Hi: I have a mid 70s 720 with a vh4d. It fires on 2 cylinders only. looking at the machine from the back it's the 2 cylinders on the right side. The first cylinder will be the number 1, I'm not sure what the other one is (I think it's the 2 cylinder). I put in a new distibutor. I put in new plugs. I've checked the spark, it's getting spark. I've checked the plugs, they're sparking. I removed the head, the valves seem ok they move up and down. I checked the compression it's about 60 -70 on all 4 cylinders. The cylinders are getting gas. I've replaced the intake and exhaust manifold gaskets. Any ideas ?