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!

There is a vacuum port on the back of the intake manifold, where the power brake booster hose is connected when equipped.  If the booster hose or diaphragm leaks, there is a vacuum leak primarily at #8 cylinder.  Try unhooking the hose and plugging the manifold port and check #8 again.  Another possibility is a leaking intake gasket.

John - "The Hoosier Hurricane"

Hi John.   I'll give it a try tomorrow morning and see what happens.  I'll let you know.  Cheers, Lloyd

I plugged the vacuum pipes to the booster and to the wiper unit on top of the fuel pump.  No difference.  I even rotated the distributor 45 degrees (about the extent of the timing adjustment) and shifted the leads around to see if a different cylinder misbehaved.  No change.I reckon I can hear a light pop-pop-pop sound - possibly coming from cylinder #8 - while the engine is idling.  That may be related.

If the pop, pop, is regular and not intermittent, I would suspect an exhaust valve is not opening, due to a pushrod off the lifter, or heaven forbid, a lobe on the cam worn flat.  With the exhaust valve not opening, the trapped air in the cylinder would blow back into the intake when that valve opened, causing a pop each time.  This condition would not show up on a compression or leakdown test.  Crank it over and watch #8 exhaust valve to verify that it indeed does open and close.

John - "The Hoosier Hurricane"

Have you checked for a flat cam by watching the valves and rockers operate on that cylinder? Does the plug get wet like it is miss firing?    

  Vic Correnti

I had a 351m in a truck that had similar issues.   It would make a pop pop through the carb.  Didn't matter what rpm you were running.  It was intermittent and drove me nuts.  Tune-up,  carb rebuild, and a timing chain later I still had it.   Turns out I had weak valve springs and a couple exhaust valves were beat into the heads.   

Questions?  Does it run rough?  Does it idle rough, is it down on power, is it low on vacuum?  Does the spark jump to the plug with the boot close with the engine running?  Joe-JDC

JDC

Sounds like a possibility - worn out cam or dodgy valves.  Nothing else seems to make sense as everything else checks out.  Haven't seen a wet plug out of that cylinder, but I'll check.  I presume if the valves are stuck in the head it'd be pretty obvious with the rocker cover off.  Cheers, Lloyd (Fremantle, Western Australia)

Pull the valve covers, look for anything obvious.  If nothing is found, adjust the valves and, once adjusted, take note of the adjuster screw positions.  They should all be in the same position.  If one is in farther than the rest, then there is probably wear somewhere between the valve and cam, possibly including the cam.  If the screw is out farther, it probably means that the valve has worn into the seat.


Lawrenceville, GA