I got my 76 300d after it had sat for a few years. I also had the chuffing sound which I susspect was a stuck valve. When I first got it I could not start it and an old school mechanic told me to pull the injectors and pour a couple of ounces of ATF in each cylinder, replace the injectors, crank it over a little to get the ATF distributed and let it sit for a few days. It fired right up! I heard the chuffing sound on the intake side but it went away after awhile. There is a lot of carbon or gunk build up in diesels that may stick a valve. The rational for the ATF was to disolve the hardened carbon deposits that might be sticking the ring and reducing compression. It worked quite well.