Variator and rollers
The Multivar kit includes 16×13 mm / 7 g rollers and a white contra spring. This is the kit’s starting setting; choose a different weight based on engine behaviour.
MV21 selection guide · GY6 / 139QMB · 4T
Four strokes.
More possibilities.
From a 50 cm³ base to a 72 cm³ conversion. Start with verified Baotian 139QMB versions with a carburettor and two-valve head.
139QMB · air-cooled50 → 72 cm³CVT drivetrain
Build my setup ↘
GY6 / 139QMB50 cm³ is the starting point.
01
Engine code first
139QMB with a carburettor?
The GY6 name covers different engines. These kits are for an originally 50 cm³, two-valve 139QMB 4T engine. This selection excludes 125/150 cm³ GY6, fuel injection, 2T and SYM XS1P139QMB.
These parts may suit other brands with 139QMB, but we check the exact version and installation dimensions before confirming a kit. Send a photo of the engine marking.
02
Start with the goal
CVT tuning or more displacement?
A variator helps make use of the existing engine. More displacement requires a cylinder conversion and checks of the head, valve timing and fuel mixture.
Prices cover the selected products. Installation, tuning, oil and additional materials required for the engine are not included. We do not promise power or speed without measurements.
03
Your starting selection
GY6 kit, part by part.
A bigger 4T conversion needs a shared plan.
Head height, valve length, camshaft, chain, piston clearances and crankcase dimensions can vary. Send the engine code and current parts so we can choose a matched kit.
Discuss your project with MV21 ↗Before the 72 cm³ conversion
Before conversion, check engine condition, lubrication and the cooling fan. Retain all cooling shrouds. Follow the specific kit’s installation and running-in instructions.
Tuning conversions in this guide are intended for a closed track. Physical part compatibility is not approval of the vehicle conversion for road use.
04
Drivetrain and intake
Four components to consider together.
Belt length
139QMB comes with several crankcase lengths. 669, 729 and 788 mm belts are not interchangeable. Check the old belt’s code and the exact version’s catalogue, not just wheel size.
Clutch and bell
Check friction surfaces, bell size and shaft connection. The complete Naraku CVT kit is an alternative to Multivar; we do not put both variators in one kit.
Open sports filter
Measure the carburettor’s filter connection and clearance, arrange crankcase ventilation and adjust the fuel mixture. “Fits GY6” alone does not confirm diameter. In this guide, you can directly select a Naraku filter element for the existing airbox.
Naraku Racing CVT
KIT.A.139QMB.R
Variator, clutch and 107 mm bell. Choose the belt separately. Not added automatically because it is an alternative to the included Multivar.
…Checking availability…
View product ↗Matched to frame and head
The exhaust must match the head flange, frame mounts and scooter layout. Two-stroke expansion chamber selection does not apply to GY6.
The first kit retains a suitable existing exhaust. A specific sports exhaust is selected by model.
05
Electronics and spark
The CDI plug is not the whole answer.
Check AC/DC supply, pin layout, stator and ignition curve. A fuel-injection ECU is not a conventional carburetted-engine CDI. A larger cylinder alone does not require a random “sports” unit.
Naraku kit
KIT.E.44137
Coil and iridium spark plug kit for comparison. It is not a CDI; connections, mounts and head plug requirements must be checked separately. Not added automatically.
…Checking availability…
View product ↗NGK CR7HSA
CR7HSA 4549
Listed in the NGK catalogue for several Baotian 50 cm³ versions. M10×1 mm thread, 12.7 mm reach. Check your exact model and head, especially after modification.
…Checking availability…
View product ↗06
4T specifics
Head, valve timing and lubrication.
Must the carburettor be replaced with the 72 cm³ cylinder?
Naraku permits the original carburettor with this kit and recommends adjusting the main jet. Choose the size based on the fitted carburettor and tuning results. We do not offer a universal jet number for every 139QMB.
Is the head included in the cylinder kit?
This 72 cm³ kit does not include a new head. Check the condition and fitment of the existing two-valve head. A different head or camshaft requires separate checks of valve length, timing chain and piston clearances.
Should I add 2T oil to the petrol?
No. 139QMB is a four-stroke engine with separate engine oil in the crankcase. Do not mix 2T oil into the petrol. Final drive oil is also separate from engine oil; check types and quantities in the model’s manual.
What should be checked after installation?
Timing marks, chain tensioner, valve clearances, oil level, leaks and cooling shrouds. Then carburettor and CVT settings. Use the installed parts’ instructions for clearance values and tightening torques.
More guides to come
Next platforms and models.
Other Derbi D50B0 versionsGeared mopedsOther GY6 modelsBy engine code and fitmentGY6 intake conversionCarburettor, filter and specific connections
How was this selection checked?
Starter kits were checked in the store finder against Baotian BT49QT-9 Sprint, BT49QT-12A1 Rebel and BT49QT-7 Smart Rider 139QMB carburettor versions. Prices and availability are loaded from the store.
