Tuning the main jetīy 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.
MIKUNI BSR42 TUNING FULL
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 constantįurther 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. 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. 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. 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. 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.
MIKUNI BSR42 TUNING MANUAL
Here’s the graph from their manual showing the fuel flow at different throttle open % (refer to the initial diagram to locate your 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. 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. The needle has multiple slots, so you can take the needle out of the throttle valve and make an adjustment. 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. 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. 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. Front and Side view for the mikuni VM Source: MikuniVM User Manual Throttle Valve and Needle jet
MIKUNI BSR42 TUNING HOW TO
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. 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. If you remove the governor or make other adjustments to increase rpm then you will need to get a Mikuni or clone carburetor for best performance. The stock carburetors on a predator 21 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. Have you upgraded your Predator 212 to get more rpm? If so, then the stock carburetor is not made for those higher RPMs.