This is a homemade bottle style carburetor synchronization tool ( homemade manometer ) and balancing tool. If you are going to attempt to, or want to know how to sync your carburetors, there are a few things to know first.
1. Carburetor Synchronization = Adjusting your carburetor throttle plates or throttle slides so they move from your carburetors idle setting to wide open throttle simultaneously, or in sync with each other.
2. Balancing Carburetors = Adjusting your carburetors so they allow each cylinder to flow or pull the same amount of air and fuel at your desired idle speed. Carburetor sync and carburetor balancing go hand in hand.
3. Idle speed screw adjustments must be made at normal operation temperatures.
4. Air fuel mixture screw adjustments must be made at normal operating temperatures.
5. Air fuel mixture adjustments only affect your carburetors pilot circuit, which means from idle to part throttle. Air fuel mixture screw adjustments have zero effect on wide open throttle.
6. Carburetor running rich = Too much gas not enough air.
Carburetor running lean = Too much air not enough gas.
7. Wide open throttle rich or lean conditions are adjusted two ways:
A. Carburetors with throttle slides have a clip on the carburetor main jet needle. Raising the main jet needle clip drops the needle deeper into the main jet and will lean the carburetor out. Lowering the main jet needle clip will raise the needle and enrich your carburetors mix.
B. Carburetors with throttle plates (butterfly style) need to have the carburetors main jet changed to a larger diameter jet to richen the mix or a smaller diameter jet to lean your mix.
8. If your engine will not idle correctly or your air fuel mixture screws seem to have no affect on the engine at idle, then most likely your pilot jet circuit is clogged and you should clean it.
The snowmobile in the video is a 1986 Arctic cat cougar 500 fan cooled, it has Mikuni vm34 roundslide carburetors which have a primer nipple on the engine side of the throttle slide. Perfect place to attach a homemade manometer to balance everything. These mikuni vm34 roundslide carburetors were common on many motorcycles, snowmobiles and atv's. Suzuki, yamaha, polaris, arctic cat, all used them at one time or another, they are good carburetors when they are dialed in.