Will a 1988 Yamaha yz250 carb fit 1986 yt125?

The answer is no. While these two models are both Yamaha motorcycles, the 1988 yz250 is a motocross bike, while the 1986 yt125 is a trail bike. The engines and carburetors are different between these two models, so the carburetor from the yz250 will not fit on the yt125.