhibit symptoms such as stalling, hard to start, low top speed, smoking or poor fuel mileage.

To perform a compression test, remove the spark plug and thread the test hose of the compression tester into the spark plug hole. Then hold the throttle wide open and crank the engine. The needle will climb every time a compression stroke occurs. Take your reading when the needle stops climbing or after 1520 compression strokes.

If you find your scooter has low compression you can determine which internal engine component has failed by using a LeakDown tester. A leak down test will utilize compressed air to pressurize the engine's combustion chamber. In an ideal situation very little to non of the compressed air will leak out. If the valves do not seal tightly because they are bent or burned the compressed air will leak out. You will be able to determine which valve is not sealing by listening to the intake and exhaust systems. If the piston rings do not create a seal between the piston and the cylinder walls the compressed air will leak into the crank case. You can detect that air leak by pulling a plug such as the dip stick. If you can hear a rush of air you know the crank case is being pressurized.

In order to use the leakdown tester the piston must be brought to top dead center of the compression stroke. With the spark plug out, turn the engine over by placing a socket on the extruded plastic nut on the cooling fan. Using a small screwdriver or something similar feel for the piston to reach its highest point. The piston is now at top dead center but you do not know if it is the compression stroke or the exhaust stroke (where the exhaust valve will be open). Pressurize the combustion chamber with the leak down tester and check to see if air is escaping out of the exhaust. If it is rotate the engine 360 degrees forward to top dead center again and retest.

Timestamps:
00:00 Intro
00:52 What is a wolf RX50
02:18 The engine has low compression
04:07 Checking for spark
05:08 Testing the engine's compression
06:51 Using a leak down tester to determine the problem
13:45 Inspecting the engine after teardown