its a 1.9 cvh, big valve head, piper 285 cam with vernier set to 2 degrees advance, its a 32/34 dtml carb, think its off a 1.8 cvh sierra, if i have the chokes open when warm it splutters at low speeds and is fine flat out, but if i close the choke 2/3rds then it fine low speeds but looses full throttle power?????