Wierd Holley issue

[Gaz64]

Originally Posted by Jac Mac

When I suggest stuff like the head gasket back to front I always feel I should apologise in advance for asking!
Your ~50 RPM idle speed may be something as simple as the secondaries not closeing properly, FR-5 plugs may be one grade too hot ( remember NGK & some other brands get colder as the number goes up as opposed to champions etc get colder with lower numbers- I use BKR6E-11 in alloy head windsor stuff with around 10/1 comp.
I dont know how your canadian fuel stacks up against our kiwi stuff, but we find it impossible to tune by plug colour with pump gas & white colour is the norm for a good mixture, but the two dark cyls in your case are a concern. No little black 'balls' clinging to the insulator on the plugs are there?
Have you double checked the timing pointer is showing 'true' TDC?
Also have you checked the rotor phasing in relation to the cap since the pertronix unit was installed ( an incorrect vac adv arm length can also cause this ). A clue to this is the tip of the rotor electrode looking like all the erosion is at one corner.

All very good points.

Other things to look at are the colour of the outer shell (exposed to combustion), since the porcelain picks up little colour from unleaded fuel.

With cylinders 5 and 6 side by side, I'd be concerned with their darker colours. I'd say do a leakdown test.