Firstly though, check your distributor for faults or failing parts.
There are basically two distributor types, Bosch and the later Lucas up grade which can be retro fitted using an adapted loom from the ignition amplifier on the side of the distributor to the coil.
Check the security of the vacuum adv/retd to the distributor, small hex head bolt on side, the unit should be tight and not rotatable. With the engine idling, pinch the vacuum pipe and the engine should slow noticeably. if it doesn't check the vacuum pipe and connections. you can suck the pipe to the distributor and you should feel a vacuum form and if you remove the distributor cap, see if the base plate moves inside. Blow through the pipe into the inlet side and check patency there. There should be no resistance otherwise it is blocked somewhere and can't pull a vacuum when the engine is running. It could just be a perished pipe!
If this all appears functional, check to see that your centrifugal weights in the distributor are free to move outwards and are not restricted by corrosion or dirt, also that the springs are not broken or weakened. These two systems work in tandem with each other to control ignition advancement.
If this is not the problem it might then pay to remove the distributor and check/service it. A new distributor cap and rotor arm are perhaps obvious but there are more checks and replacement parts you can swap. Firstly a physical check of the units function and this will require a strip down by first drifting out the pin securing the dog drive that slots into the cam shaft. Note here that you should mark that position as it is slightly offset and if returned the wrong way around you will be 180' out. this will allow you to then change out the electric pack fitted in the side and base of the distributor with three screws.
It is also possible to adjust the amount of vacuum applied by the diaphragm. remove the vacuum unit by loosening the hex head bolt on the side. you can then insert an allen key down through the centre hole. The more you wind it in, clockwise the greater the vacuum applied and wound out the less applied. If your diaphragm has failed or is failing you will need to set the new unit inline with the old one. Take the old unit and counting the turns, wind it fully in to the stop then with the new unit wind that fully in and then out by the same number of turns. This will be a ballpark position but if on acceleration you get either hesitancy or pinking you can manually adjust to suit. It's diificult to fully describe what might be wrong and how to find and rectify it but I hope this is of some help as a starting point.
