Has yours got an emissions air pump? mine did and it would pop when you change gears, took it off and it stopped.
Send me a message if you want a hand giving it a tune, ive got all the specs I tuned mine to, plug gap, points gap, optimum timing, ect all that stuff.
Would also recommend if you don't know the condition of plug leads, points, dizzy cap/rotor, ect to get some new ones as it is very cheap to replace it all.
And there is no such thing as surface rust its just rust, haha.