QMB139 Maintenance Procedures

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QMB139 Maintenance Procedures

Postby 4Strokes » Fri May 20, 2016 7:09 pm

Spark plug grounded
QMB139 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.

Parts draining oil

Oil screen drain plug

Oil change supplies

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.

Unscrew oil dipstick

Check oil level on dipstick

QMB139 Gearbox Oil Change
To avoid stripping the gearbox case threads, apply thread compound to bolts and just snug them up.

Gearbox fill and drain plugs

Drain plug thread compound


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.

Gearbox lube supplies

Gearbox oil fill

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

Recycle used oil and lube

Testing for Spark on the QMB139 Engine
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.

Spark plug access boot

Spark plug removal with socket

Reading the spark plug

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".

Brake lever rigged on

Spark plug grounded

Spark plug thread anti-seize

QMB139 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.

Coil, wire, plug boot and wire

Ignition coil terminals

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.

CDI wiring diagram

CDI wire connector

CDI, connector and wires

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.

Adjusting Valve Clearance on the QMB139 Engine
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.

Valve cover removal

Valve cover seal or o-ring

Head bolt check

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.

Timing chain and gear

Timing mark inspection hole

Timing marks on flywheel

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

Valve feeler gauge

Valve adjustment

QMB139 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.

Compression tester, plug end

Turning over engine with starter

Compression test reading

Testing QMB139 Engine 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.

Fuel in fuel filter

Vacuum port for gauge

Vacuum gauge reading

Credits: Backup of Forum post. Edited and published by 4Strokes.com.

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