Honda GX160 Engine Build
Article by Mark Trotta
As the smaller brother of the GX200, the GX160 is one of the most popular small engines made by Honda. Whether you plan on building a mildly modded engine or a screaming torque monster, this article covers the many ways to get more horsepower from your Honda GX160.
The old GX160 you see in the pictures was from a pressure washer whose pump stop working, but the motor still starts with a few pulls and runs fine. I plan on freshening it up with a few performance mods, to be ready for use on a future project.
GX160 Performance Mods
Having 5.5 horsepower may be enough for some folks, but most of us need a little more power. There's plenty ways of getting more performance from this motor. Some mods are free, while others will require a little cash.
Gen 1 vs Gen 2
Before ordering parts, you will need to know which generation GX motor you have. After a 21-year production run, Honda completely redesigned the GX160 for 2012. The newer version is referred to as the T2 and the older version is called the T1.
Externally, the two engines are similar, but internally they are very different. Not many parts interchange between the first generation GX160 and the second generation GX160.
Oil Pressure Sensor
Most Honda GX engines came from the factory with a low oil pressure sensor. It is designed to shut off the ignition if low oil pressure is detected. The sensor can be removed, but it's a nice feature to have.
Replace Stock Exhaust
The stock exhaust muffler was designed to keep engine operation as quiet as possible. From a performance standpoint, it is very restrictive. Upgrading to a high-flow header is one of the first performance upgrades to make on any small gas engine.
Shop: Header Exhaust Pipe for GX160
Headers and mufflers are the same for GX160's and GX200's, as well as many clone motors. They are simple to install and inexpensive to buy. Since they use a common NPT thread, they can also be easily modified.
Replace Air Filter
Like the factory-equipped muffler, the stock air filter is designed for quiet operation and is very restrictive. Upgrading to any high-flow air filter will improve engine breathing. K&N style air filters are a very popular choice.
Shop: Air Filter and Adapter Kit for Honda GX160
After you have replaced the stock muffler and air filter, there are several cheap upgrades you can perform on your stock GX carburetor. This includes swapping out the standard jet with a larger jet, and replacing the emulsion tube to get a little more bottom end. More serious carb mods include increasing and reshaping the radius bore.
Another performance option is to replace the stock carburetor. The Mikuni VM22 is a 22mm slide carb, and will significantly improve performance over the stock carb. The caution here is that it may require an adapter to fit the Honda GX engine, and the throttle hook-up has to be modified.
Shop: Mikuni Clone VM22 Carburetor
From the factory, the VM22 carb has a #30 pilot jet and a #100 main. Many kart engines run better with smaller pilots and bigger mains. It may take a few jet swaps to dial in the carb.
A Keihin carb is also a popular swap for the GX160 and GX200 engines. Like the Mikuni, it will add torque and horsepower.
Behind the recoil starter is a plastic fan that helps cool the engine. A common low-buck trick is to cut off every other blade. This will reduce drag and give you a little "free" power.
If you do not know the full history of your engine, it's worth your while to open it up before ordering parts. You may find that some engine modifications have already been done.
Milling Cylinder Head
Machining a cylinder head is a relatively simple job for a machine shop. Because engines vary a lot in head volume and deck height, check what you are starting with before you do this. Common (safe) practice is to shave 1mm off the head and then reinstall with a GX200 head gasket, as the 200 head gasket is thinner than the 160 head gasket.
Another way to boost engine compression is to replace the stock dished piston with a flat-topped piston, and swap the low-volume cylinder head and a higher-volume head.
If you're not removing the governor, a compression increase will do little at 3,600 rpm, so this is something to be done in combination with other performance modifications.
Should I Remove The Governor?
By allowing the engine to rev higher, removing the governor unleashes more power. But, it should be done in conjunction with upgrading both the flywheel and valve springs. This is done not only the engine's safety, but your safety as well.
The factory-installed governor was designed to limit RPMs to 3,600. Once removed, a Honda GX160 without any other mods will rev to about 5,200 RPM.
GX Cam And Valvetrain
There are several aftermarket cams that will help you achieve more power. The key here is selecting the one that best compliments your other mods.
Stock Honda rocker arms are very good and do not need upgrading. Stock Honda cam followers are very good provided they were properly lubricated when assembled.
Install Stiffer Valve Springs
The original factory valve springs were never designed for high revs, and will begin to "bounce" and weaken at higher RPM's. This leads to lost power and eventual spring breakage.
Installing stiffer valve springs is the answer. A valve spring with more spring pressure will allow higher RPM's.
Performance valve springs are the same for Honda GX120/160/200, as well as Titan and Predator engines. They are available in different ratings and identified by different color paint splotches.
For mild performance, there's the 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 5,200 RPM and a little beyond.
The common rev limit in pro-kart racing is 6,000 rpm. Upgrading to 18-pound valve springs (blue splotch) will prevent valve float and get you to 6,500 rpm.
Shop: 18-pound Valve Springs
There are also 26 pound valve springs (white splotch), but they may be too heavy. Installing springs stiffer than necessary robs the engine of power.
Changing valve springs without removing the cylinder head can be done, it's just a little harder. Valve lash is set to .004" intake and .008" exhaust when the engine is cold.
If you're building a serious hi-performance motor, get an aluminum or billet flywheel for your own safety. If you're racing on a track, many organizations require it. Aluminium/billet flywheels don't give any more performance, aside from saving a little weight and inertia.
A stock GX flywheel is good til about 6,000 rpm. After a governor is removed, and the motor is revved past that, the stock flywheel lets go at high RPM. When it does, it shatters everything around it.
If you're lucky, you may find a good used Honda GXV160 flywheel. These are cast aluminum, which are much better to run at high RPMs than the cast-iron GX flywheel.
Engine timing can be slightly advanced for better cylinder pressure. For a little more performance, when re-installing the flywheel, switch to a six-degree flywheel key.
Stroker GX160 Kit ?
Although stroker kits are available for the Honda GX200, there aren't any for the GX160. Although the 160 and 200 share the same basic block, the rod would need to be shortened or the top of the piston will stick out of the bore. The simple truth is, stroking a GX160 with a GX200 crank would require machine work, and it doesn't make much sense to do all that work for only 196cc.
GX160 Connecting Rod
Like a billet flywheel, a billet connecting rod doesn't give you any more power. What it does give you is lightness and strength over the factory rod.
Several companies offer "bundle" packages, where many of the aforementioned parts are bought together, saving you money.
Clone vs Real
Honda clone engines are made with less precision and accuracy than real Honda engines. Clone engines are made in China, where labor is cheap and copyright laws seem non-existent. Any parts made from plastic or rubber, such as the rocker cover gasket, recoil handle and recoil parts, are of inferior quality. There is no quality control, so you don't know how long it will last.
Price vs Longevity
If you're truly concerned about longevity, get yourself a genuine Honda GX motor. With the exception of price, a Honda GX160/GX200 is better in every way than the best Honda clone engines. The additional cost will pay for itself in longevity and the ability to be rebuilt.
Change Oil Frequently
Last but certainly not least, regular oil changes preserve the life of any internal combustion engine, and even more so with a performance engine. Honda GX engines with a low oil sensor take 14 ounces. Without the low oil sensor it will hold 16 ounces. Be sure to use a name-brand quality oil.