Chinese QMB 139 Service & Maintenance Procedures on 4Strokes.com

4Strokes.com Technical: Chinese QMB 139 Engine Service & Maintenance

QMB 139 Engine Service
Technical Article Sponsor

Engine Oil Change

A kitchen strainer or sifter from Wal-Mart or a thrift store is useful for catching the engine oil drain plug and screen assembly. Change the oil several times during the first 1,000 miles or 1,609 kilometers to flush out any suspended metal particles. Wal-Mart SuperTech 15W-40 oil or similar works fine for the crankcase.

While positioned on the center stand, warm the engine up. The oil level on the dipstick should be around midpoint on the cross-hatched tip when placed against the case threads, not screwed in.

Gearbox Oil Change

To avoid stripping the gearbox case threads, apply thread compound to bolts and just snug them up.

Using a squeeze bottle (Wal-Mart kitchen gadgets, gear oil pump for marine engine lower unit, etc.) is one method of refilling the gearbox until oil runs out of the hole. Wal-Mart's SuperTech 80W-90 gear oil works fine.

A rinsed and dried 2-liter soda bottle works well for collecting used oil for recycling.

Testing for Spark

Pull the spark plug access boot. With a 5/8" deep socket, extension, and perhaps a universal joint, unscrew the spark plug. Read the spark plug electrode color: Black indicates a too rich fuel mixture and very light beige to white usually indicates a too lean of fuel mixture.

To free up one hand, bungee cord the left brake handle or lever in order to enable the electric starter interlock. Ground the spark plug against the engine case and energize the starter to observe spark. Ensure the Emergency-Off switch is in the Run position. Apply anti-seize to the old or new spark plug threads when reinstalling. Consider an NGK C7HSA (or resistor version) spark plug gapped at 0.028".

No Spark Troubleshooting

Verify the spark plug wire ends are securely twisted into the screw studs in both the spark plug boot and coil. Check if the coil wire connectors are loose.

The first thing you should try is to disconnect the black/white wire from the CDI connector and then check to see if you get a spark. This test will indicate if the problem is with ignition components or in the scooter's wiring. With the black/white disconnected and you still do not get a spark, the problem is within the ignition system component. If you do get a spark, then the problem is with the ignition switch, kill switch, side-stand switch, or the wires going to those switches. A miniature flat-tipped screwdriver can be inserted into the plastic connector housing to bend down the tang for temporary ground wire removal.

The lesson I learned from all of this is that you can't determine anything by measuring voltages out of the stator with the CDI hooked up. Also, I've tested a stator before that ohm'ed out fine but after it ran for a while it lost spark because of low quality windings. Replacing the QMB 139 stator requires removing the flywheel with a 24mm x 1.0mm right thread puller or cross puller.

QMB 139 Engine Service
Technical Article Sponsor

Adjusting Valve Lash / Clearance

On a cold engine, remove the spark plug and valve cover. Apply silicone, ArmorAll or somthing similar to the valve cover gasket to keep it supple. Check to make sure the head bolts are tight.

Use the kick-start lever to rotate the engine and align the cam sprocket holes with the top surface of the head. Remove the right side fan cover inspection plug and verify that the flywheel "T" mark is aligned with the case protrusion.

Service manual specifications state 0.002" for valve clearance or lash.

Cylinder Compression Test

Verify compression with a test set that includes a 10mm adapter. The engine should be warm before testing compression. The removed spark plug must be grounded or the Emergency-Off switch must be set to the Off position to avoid ignition coil damage. Twist the throttle to full open to allow air into the cylinder then energizing the starter for a few seconds, and read the compression gauge.

Testing Vacuum

Verify that fuel is visible in the inline fuel filter. Connect the vacuum gauge hose to the intake manifold vacuum port. Readings will be highest at idle.

Credits: Backup of a deleted forum post. Edited by 4Strokes.com.

4Strokes.com Technical: All Other Models