The control angles change the most when the additional stroke at the cylinder base is compensated. With a lift increase from 52 mm to 55 mm, i.e. 3 mm, 1.5 mm would have to be placed on the foot OR head. Therefore, no 3 mm compensation seal is used but only 1.5 mm as the piston moves 1.5 mm upwards AND downwards.

If 1.5 mm is now backed at the foot, the overcurrent time increases disproportionately to the outlet angle, due to the angular function of the crank drive. However, both angles, overcurrent and outlet angle, should always be in proportion to each other for a well-functioning flush. The outlet channel should always be opened for a certain period of time in front of the overflow channels to give the old gases enough time to escape and make room for the incoming fresh gas. If this process, called pre-release angle, is disrupted in its function due to an overcurrent angle that is too high (or an outlet angle in relation to a short outlet angle), power delivery in the upper speed range will be massively disrupted. Then the best exhaust system simply doesn't work anymore because it literally lacks the space to breathe. If the exhaust is also a real racing exhaust that only plays music in the upper engine speed range, this is usually accompanied by a pronounced lack of torque in the lower engine speed range. As a result, the engine would then be slower than an original engine, despite the multi-stroke and expensive exhaust, as the exhaust steals the torque from the bottom and the cylinder cannot deliver out at the top.

It is therefore always necessary to aim for a value of between approx. 23°-32° for the pre-release angle. The smaller the angle, the less flushing losses the cylinder has, but also correspondingly less power in the upper speed range. Conversely, a large pre-release angle is perfect to support powerful exhaust systems, but it has the risk of losing a lot of power in the lower engine speed range.

For starters, it is therefore usually more advisable to compensate for the additional stroke at the cylinder head in order to avoid excessive changes in the control angle.

If the aim is to balance the cylinder base, which is usually also easier to achieve with water-cooled motors, the outlet angle must often be adjusted mechanically, i.e. the outlet must be milled to achieve the desired pre-outlet angle.

However, depending on the type of cylinder, this requires skill, tools and know-how.

Measuring the control angles is just as essential as subsequently adjusting the ignition angle and the carburettor spray.

About the manufacturer BGM

BGM is a leading brand for high-quality spare parts and accessories for scooters, motorcycles and classic Vespa models that stands for quality, precision and innovation. With an uncompromising “Made in Germany” philosophy, BGM develops durable products in collaboration with leading engineers and under strict quality controls. Whether restoring vintage cars or tuning modern scooters — BGM is the first choice for discerning drivers. Thanks to first-class workmanship and a wide range of products, BGM offers ideal solutions for performance, everyday use and individuality.