Manifolds are made of rubber, a component that often deteriorates and cracks as it ages.
When the engine malfunctions, it is often the case that the manifold is torn and sucking secondary air. This should be checked at the same time as plug replacement/carburetor OH.
NTB product No. Compatible model Genuine parts number (top) YAMAHA JOG 3KJ (bottom) YAMAHA JOG 5BM