GY6 4t engine - everything you need to know about the Honda powertrain

Two versions can be found on the market: 50 and 150 cc engines. In the first case, the GY6 engine is designated QMB 139, and in the second, QMJ157. Find out more about the drive unit in our article!

Basic information about the motorcycle Honda 4T GY6

After its premiere in the 60s, Honda could not implement new design solutions for a long time. In the 80s, a completely new scheme was created, which turned out to be successful. It was a four-stroke single-chamber unit with air or oil cooling. It is also equipped with two top valves.

It had a horizontal orientation and was installed on many small motorcycles and scooters - the everyday means of transportation for Asians, such as Taiwan, China or the countries of the southeastern part of the continent. The project met with such interest that soon other companies began to produce units of a similar design, for example, the Kymco Pulsar CB125, which was a modification of the Honda KCW 125.

GY6 engine in QMB 139 and QMJ 158 versions - technical data

The smaller four-stroke unit uses an electric starter with a kickstand. A hemispherical combustion chamber was installed and the cylinder layout was done in SOHC format with a camshaft in the cylinder head. Bore 39 mm, stroke 41.4 mm. The total working volume was 49.5 cubic meters. cm at a compression ratio of 10.5:1.. He gave out power of 2.2 hp. at 8000 rpm. and the capacity of the oil tank was 8 liters.

The QMJ 158 variant also has an electric starter with stand. It is air-cooled and has a total displacement of 149.9cc. The maximum power is 7.5 hp. at 7500 rpm. with a cylinder bore of 57,4 mm, a piston stroke of 57,8 mm and a compression ratio of 8:8:1.

Drive design – the most important information

The GY6 uses air cooling as well as an overhead camshaft chain driven camshaft. The design also included a semi-cylindrical cross-flow cylinder head. Fuel metering was done by a single side-draft carburetor at a constant speed. This component was an imitation or 1:1 conversion of a Keihin CVK part.

A CDi capacitor ignition with a magnetic flywheel trigger was also used. Due to the fact that this element is located on the flywheel, and not on the camshaft, ignition occurs during the compression and exhaust stroke - this is a spark type of ignition.

Power and continuously variable transmission

The GY6 motor has a built-in magneto that supplies 50VAC to the CDi system as well as 20-30VAC rectified and regulated to 12VDC. Thanks to him, power was provided to accessories located in the chassis, such as lighting, as well as to charge the battery.

The centrifugally controlled CVT transmission is housed in an integrated swingarm. It uses a rubber strip and is sometimes also referred to as VDP. At the rear of the swingarm, a centrifugal clutch connects the transmission to a simple built-in reduction gear. The first of these elements also has an electric starter, rear brake equipment and a kick starter.

It is also worth mentioning that there is no clutch between the crankshaft and the variator - it is driven by a centrifugal type clutch located on the rear pulley. Similar solutions have been used, for example. in products such as Vespa Grande, Bravo and modified Honda Camino/Hobbit. 

GY6 engine tuning - ideas

As with most internal combustion engines, the GY6 variant can be made with many design changes to improve its performance. Thanks to this, the scooter or kart in which the drive is installed will be faster and more dynamic. However, it must be remembered that this requires special knowledge and experience in order not to adversely affect safety.

Exhaust flow increase

One of the most frequently performed modifications is to increase the flow of exhaust gases. This can be done by replacing the stock, standard mufflers with an upgraded version - these can be found in online stores. 

This will increase the performance of the engine - unfortunately, components installed at the manufacturer's factories limit the ability of the engine to get rid of exhaust gases at low throughput. Because of this, air circulation in the power unit is worse.

Head milling

Other ways to improve the performance of the power unit include increasing the compression ratio, which will positively affect the torque and power generated by the power unit. This can be done by milling the head by a specialist.

It works in such a way that the machined section will reduce the volume of the combustion chamber and increase the compression ratio. Be careful not to overdo it, as this can result in more compression, which can lead to interaction between the piston and engine valves.

GY6 is a popular device that offers a lot of possibilities.

 It will work both in standard use and as a motor for modifications. For this reason, the GY6 engine is very popular. Fits both scooters and karts. The car is an attractive price and the possibility of making improvements and high availability of the so-called. modification kits to increase the performance of the unit.