Small Engine Performance
Article by Mark Trotta
There are many ways to get more power out of your small engine. Some cost money, some are free.
Some performance upgrades are stand-alone, meaning they will improve power by themselves. Other ways require a combination of things to be the most effective.
Manufacturers of performance parts often use the terms Stage 1, Stage 2, and Stage 3 to denote what state of tune the part will provide. Although opinions vary, the following is a general guideline.
Stage One Engine Mods
* Bolt-On Items, Engine In Good Condition *
Stage One modifications for small engines are simple bolt-off, bolt-on performance upgrades, such as air cleaners and exhaust pipes. Maximum results are seen when these are accompanied with replacing the stock-size main jet.
These will be the least expensive mods with minimal labor and improve performance on any engine (provided it's in good condition). This means compression is good, spark is good, and there's no excessive oil usage.
Stage Two Engine Mods
* Partial Engine Disassembly Required *
Moving higher up on the performance ladder, Stage Two engines will have the Stage One mods, plus the governor and low oil sensor are removed and valve springs will be replaced. Billet connecting rods and billet flywheels are recommended but not required.
Many Stage Two parts work best in conjunction with other parts. For example, 18-lb valve springs don't give more power by themselves, but when installed, will allow a higher-lift cam to operate at it's intended performance level.
Stage Three Engine Mods
* Complete Engine Disassembly Required *
A Stage Three engine is usually a track-only engine, or one that gets overhauled and serviced at regular intervals. Internal engine upgrades include special piston and cam, 18-pound or heavier valve springs, and carburetors up to 22mm in size. These motors generally see extensive machine work, such as ported/milled cylinder heads for more power and higher compression. Billet connecting rods and billet flywheels are mandatory.
NOTE: Unless you're involved in competitive racing, don't be tempted to build a Stage Three kart motor. The power band is all top end, making it unusable for the street (not to mention loud).
Before you start removing and changing parts, it's best to have a plan. What is the kart/minibike going to be used for; street, track, or off-road? Are you willing to sacrifice reliability or driveability?
Also consider a budget.
Replace Stock Exhaust
One of the first performance mods to make on a small engine is upgrading to a high-flow header. The factory exhaust muffler was designed to keep the engine as quiet as possible and is very restrictive.
Aftermarket headers and mufflers are a simple bolt-on installation. Since most small engines use a common NPT pipe thread, you can get creative and make your own custom exhaust.
Replace Air Filter
Just like the factory-equipped muffler, a stock air filter is designed to give quiet operation and is very restrictive. Don't be tempted to run without an air filter. A gas engine is like a big air pump, it pulls in air, along with any dust or dirt that may happen to be nearby.
Upgrading to any high-flow air filter will improve engine breathing. K&N style air filters are a very popular choice.
The factory-installed governor was designed to limit rpm's to about 3,500. Removing it allows the engine to rev higher, which "unleashes" more power.
Once removed, an average 5hp small engine without any other changes will rev to about 5,200 rpm. But this mod should be done in conjunction with (at least) upgrading the valve springs.
Valve Spring Upgrade
The factory springs are too soft for anything more than the stock 3500 rpm. Replacing stock valve springs with stronger ones will prevent the valves from bouncing at higher RPM's.
Valve springs are available in different strength ratings and identified by different color paint splotches. Aftermarket springs are the same size as the stock ones, it's just a stronger spring.
10.8 Pound Valve Spring
The first step up from a stock valve spring is a 10.8 pound spring (red splotch). These work well with mildly modded (Stage One) motors with no governor, and will allow the engine to spin to 5,000 RPM and beyond.
18 Pound Valve Springs
Upgrading to 18-pound valve springs (blue splotch) will prevent valve float and get you to 6,500 rpm.
Not recommended for any cam with a .274" or more lift.
NOTE: You don't want to run heavier springs on a stock engine. It will put additional force on the cam lobes, forcing it to wear prematurely.
Stock vs Billet Flywheel
Although they're lighter in weight, the major benefit of a billet flywheel is strength. Whereas a stock flywheel is "cast" in a mold, a billet flywheel is machined from a solid chunk of steel.
Cast flywheels were designed to operate at 3500 rpm or less, so revving it twice that much is inviting trouble. Simply put, when a motor is wound too tight, the weakest part breaks. Although flywheel breakage doesn't usually happen under 6000 rpm, it can be a very dangerous situation if and when it does.
When you are making more power than stock, upgrading to a billet flywheel is highly recommended - not only the engine's safety, but your safety as well. Aftermarket companies take liability seriously, and they all state, in some form or another,
"If you have taken the governor off of your motor use a billet flywheel."
The most popular carb upgrade is swapping the standard jet with a larger jet. This goes hand-in-hand with a less restrictive exhaust and air filter.
A more advanced carburetor mod is to replace the emulsion tube to get a little more torque. This is often done at the same time as re-jetting the carb.
What Is An Emulsion Tube?
An emulsion tube, along with the main jet, control the carburetor's air/fuel mixture. As you can see in the picture, there are small holes in it. The size and amount of these holes regulate the amount of flow it can provide.
Shop: Performance Emulsion Tube
Switching to an E-tube with a slightly larger main bore and less air bleed holes will provide a better fuel signal at the jet. Best results are seen when also replacing the stock-size main jet.
Major carb modifications like reshaping and/or increasing the radius bore are best left to an experienced engine builder. Many carburetors have been ruined by well-intentioned kart owners. Although experienced builders can get 10 to 15 horsepower from a modified factory carb, this is a very specialized skill.
Cylinder Head Mods
Incorrectly done or over-done, porting your own cylinder head can easily do more harm than good. To be safe, all you really want to do is smooth the ports slightly and remove some of the short-side radius.
Everyone has a different idea of what "mild" porting is. Either practice on a spare head you don't care about, or leave this to someone with experience.
Use Proper Oil
Check the manufacturer's suggestions; most small engines require oil specifically designed for air-cooled equipment. Most automotive oils manufactured after 2009 do not meet these specifications. This results in higher operating temperatures and reduced engine life. Oil should be changed at recommended intervals (or once a year minimum).
If your motor is already broken in and still fairly new, a switch to synthetic oil may help performance slightly. Because synthetic oil is more "slippery" than conventional oil, there is less friction for the crankshaft, connecting rod, valve guides, to ride in. Any name brand oil will do -- they're all rated the same, it's just personal preference.
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