Bicycle motor tuning
This is a portal to everything you ever wanted to know about tuning 2 stroke bicycle engines.
We follow scientific principles used to tune racing 2 stroke engines.
Every section of this page is relevant to tuning 2 stroke engines of all kinds, although we focus on tuning 80cc bicycle motors.
Tuning a 2 stroke engine can be a simple process. Fit a compressed head, expansion chamber and reed valve. Raise the exhaust and transfer ports by a couple of hundredths of an inch. Cut a 1 inch piston window for the inlet and you have a rocket.
In general, this is all that’s required. Then you race your friends and find that although your powered bicycle is fast, theirs are faster. The following sections of this article explain in more depth how to finesse your bicycle engine to find that extra power.
Expansion chambers form the cornerstone of tuning 2 stroke engines.
- Why you should use expansion chambers
- How much power will and expansion chamber add
- The best expansion chambers for your engine
- Expansion chamber design and
- Expansion chamber formula
Sports carburettors have many benefits over NT carburettors.
We break down the components of sports carburettors and explain why you can expect more from the improved design.
high performance cylinder heads are the easiest way to add power to a 4 stroke engine. On 2 strokes, it isn’t quite that straight forward.
We test the performance of a variety of cylinder heads available for bicycle motors and detail how they can increase performance.
Reed valves can make a huge difference in performance. They allow us to tune the engine differently to create more power. We step you through how to fit a reed valve and how to change the engine porting to make use of the valve.
We explain the problems that are not obvious if you have not fitted NOS to your bicycle engine before. There are many common issues that you are likely to encounter with NOS and we explain the actions you can take to prevent them.
You can change the ignition timing simply and cheaply. We detail how to adjust ignition timing. We also look at power ignition options and the benefits that they bring.
bicycle engine fundamentals
Bicycle engines are the most basic of engines. The fundamentals of the 2 stroke engine have changed little since it’s first design by a Scottish engineer in 1878 (Dugald Clerk).
Modern construction techniques, and tolerances, are much improved. Engine porting has become more efficient. Bearings and lubrication have increased reliability. The basics of the engine, however, are still the same as they were almost 150 years ago.
The principles of tuning 2 stroke engines are identical whether you are tuning a race bred Suzuki RG500 gamma or a bicycle engine. The over riding principle of 2 stroke tuning is that all components have to balance.
4 stroke engine, components are typically tuned in isolation from each other. In other words, improvements in one component results in a benefit to performance. In a 2 stroke engine, improving one component can have a detrimental effect on the performance of the engine.
We can be frustrated when we purchase a new power part and, after fitting, find that there is no power improvement. Often, there can be a reduction in power when fitting a performance part.
The tuning principle here is that we have to balance carburation, port timing, ignition timing, exhaust and inlet each time we change a component.
The next principle that differs from 4 stroke engines is fuel : air mixture. This is very different on 2 stroke engines. Bicycle engines will run faster with lean fuel : air ratio. By running a very lean mixture you will, typically, improve the performance. There is, however, a trade off. The leaner you run the carburation, the hotter the engine will run. This is why fast 2 stroke engines have a tendency to blow pistons. The engines seem to be running great but they soon overheat and melt pistons.
The third principle is to know the limitation of your engine. An issue with tuning a low cost bicycle engine is that it can become more costly to tune than purchasing a purpose built engine.
The limiting factor of bicycle engines is the engine crank. In most bicycle engines, the limit is around 4,000 RPM. You will find that most engines purchased from Amazon or eBay are tuned to max out at these revs. If you try tuning your engine for higher RPM vibration will be severe and often destroy the motor.
Better motors have “single piece” cranks which will allow the engine to rev to around 6,000 RPM. This is not to say that tuning is pointless. Tuning emphasis is usually different on bicycle engines. We are not looking to obtain the 9,000 RPM of a thoroughbred motocross machine. In bicycle engines we are more often looking to maximize torque in the 4,000 to 6,000 RPM range. We then gear the bicycle to obtain the speed and acceleration that best suits our needs.
With the above principles out of the way, we can then take a logical approach to tuning. The important decision to be taken next is the amount of money we want to spend. Big performance benefits can be obtained reasonably cheaply from a basic engine. The price for every fraction of BHP grows exponentially once some basics have been completed.
Assuming that you have a reasonable budget. A fair price for some performance in my view would be around $250. This then would double the price of a basic bicycle engine.
This budget would allow you to fit all the best performance components for these engines. If you have limits of spending then I would strongly recommend the following components in order of purchase:
- Expansion chamber
- Sports carburetor
- Speed sprocket
- Reed valve
- Performance cylinder head
- Power ignition
- Bell air intake
- NOS (with some reservations)
There are a lot of assumptions with the above list. For 90% of bikers who want some extra performance the order of the list is good.
Note here that many of the components require additional work to help them perform properly. Reed valves for instance do little unless the piston or barrels are ported correctly. Expansions and cylinder heads do little unless carburetion is adjusted.
You will note also that I am reserved about the use of Nitrous Oxide Systems (NOS). If fitted correctly, they can provide a great burst of power for a few seconds. They do, however, stress the engine. A little is great for a much needed boost up that steep hill but I would not recommend frequent use.
note also that the power burst from NOS systems typically only lasts for up to 3 seconds. If you hold the NOS trigger for longer, the engine starts to miss-fire and choke.
Engine tuning is great fun. For just a few dollars and a little bit of time and knowledge you can have a bike that can reach 50 MPH or more.
An added advantage of performance tuning is that increased power can also reduce fuel consumption.
Since the 1990’s, consumers have had little choice of combustion engine other than 4-stroke engines.
2-stroke bicycle engines have, however, made a resurgence as a popular, low cost bicycle engine option. We analyse the differences between 2-stroke gas bicycle engine kits and their 4-stroke counterparts.
If you are considering a gas bicycle engine, this article will highlight some key differences that you may not have known about.
The worrying speculation about climate change catastrophe has been driving the uptake of e-bikes around the world. Suddenly, COVID 19 is escalating demand for clean air. We link to this interesting article about how COVID is pulling e-bike adoption forward.