I have a 1986/87 Yamaha Moto 4 that lurches when you give it gas. It jerks badly and at higher speeds its shakes horribly. If you give it gas and don't hold the handlebars it automatically veers off to the left gradually. Please help me with this issue. I have replaced front brake shoes, rear brake pads, battery, carb clean, gas tank clean, air filter, oil change, oil filter, new wheel bearings in front (Didn't alleviate shaking).