My wife thought it would make a good tractor to ride in tractor drives so when she got it home I tried to start it but it started hard and every time it was a chore to start . So I decided to check into it and I tore it down replaced cam as it was chipping away took and had the block bored and sleeved the crank turned put new bearings in and bushings put it all back together and had the injection pump done 3 times the head checked and redone 2 times , put it all back together then put new rocker assembly in it then started and it still does not have good throttle response and when you go full throttle it pops so checked the timing again and it is right on for the pump and the engine anybody have any ideas I’ve rebuilt many engines before but this one has me stumped?