I can not get my 390 Ford engine to "hit" on cylinders 2, 3, 5, and 8. The engine has a new cam, lifters, heads rebuilt with new valves, seats, and guides. The vacuum is only about 10 at idle and the needle shakes. Compression on all cylinders is about 115. I have tried the following: Switched spark plugs from firing cylinders to those not hitting, new plug wires, new spark plugs, new points, condenser, coil,and rotor, switched distributors, switched carburetors (4 barrel Edlebrock to 4 barrel Holly and back) , watched the rocker arms move while running the engine with the valve covers off. I have jumped across the resistor momentarily with no change, Voltage at the coil is around 11 without the jumper (power going through the resistor). The valves seem to be seating OK, I can turn the push rods with the lifter on the low part of the cam and there is no "play" with the engine running. I can't measure the valve lift but it looks OK idling, all rocker arms seem to have the same travel. When I first started the engine after the rebuild (rings, heads rebuilt, cam, lifters, timing chain and gears). The engine seemed to run OK and the vacuum was at about 17 at idle and steady. After driving about a mile the four cylinders no longer hit.

I have also hooked the plug wire from a non-hitting cylinder to a spark plug laying on the engine and the spark looks fine. Timing is at 6 BTDC. I have hooked a timing light to each plug wire and the flash is even (consistent)in all cases.

The engine ran poorly prior to the overall and runs about the same now. Sometimes it backfires back through the carbureator as if it is too lean. I have a manual choke and apply the choke does not improve how the engine runs. I do need to choke it while the engine is cold but not after warming up past 140 degrees.

I give up!! Can someone give me some help?

 

Look for either a big vacuum leak or a restriction in the intake manifold . The intake bore you should be checking is the one on the driver's side. I am pretty sure the cylinders with the possible lean condition are served by the driver's side bore.

 

Thanks Hanky! I have not been able to locate an external leak using starting fluid. I think I will have to pull the manifold. Is the sheet metal shield on the bottom side of the manifold necessary? I'm thinking that I might have a crack on the bottom of the intake manifold.

 

I suppose it is possible. Let me know what you find. Thanks