I'm trying to diagnose misfiring at low revs on a 1980s Mazda. Things start sounding rough below about 1200rpm. I noticed in the 'Focus misfiring' thread MrCheerful suggested replacing Bosch spark leads with original Ford ones. Does it really make such a difference? Mine is an old-school ignition system: capacitor, ignition coil, distributor cap, rotor arm, spark leads, plugs. I've replaced the lot (except the capacitor which I measured OK): each stage of replacement did seem to improve things. The carb has also been cleaned and adjusted.
The leads I have on there are new-old-stock random-brand from eBay. When I gently wiggle them on the distributor with the engine idling I get a shock through the boot. Is this bad, or to be expected if I wiggle them? I don't feel a shock normally when gently touching them. Replacing the 20-year-old leads definitely made a difference, but I'm wondering if these new ones are somehow leaky. Silicone and rubber looks new. The boot on the ignition coil has slightly split: I need to do something about this, but the lead is firmly plugged into the coil so should be electrically OK. HT leakage faults are just so annoying to track down.
I can buy Bosch or another random brand from the autofactors, but a Mazda parts database quotes EUR134 for the genuine article, assuming Mazda has any stock at all (they often don't for this age).
Any other ideas? I pulled the plugs out after a few hundred miles and they look as I'd expect (slight pink dusting). I've poked a hose around listening for vacuum leaks and can't hear anything obvious.
Cheers, Theo