Predator 420/GX390 Mods – A Beginner’s Guide

Looking for ways to get more power out of the big block go-kart engine? If so, you’re at the right spot! I’ll be listing out the parts you can upgrade, how they will improve the performance and how much they cost. Bear in mind, big-block engines are expensive to modify, unlike small engines.

Top Speed Calculator

Since Top speed depends on your exact setup, there is no way for me to tell you the exact top speed you can achieve after each mod.

However, I will provide you with a rough estimate of the max rpm of each mod and you can check the top speed with this calculator. (Wheel Diameter is in inches and Top speed is in mph)

You can learn how to calculate the gear ratio here.

Governor Removal

• Max-RPM : ~5000 rpm
• Cost: $0

A governor is a mechanical device present inside the engine that limits the Engine to 3600 rpm, which is an ideal rpm for Industrial use.

To remove the limit, we remove the Governor. It consists of 2 parts, the governor gear, and a governor arm. You need to take them both off the Engine and re-link the throttle cable. (Here’s a video of the process)

After removing the governor you can expect the max rpm to reach ~5000, after which the valve floats.

Will It Damage the Engine?

Generally speaking, it won’t damage the engine, the parts that are at risk at high rpm are connecting rod and flywheel. However, since your Engine will still be limited to 5000 rpm, you won’t have to worry about Rod and Flywheel just yet.

If you’re still worried and want to be on the safe side, you can get yourself the billet flywheel and billet rod kit.

Can you Bypass Governor instead of Removing it?

This is a very common question since bypassing is much easier. But it’s not recommended. The Governor gear is not designed to handle high rpm, which can result in it snapping. Gear and washers moving freely inside your engine will almost certainly cause the engine to fail.

Stage 1

• Max-RPM : 5000-5500 RPM
• Cost: $99

The Kit Includes:

• Upgraded Air Filter
• Upgraded Header Pipe
• High Performance Spark Plug
• 0.042, 0.043 Main Jets
• Adapter, Gaskets, Choke bracket for Installation

These are the basic upgrades once you have removed the governor and want extra power. It won’t increase the top-end speed by much but it will make the engine more powerful in the lower rpm range.

The main parts that the kit includes are air intake, header pipe, spark plug, and main jet. Adapters and Gaskets will help in the installation.

Air Filter

The filter that comes with Stage 1 kit will increase the amount of air entering the engine for combustion.

If we combine this with more fuel, we can increase the amount of combustion taking place inside the engine which will naturally increase power.

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Main Jet

Stage 1 kit comes with 0.042 and 0.043 main jets. It will increase the amount of fuel entering the combustion chamber for combustion.

This is necessary once you have an upgraded air filter that increases the air entering the engine since too much air in the air-fuel mix will cause improper combustion, i.e, the Engine will run lean.

Header Pipe

Naturally, the last piece is the header, since we are burning more fuel we will also have to release the burnt gas quicker. Which is exactly what the header does.

The kit should come with a muffler, however, if it doesn’t, you should really consider getting one. Your Neighbours will hate you if you don’t!

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Mikuni Carburetor

Once you have the stage 1 kit installed, getting a 34 mm round slide Mikuni Carb will take your motor to the next level!

Mikuni Carbs have better throttle responsiveness and a bigger venturi that will allow it to supply more fuel to the engine.

The Predator 420 and GX390 have 38mm intake valves, so a 34 mm venturi is very close to the ideal size for these engines.

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Stage 2

• Max-RPM: ~6200 RPM
• Cost: $320

Kit Includes:

• Everything From Stage 1
• Billet Flywheel
• 29 lbs Valve Springs

Stage 2 Kit

Stage 2 Kit consists of everything from Stage 1 and in addition, has 29 lbs valve spring for both intake and exhaust valves, and a billet flywheel that will not fail at high rpm.

Valve Spring

A heavier Valve spring will remove the rpm limit caused by valve float that happens at about 5500 rpm in a stock spring.

The heavier the spring is, the higher the rpm limit will be. Note: Valve Springs does not increase power by themselves! All they do is remove the speed limit which will naturally increase top-end power.

You can get heavier springs like 50 lbs spring or 65 lbs but that will be a waste of money unless you also upgrade your Camshafts (more on that later).

Stage 3

• Max-RPM: ~7200 RPM
• Cost: $500

Kit Includes:

• Everything from Stage 2
• Billet Connecting Rod
• .308 Camshaft
• Stock Gaskets

Stage 3 Kit

A stage 3 kit will essentially double your stock rpm and power! You can expect a stage 3 Predator 420 to make 24 hp.

The major upgrade in this kit is the camshaft. The rod that comes with this kit won’t improve performance, it’s a requirement because a stock rod can fail at high rpm.

Camshaft

Camshafts are essential in a 4-stroke engine’s working. Installing a cam that will open the valves for a longer time period increases the amount of fuel burning in the same time period and therefore produce more power.

There are several things to consider while deciding on a camshaft, you can read in more detail here.

Dynocam sells camshafts from 0.255″ lift all the way up to 0.355″ lift. Stage 3 from OMBwarehouse comes with a 0.308″ lift.

A Predator 420 with a 0.3555″ lift can produce 7800 rpm!

Once you install cams with high lifts and duration, you’ll have to match them with an appropriate valve spring so that the valves can close quicker, i.e, they don’t float.

Additional Mods

As mentioned above, you can choose a cam with a higher lift with stiffer valve springs to gain even more power. But apart from that, there are a few other changes that you can make.

Advance Timing Key

Advanced timing key will advance your ignition by a certain degree.

Many people believe that combustion takes place when the rod is at its topmost position, but that is not practically possible. The timing is always set 30-40 degrees before the top dead center.

You can get a 4, 6, or 8-degree advance timing key, which will boost your engine’s performance by a noticeable amount.

Where to Buy From?

GoPowerSports and OMBwarehouse are the most well-known suppliers. However, they don’t manufacture the parts, they sell the parts in kits such that all the parts in the kit are compatible with one another.

However, if you believe you understand all the parts and what they do, you can directly buy from manufacturers such as ARC Racing, NR racing, Dyno Cam, Mikuni, etc.

Where to Start?

If you’re new to engine building, start with Governor removal and Stage 1 kit. They are easy to install and won’t require a lot of adjustments. If you’re still not satisfied with the performance, you can then buy the other parts separately.

Which Engines will these Parts Work With?

All of the parts mentioned will work with Predator 420, GX 390. Most of the parts will also work with GX390 clones, Predator 301, GX270, and GX240. Contact your supplier before you order any parts, they will ask you to make measurements of your existing parts and help you choose the right size.

Have any more questions? Feel free to leave a comment below!