I just bought a 1980 300d with 200k miles. It looks very well taken care off very clean and even the AC works. The problem is it will only shift the first 3 gears and will not do the last. I've been reading about checking the linkages for the Bowden cable to the transmission and throttle linkage to the injection pump,and vacuum hoses but how exactly do I calibrate or adjust the linkages? They look like they have threads to adjust but where is the right position? How is it adjusted clockwise or counter...Thank you.