Your 1973 Yamaha 125cc may surprise you with renewed engine performance after basic adjustments of the ignition points and carburetor. Include a new spark plug with the tune-up and be prepared for improved fuel efficiency and a bit more top speed. In the event you are unfamiliar with motorcycle systems, enlist the assistance of an experienced mechanic to walk you through the process the first time. Otherwise, park your Yamaha 125 cc on its kickstand and break out the metric tools.
Ignition Points and Spark Plug
Remove the spark plug wire cap from the plug. Remove the spark plug from the cylinder head using a spark plug socket and a ratchet.
Loosen and remove the bolts that hold the timing cover at the left side of the engine crankcase using a metric socket and ratchet. Set the bolts and cover aside. Look inside the timing case and locate the timing marks on the face of the timing rotor.
Insert a drinking straw partially into the spark plug hole on the cylinder head. Rotate the engine using the kick starter until you feel the piston contact the end of the straw. Continue to rotate the engine until the piston is at the uppermost position (top-dead-center) in the cylinder.
Notice the position of the timing marks on the timing rotor. If the marks line up, the piston is at the top of the combustion stroke, as it should be to adjust the points. If not, rotate the engine one revolution so the piston is at TDC (top-dead-center) and the timing marks line up.
Loosen the small adjustment screw at the base of the points plate in the timing case. Insert a 0.4 mm feeler gauge into the gap between the firing tips of the points. Insert the tip of a flat screwdriver into the adjustment slot on the edge of the plate. Twist the handle of the screwdriver to the right or left as needed to set the point gap. Tighten the adjustment screw.
Reattach the timing cover and tighten the bolts in a crisscross pattern. Do not over-tighten the bolts. Snug is fine. Install a new spark plug in the cylinder head and reattach the plug wire cap.
Throttle Cable and Carburetor Adjustments
Twist the throttle grip open slowly and note the amount of free-play before the throttle cable begins to engage, or tighten. If necessary, hold the cable just below the handlebars as you twist the grip and feel the cable as it engages.
Turn the throttle cable adjuster nut below the throttle grip clockwise or counterclockwise with a metric wrench as needed so the free-play is 1/16-inch for maximum throttle response.
Start the engine and allow it to warm up for five minutes. Locate the idle adjustment screw at the right side of the carburetor. Identify the screw by a slot-head and a spring on the stem of the screw.
Turn the idle adjustment screw to the right or left as you listen to the engine's speed. Turing it too far either way slows the engine's speed. Find the point where the engine idles at the fastest speed.
Note the engine rpm on the tachometer. Identify the larger idle stop screw above and behind the idle adjustment screw at the right side of the carburetor. Turn the idle stop screw to the right or left as needed until the tachometer reads 1,100 rpm.
Items you will need
- Yamaha 125cc repair manual
- Spark plug socket
- Metric sockets and ratchet
- Drinking straw
- Feeler gauge
- Metric wrenches
- Spark plug
- Refer to your repair manual for air filter information and install a new filter.
- Replace the ignition points if you notice burned or pitted spots on the firing tips.
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