I have a 56 mercury montclair I got my hands on. I can not get it to go past 15 mph or so. When i put the pedal to the floor it shakes a lot and just wont go. Ive given it a tune up, plugs, wires, cap and rotor. Ive also cleaned the carb. I'm wondering if it can be a transmission problem. In which case I feel like I'm screwed because its an original merc-o-matic.