My '57 T-Bird 312ci has a problem I can't identify. If I pull off leads one at a time at the dizzy cap, the rpms drop about 40-50rpm, except on cylinder #8. I've changed the lead, swapped leads with ones that I know are good, swapped plugs, bought new plugs, etc. I've also replaced the distributor cap and rotor, just fitted new points and condenser (it was running a Pertronix electronic ignition module - so I removed that just in case). I've run a compression test - getting around 130psi or so on #8. Pretty much the same on all cylinders. I've done a leak down test - nothing leaking past the valves, although there is leakage past the piston rings, same effect on other cylinders. I can't get a percentage leakage as my leakdown tester gauge is dodgy. I'm at a bit of a loose end as to what else to check. I haven't checked the valve clearance on my solid lifters, but I did take the rocker cover off and there was a similar amount of play when the valves were closed, like other cylinders. I'm missing something! Can anyone else suggest what I can look at or test. I'm assuming an inlet manifold leak wouldn't affect a single cylinder. Maybe a valve isn't opening?? This one has got me befuddled!