alright im havin some problems. i have a 74 d 100 with a stock 440. just bought it about a month ago. the only problem im havin with it is it has a thump kind of sound in the bottom end. i changed the oil in it hoping it would help i used royalpurple 15w40. it runs and sounds good cold but only has about 20for oil pressure. when it warms up it goes down to about 2 unless ya get on it but then it only goes up to about 18 and the thump is alot worse. were should i start?

 

The first thing I would do is remove all the plugs and belts then turn it over by hand both ways and see what I could "feel".