how can you tell if your spark plugs or HT leads are failing??

 

u can measure the resistance with a multimeter.

This wont tell u if the insulation had broken down

Then, i's just a case of swapping over

 

would the car struggle if the leads are breaking down??

kinda like a fuel block.....stuttering??

 

yes if leads are knackerd this can cause problems underload, when you give it a boot full

leads can also break down under heat, so if you carry out a resistance test on them when cold it doesnt mean they are ok because they could be breaking down when there hot

 

Id use a large megger to test insulation at a few kv, that should show any weekness up. What voltage do coils give out on average?