Well I replaced my spark plugs today , and noticed one of the caps was very lose !
It was a lose fit on the plug , so took it off the HT lead and had a look
Only to find the center had come undone , so took it apart gave it a clean and put it back togeather , making sure it was tight !
I never knew they come apart !!
One of the other electrically minded guys on this forum might be able to help you with the figures for the amount of resistance each HT line should carry. You might be running low on power to the spark plugs with any HT wires that are older. I am using a thicker mm HT wire with the Petronix dizzy with great results. No affiliation, etc. Don’t forget a Flame Thrower type coil to really zap the plugs!
Gerard
Hi Gerard , I was and have no problems with the spark plugs , just thought after 40 years it was about time they was changed 
You’ll probably go much faster now!!
Gerard