Index
Overview
Tuning
Tuning Overview
Lighter Roller Weights
Remove Variator Restrictor
Stiffer Clutch Springs
Move Needle Jet Washer
Increase Fuel Jet Size
Carb De-restriction
Un-Restricted CDI Unit
Un-Restricted Air Inlet
Trouble Shooting
Compression Test
Fuel Enricher Test
Seized Brake Caliper
General Maintenance
Replace Air Filter
Check & Change Oil
Spark Plug Change
Idle Speed Adjust
Change Brake Pads
Adjust Valve Lash
Electrical
Testing the Battery & Charging System
Battery Change
Coil Change
HT Lead Change
Transmission
Remove Starter Motor
Refurb Bendix Gear
Engine
Camshaft Removal
Valve Removal
Replace Valve Guides
Engine Removal
Return to Transport Listings
Models Covered
Lexmoto, Pulse, Sukida, Baotion, Lifan, Direct Bikes, Huatian, JMStar, QT50, LF50QT, YH50QT,
BT49qt, 139 QMB, Honda, GY6, Motorbike, Taishan, Jonway, Peugeot V-Clic, QT-4, QT-7, QT-9, qt-11, QT-50
Kymco, Tank, Quinqzi, PGO, Roketa, Strada, Vento, Yamati, QMB139, SunL, SYM, QingQi, Volano Plus Many More Chinese Motorcycles, mopeds and
scooters!
Cylinder Compression Check
As with all combustion engines, including those manufactured in China and Taiwan such as the Baotian QMB139, Tank, Kymco and Sukida brands, require a certain amount of cylinder compression in order for the spark to ignite the spark and cause combustion. Worn piston rings on your 4 stroke scooter or worse still, a worn big end bearing can cause low compression on your moped, as can leaking valves and Head Gasket. Follow the instructions on this page to check your cylinder compression and diagnose your motorcycles starting problems.
How to Check the Compression on your Scooter
Applies to: Baotian, Sukida, Hoatian, Lifan, JMStar, Jinlun, Kymco, Tank, Quinqzi, PGO, Roketa, Strada and Taishan QingQi and many more...
Testing Cylinder Compression
One common reason for poor running or non-starting on imported 50cc Chinese scooters is poor compression, normally caused by poor valve adjustment, worn piston rings or very worn Big End bearings. A typical complaint will be, 'my scooter was running fine, I rode it home from work and the next day it wouldn't start!'. Typically the wear is caused by poor oil, low oil levels, over tuning and finally, poor quality parts. To test the compression you will require a Compression Tester that screws into the spark plug hole on your scooter. NOTE: Not all compression testers give the same reading, depending upon the quality of the tester, the length of the hose and whether the seals leak.
Because of this, compression testers are often better used for comparison between a working engine and a none working scooter engine.
TIP: On 50cc QMB139 type engines, the piston and cylinder are so small that the volume of air in the compression testers hose is greater than the volume in the cylinder when the piston is at the top of its stroke! This will lead to quite low compression readings in many cases, giving the impression that there is a compression problem when there is not!
- Remove the spark plug from the engine and screw in the compression tester, using the correct adapter.
- Leave the spark plug attached to the HT Lead and rest it on a metal surface. This is to stop the coil being damaged.
- Now, turn the scooter engine over a few times, either using the electric start or the kick start. You should get a reading on the compression tester.
Note: Many manuals will give a suggested compression over over 100psi which would be correct. However, as stated earlier you may well get a much lower reading depending upon the hose length on your compression tester as the cylinder is compressing the air in the hose as well as the air in the cylinder! In the picture below you can see that the tester gives a reading of about 65psi, but this was on a fully working scooter!
- If the compression reading compares well to a working scooter then there is no problem. If it is very low then first check the valve clearance to make sure the valves are both fully closing.
Notes
Poor valve clearances are one of the most common reasons for poor running on chinese import scooter clones, particularily if the mileage is quite low. It seems that the motorbikes valves take a few hundred miles to bed in, then they need to be re-adjusted.
If replacing the piston rings, it is advisable to check the cylinder barrel on your QMB139 Scooter for wear, scuffs or worse. If the cylinder barrel looks ok, give it a quick hone then reassemble the scooter with new piston rings.