I was going to tune up a 1965 220 se coupe today.It has a 6 cyl fuel injected engine,no air cond,no frills.I loosened the allen head bolt under the distributor so I could turn it.Suprise! although I loosened it almost completly.the distributor will not budge.I have consulted the cd manual,it says only ...turn distributor.... Please help.I have done this on several mbs,never a problem.What can it be?