Predator 212 Carb Upgrade: Mikuni VM22 Overview

Have you upgraded your Predator 212 to get more rpm? If so, then the stock carburetor is not made for those higher RPMs. The stock carburetors on a predator 212 or Honda GX160 or any other smaller engines are designed for the stock engine which has a governor to limit the rpm to around 3600.

If you remove the governor or make other adjustments to increase rpm then you will need to get a Mikuni or a Mikuni clone carburetor to get more power out of the engine. Why? Because, when you’re increasing the rpm, the amount of air entering the carburetor will increase, and the air intake hole for the stock carbs is just not big enough for this purpose.

In this article, we will go through the parts and the working of the Mikuni VM2 and how to tune them, then we will make some comparison with the stock carb. If you want to have a look at other upgrades for Predator 212, you can check out this article.

Front and Side view for the Mikuni VM

Throttle Valve and Needle jet

One major difference between the stock Carb and Mikuni is the throttle valve, the stock carbs comes with a butterfly type of throttle valve, whereas the Mikuni has a cylindrical throttle valve and goes up and down as you open or close the throttle.

The throttle valve has a needle connected to it, that needle goes into the needle jet (aka emulsion tube) in idle condition, as the throttle opens, the needle moves out of the needle jet.

The needle jet has multiple holes, which are there so that air can flow through them, so when the engine is in idle condition when the throttle is fully closed, the needle is inside the needle jet blocking all the holes and therefore preventing any air to flow through them.

Needle Adjustments:

The needle has multiple slots, so you can take the needle out of the throttle valve and make an adjustment.

If you move the needle upwards from its initial position, then the needle will leave the needle jet at a lower throttle and vice versa if you move the needle down from the initial position, the needle will be in the jet till a higher throttle opening.

Pilot Jet

When the engine is idling with the throttle closed to the point of 1/4th throttle open, the fuel supply is controlled by the pilot jet and the air screw will adjust the amount of air available for mixing with the fuel. (refer to the initial diagram to locate your pilot jet)

Here’s the graph from their manual showing the fuel flow at different throttle open %

The pilot jet controls the majority of fuel flow u until the intersection point in the graph, after that the fuel from the pilot remains constant and the main jet becomes the primary source of fuel.

Tuning: The pilot jet ranges from size 10-50 with 10 being for lean mixture and 50 for richest, so if you change the pilot jet to a larger size, you’ll get a rich mixture and that will be very noticeable from idle to 1/4 throttle position.

Likely you’ve a 15 size installed by default if you have a Mikuni or clone, for a predator 212 you’ll likely not need anything over 15-20. If you are having issues from idle to 1/4 throttle position, then the culprit is the pilot jet and that’s when you’ll need to try different size.

Main Jet

The main jet is the major fuel supplier from throttle 1/4 to full throttle, you can see from the fuel flow to throttle graph above, after a certain point the fuel from the pilot jet becomes constant

Further increase in throttle will result in the needle pulling out of the needle jet/emulsion tube and therefore creating pressure for the fuel to be pulled up via the main jet into the needle jet where it mixes with some air that is supplied through the holes in needle jet, after that the fuel gets atomized as it reaches the main bore and comes in contact with air in the bore.

Tuning the main jet

By default, the main jet on Mikuni and clones comes in at #98 size, that size is smaller than required for the predator 212 and it will limit the engine’s performance.

Generally speaking, you’ll need a 125 main jet for a predator 212 with no governor and if you’ve made other changes then you can go for a higher main jet, again the same principle applies, the higher the number, the richer the mixture will be.

One important thing to note: Measure the dimensions (diameter and length of the jet) of your current main jet before you order a new jet, some of the clone’s main jet has different dimensions to Mikuni.

Float

The float is there to maintain the proper amount of fuel in the bowl at any given time, its very similar to how it works in a stock carburetor of a small engine you can read this guide on small engine carb’s working if you want a detailed explanation on how they work.

But in short, the float is connected with a inlet needle and a seat, when the fuel rises, the float moves up and inlet needle moves along with it and sits on the seat, resulting in a blockage of any further fuel entrance.

Other parts and their workings

Apart from the throttle and needle, the Mikuni carbs are not much different from the stock carburetors, if you’re looking for a more detailed guide on how the whole carb works then use this guide.

The Pilot screw and air-fuel mixture screw will determine the idle speed and the air flow, you can make a quick adjustment in these Carburetors, which is the major benefit as compared to the stock carburetor for which the only solution is either cleaning of carb or changing the jets.