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!
Carb Restrictor
Some of the imported Chinese Scooters, like those found on eBay, are restricted to 30 mph by the use of a restrictor screw or similar in the cap on top of the carburetor.
This is a simple form of restriction that prevents your chinese gy6 scooter clone from reaching full throttle.
How to Remove the Carburetor Cap Restrictor Screw
Applies to: Baotian, Sukida, Hoatian, Lifan, JMStar, Jinlun, Kymco, Tank, Quinqzi, PGO, Roketa, Strada and Taishan QingQi and many more...
De-Restricting the Carburetor
Almost all GY6 style 50cc 139QMB chinese scooters are fitted with a generic style carburetor. This is a slide type carb where the vacuum created in the inlet ducting pulls up on a diaphram which in turn lifts the Needle Jet and Plunger to allow more fuel and gas into the engine.
In an effort to comply with regional speed laws, some scheming motorcycle manufacturers fit a screw or similar obsticle to the cap on the carburetor to stop the diaphram from pulling the plunger and Needle Jet fully open.
Fortunately it is quite easy to see if your chinese scooter has such a restriction fitted to the carb. Just follow these simple steps.
- Remove the plastic panel under the seat to gain access to the carburetor. Undo the two screws that secure the carburetor cap to the carburetor.
- Carefully lift the cover from the carburetor, taking care not to loose the spring underneath. Turn the cap upside down and inspect the plastic spring retainer in the middle of the cap.
- The spring retainer should be about 2 cm long, if it has a screw fitted to the end that extends its length or it has a very long plastic retainer then that is a restrictor which will stop the scooter or motorbike from reaching full power. If so you will need to remove the screw or shorten the plastic retainer to remove the restrictor.
TIP: You can test if the plunger is fully opening. Refit the cap and remove the air inlet pipe. Stick your finger in the air inlet and push up on the plunger. Ideally it should stop flush with the top of the carburetor venturi.
Notes
Chinese scooter manufacturers use an assortment of different techniques to restrict the speed of their scooters, not just carb cap restriction! If your plunger lifts all the way up when tested with your finger then do not adjust the cap, if the plunger is allowed to lift too far it could lead to poor running of your GY6 139QMB 4 stroke scooter.
When tuning your 50cc motorbike, remember that the inlet port in the cylinder head is only about the size of your finger! Bigger carbs and performance airfilters are no use if the air is then restricted by the port!