Thanks guys.  A few things.  As Sandbird mentions, there is a longer bolt than acceptable in the last back hole of the intake manifold.  Doesn't hit the pushrod, but is a little close.  John - I'll try spraying some volatile liquid around the manifold near cylinder #8.  I re-ran compression tests on all cylinders - all got within 135-140psi, so pretty good.  As Charlie suggested - I've checked tappet clearances, screw adjuster positions, etc. - all good too and valve lift seemed fine.   I also used my leak down tester to check that both inlet and exhaust valves open and close, which they do.

It really has to come down to a vacuum leak near that cylinder.  I've ordered a gasket set from the east coast (of Australia).  Hopefully I'll get that later this week.  I guess the inlet manifold will get a new coat of paint in the meantime.

Yes, a gasket leak could cause it.  Try spraying some volatile liquid, like choke cleaner around the area with the engine running.  If the engine reacts, there's a leak.  Do this outside, you'll be spraying near the distributor where there are sparks.  I usually use an unlighted propane torch to find a vacuum leak, but have had an occasional flash of flame if I allow the gas to accumulate.  No harm done, but my heart rate jumped a little.

John - "The Hoosier Hurricane"

Thanks for all the advice.  It was tricky actually measuring valve lift.  In the end I ran a long straight rule across the rockers above the springs.  They all seemed to move equivalent amounts, but I'd need to confirm that.  It doesn't run rough, although it could be a little smoother.  It accelerates well, but driving it around it sounds like there's a miss.  I'm going to accurately measure all compressions to be absolutely sure.  I cranked the engine for about 20 seconds.  No wetness on any of the plugs on the right bank - a little on the left bank.  I'm wondering whether there's a vacuum leak around the inlet manifold to the head near cylinder #8.  Would that cause this problem?

Twice in my life I have seen where too long of a bolt was used in the last hole back on the left side of the intake manifold. The bolt hole is drilled all the way through, when too long a bolt is used it goes all the way through and hits the push rod on #8 cylinder.

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

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)

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

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.   

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

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"