Category:Performance Tuning

Required Reading

To familiarize yourself with two stroke engine tuning, read the Two Stroke Tuner's Handbook by Gordon Jennings as well as Two-Stroke Performance Tuning by A. Graham Bell.

Click here for a .rar file that includes both of these sources.

A slightly better scanned version of the Jennings Handbook may be found here.

Another link to Graham Bell's book here

Better, Faster, Stronger

You'll get noticeable power improvement on most mopeds by the following performance modifications, however, the system is tuned to work together overall, so changing one variable may affect other components and require further tuning. The most common example is the need to upjet when changing any part to one that allows the engine to consume more oxygen, such as a bigger exhaust, carb, cylinder kit, etc.

Upjetting is a technique used to achieve an appropriate mixture of fuel and air. The main technique for determining whether the jet is the correct size is the plug chop. However, with more complex carburetors (such as the Dellorto PHBG), this technique on its own will not suffice since it mainly gives information relating to the mixture at WOT while the mixture at other throttle openings can still vary widely.

Moped Performance Tuning Spreadsheet

The spreadsheet contains a variety of successful setups. All contributors post in the Moped Army forums, and their information will give you a good starting point to get maximum performance from whichever aftermarket part(s) you have purchased. Please feel free to contribute to the document.


High Flow Air Filter

A high flow air filter will increase the amount of air flow capable of flowing through your intake. Most of these filters are made out of foam or another similar material. You must upjet if you install a high flow filter in order to match your increased air with an increased amount of fuel. Metal mesh and fabric K&N style filters are available and also covers ordered to restrict rain or water from entering the intake system. You can design your own airbox to house the filter, although the Airbox Calculations are quite complicated. Upjetting is required with any modification.

Performance Exhaust

For most mopeds, an aftermarket exhaust is the best value for a performance mod.

Mopeds were designed to produce around 2hp or less. Replacing the stock muffler with one that has an expansion chamber improves the performance of the engine by scavenging mixture through the cylinder.

You should upjet your moped when you install a performance exhaust and then perform plug chops to make sure your moped is getting the right fuel:air mixture.

Bigger Carburetor

A bigger carburetor is able to supply more air-fuel mixture for larger kits and/or increased RPM. Oversized carbs need a intake manifold with a minimum ID of at least that of the ID of the exit of the carb. Controlling the fuel mixture can be done by changing the size of the jet.

You can also try over-boring your carburetor. The process isn't very hard and saves a lot of money.


By changing the front and/or rear sprocket you can redistribute speed to top or low end depending on your needs. Usually increasing top end with this method decreases low end, and vice versa.

Changing the size of the rear tire will have a similar effect. On most moped sized tires width and sidewall height are the same. Increasing your tire width 1/2" results in a 1" taller tire and a 3.14" gain in circumference. Therefore each 1/2" increase in tire size results in an increase of 3" per wheel revolution.

  • For more information concerning gearing and performance, please see the gearing article.

High-Compression Head

Cylinder heads can be obtained that increase the compression. In addition, aftermarket or custom cut heads have altered squish geometry to better match the piston used in a kit.

  • Want to raise the compression on your stock cylinder head? Read up about the technique in the head milling article.

Reed-valve intake

A reed valve intake has a lot more tuning potential than a piston-controlled intake. Read how to make your own on a Puch engine (and any other engine if you're handy).

Cylinder Porting

Cylinder porting is one of two arts - the white art of Port Matching, and the black art of altering the port size, which affects open duration and event timing.

Good resources on cylinder porting and tuning in general include Jenning's Two Stroke Tuner's Handbook, and Two Stroke Performance Tuning by A Graham Bell.


These kits replace your cylinder and piston with a larger bore. Generally, you will also need to re-jet your carburetor to allow a greater volume of fuel:air mixture to enter your engine. You will also need to re-jet to allow the carburetor to deliver enough mixture to sufficiently lubricate and cool the cylinder and piston. If you do not supply enough mixture, the piston and cylinder will overheat, typically resulting in seizure.

Installing a kit can be done in weekend, and it gives good performance when installed and broken-in properly.

Most kits recommend a 300 to 800 mile break-in period after installation to allow the rings on the piston to use the fine scratches (called honing) in the cylinder to gently polish into a smooth, well seated finish. Many theories and break-in techniques have surfaced over the years. You should do some research; Google it or search in the Performance Tuning forum and then decide for yourself which one to use. One method is listed here. Sources agree, though, that over-revving the engine and showing off your new kit to your friends by throttling all the way up is a sure way to destroy your new kit if it's not properly broken in.

Note that some kits can increase your engine displacement above 50CC, which may change your moped to a motorcycle due to your state's laws. Make sure to check with your DMV before you spend the money on a kit!

  • Never installed a kit or taken off cylinder before? Read Install a kit.

Performance Crankshaft

The main advantage to aftermarket crankshafts are the use of roller bearings for the wrist pin, which increases reliability at high RPM.

A performance crankshaft can influence the primary compression and transfer of fuel mixture into the cylinder. A crankshaft with full webs can increase the pressure inside the case by reducing the case volume. Sometimes there are holes in the webs, filled with aluminum or plastic to remove weight and change the balance of the crankshaft.

Some performance cranks have a different stroke which changes displacement. A common misconception is that a shorter or longer connecting rods would influence the length of the stroke. This is not true. The stroke is determined by where the shaft is connected to the crank, how far it is off center. The further off center, the longer the stroke. Shorter or longer con-rods, along with fabricated spacers, can be used to fit cylinders that are not meant to go onto a specific engine.


The stroke has a major part in how an engine performs. It's not easily changed (as it requires a different crankshaft) but some people have managed to machine cylinders to engines with a mis-matched stroke.


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Pages in category "Performance Tuning"

The following 106 pages are in this category, out of 106 total.