The Mikuni BS25 is a round-slide, constant velocity (CV) carburetor commonly found on mid-sized 150cc 4-stroke engines, such as those in GY6-equipped vehicles and Yerf-Dog utility carts.
When tuning or modifying your Mikuni BS25 carburetor, keep in mind:
If you are looking at a diagram because you need to repair a patched carb, stop patching. Follow this flowchart of salvation: mikuni bs25 carburetor diagram patched
Before looking at the diagram, let’s establish the hardware. The BS25 (also labeled as BSR or BST in some variants) features a 25mm bore. It is a CV (Constant Velocity) carburetor, meaning the slide is lifted by vacuum via a rubber diaphragm, not directly by the throttle cable.
Air/Fuel Screw: Start by lightly seating the screw clockwise, then turn out 1.5 to 2 turns. The Mikuni BS25 is a round-slide, constant velocity
Idle/low-speed hesitation
Let’s look at the anatomy of the BS25 and explain why "patched" is often a four-letter word. A Quick Profile Before looking at the diagram,
Check forums: