Category:Performance Tuning

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Revision as of 11:41, 20 August 2010 by Marozie (talk | contribs) (→‎Kits)
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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.

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 following link is a spreadsheet of 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 at any time. It is advisable that you use the formating already established to maintain document consistency.

Performance Tuning Spreadsheet

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. Upjetting is required with any modification.

Performance Exhaust

For most mopeds, this will be the best performance improvement, and the easiest for the average person to do in an afternoon.

Many mopeds of 1970-1985 vintage had exhausts tuned to deliver the most power at approx 25mph. As the engine goes faster, its cycle rate moves out of the power band of the exhaust, and the exhaust begins to restrict the amount of fuel available for combustion, which self-limits the top speed.

However, if you replace the stock muffler with one with a power band that is higher, your top-end speed will increase, but at a cost of the low end power. For most moped riders under 200lbs, this really isn't an issue.

For example, on a Honda Hobbit, you can get up to an 8mph improvement, just by swapping out the stock muffler with a Proma Circuit exhaust.

Of course, every model of bike is different, and you'll need to use the forum search function on the 'Performance Tuning' forum to find what people recommend for your model. 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

Installing a bigger carburetor will increase the amount of fuel leading to more power. Oversized carbs need a intake manifold with a minimum ID of at least that of the ID of the exit of the carb. The hard part is getting the air to gas ratio correct so that you don't cause a seizure or foul plugs. Controlling the gas flow can be done by changing the size of the jet. Controlling the air flow can be through the use of special air filters or by drilling holes in your air box.

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.

  • 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 the combustion chamber leading to faster strokes leading to increased speeds.

  • 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 Port Reshaping.

  • See Smitty's full article, as transcribed from the forums, for a guide to cylinder porting from the guy who does it for a living.
  • For information about calculating timings, see Calculating timing
  • You can use this degree tape to help you figure out your port timing if you have a Puch or Tomos-sized flywheel.


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 Crank

Installing a performance crank alone does not increase your speed. A performance crank can, however, influence the carter flow and the combustion chamber size.

A performance crank with full "cheeks" can improve the carter flow. Sometimes there are holes in the cheeks, filled out with nylon or cork. This is done to reduce the weight of the crank, increasing gas reaction but reducing low end torque. This is mostly done on high-rpm machines, since they don't need a lot of low end torque.

Some performance cranks have a different stroke that allows for greater or less displacement. A common misconception is that a shorter or longer shaft 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 shafts are used to fit cylinders that are not meant to go onto a specific engine. Although it does not fix the timing-problems, it does fix the piston coming above or too much below the cylinder.


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.

Really Fast, But Not For Long

Nitrous Injection

Makes you go real fast for up to 5 seconds.

The problem? You run really lean for those five seconds, and every time you use the nitrous, you have a good chance of causing your engine seize from lack of lubrication. Nitrous is easy to get in the form of whip cream cans and best to inhale it fresh. gigggles....

side note: N2O contains 33% oxygen, only about 11% more oxygen than air which has about 22%. Running a dry shot after the carb leans the fuel mix in proportion to the amount of nitrous injected this is why manifold injection needs to always be a wet shot or very very small. Running nitrous before the carb will allow it to take some fuel along resulting in a consistent 25% leaning effect when comprising of 50% of the total incoming cfm, though the longer and more nitrous used the more the cooling effect may cause a slowing in fuel travel through the carburetor's fuel circuit. (example: If you inject nitrous at 1/10 the volume of the incoming air you will only be 5% lean.) Leaning may be good though; 2 strokes generally make more power at HIGH rpm's when lean and also this can be used to counteract a carburetor's nature of over richening as rpm's rise/vacuum rises for a given throttle position, generally noticeable at max rpm and WOT. Though the MAJOR ADVANTAGE of nitrous isn't the added oxygen but the cooling effect of being a liquid when compressed and quickly becoming a vapor when released, in a wet shot to the intake manifold this causes a increasingly HUGE vacuum drawing in more air and fuel trough the carburetor and also cooling the air/fuel mix making the volumetric efficiency much better, sometimes too good... BLAMO!!!

JATO Rocket

A Jet Assisted Take Off unit will make you go really really really fast for up to 7 seconds... straight into a tree. And fry everybody behind you.


Nitromethanol, also known as RC-car fuel is an alcohol based fuel that burns faster and hotter than gasoline. This increases the performance greatly.

But as most people know, alcohol dissolves oil. And oil is what keeps your 2-stroke going. Also the added heat can cause your piston to over expand and instantly seize. So you will need a good lead-replacer (The type that goes into old cars that run on leaded fuel) to insure a good heat-conduction of the piston and cylinder wall. Also you will need special nitromethane 2-stroke oil, so the oil doesn't dissolve.

Don't use more than 15% nitromethane, as it will surely kill your engine. Mix the oil and lead-replacer with the fuel in the right proportions, shake and take for example 850ml's of that mixture and add 150ml's of nitromethane. Start the engine on normal fuel and run it till it is completely warmed up. Then switch to the nitromethane mixture.

Beware: You can't kill your engine by stopping the ignition anymore, the nitromethane mixture is self-combustible under pressure. So make sure you can cut the fuel off at any time!

side note: Ready to run "nitro" fuel (as intended for rc cars) is a mixture of mostly methanol (practically alcohol), oil (caster, synthetic or both), and typically has around 20% nitromethane added in though that can vary from 10-40% by brand or type. The percentage of oil might range anywhere from 8% to 25% with 15%-20% being the typical amount of oil found in nitro fuel. Up-jetting is a necessity to obtain a stoichiometric balance. A loose rule is an engine must burn twice as much alcohol as gasoline.


This category has only the following subcategory.

Pages in category "Performance Tuning"

The following 104 pages are in this category, out of 104 total.