Hoping someone can provide some advise on where to look to correct engine misfire problem.
Engine is ‘57 factory McCullogh supercharged 312, running the -C camshaft. Was recently assembled.
Here is what I have found so far.
Vacuum is steady but reading 15” at idle.
Initial timing 10BTDC.
Plugs all look good but possibly a little lean.
Checked for vacuum leaks with propane, no leaks found.
Cold valve lash in spec for -C cam intake .022 and Exhaust .027.
Teapot Holley Carburetor was rebuilt and run tested.
Cam was not degreed when installed, set at factory location.
Suggestions on where to look next are appreciated. Randy

Randy,
Can you describe your misfire? Is this at idle, off-idle, under load, throughout the RPM band?
Usually misfires end up being something in the ignition circuit.

Daniel Jessup

Lancaster, California

aka "The Hot Rod Reverend"
check out the 1955 Ford Fairlane build at www.hotrodreverend.com

Somewhat rough idle, was missing pretty bad at part throttle, didn’t try more RPM as this miss was pretty bad. Had a couple of backfires through carb.

I’ll suggest a cranking compression test and/or a leak down test.  While the plugs are out for the compression test, look at them for one or two that’s running differently.  This may help to isolate the cylinders to look at in detail.  A leak down test can isolate if the issue is with the rings, intake valve, or an exhaust valve.  With the valve covers off, double check that all the valves are working up and down as they should be.  Could be as simple as a pushrod off or as bad as a flat exhaust lobe.  Double check the valve lash if you haven’t rechecked it since this all started.  If you've already double checked some of these, then simply disregard.
  
Here are some other items to consider:
Too much vacuum to distributor at idle – lines hooked up correctly?
Spark plugs – new? old?
Spark plug wiring order incorrect
Spark plug wires tie wrapped together (induction firing)
Faulty Spark PlugWires – new? old?
Stopped up fuel circuit(s) in carburetor
Crack in distributor cap

Lorena, Texas (South of Waco)

Thanks for input.
Just rechecked valve lash on all valves and all are operating. All plugs looked about the same but maybe a little too white.
Vacuum off distributor when initial timing set to 10degrees. Vacuum line is as original from front of carb above throttle plates.
All new plugs.
All new wires in the stock type insulators none tied together.
I will try another distributor cap, check compression and recheck firing order. don’t have a leak down tester, yet.
I was leaning to vacuum leak as it was low and plugs are a little lean but can’t find one.
Will post what I find. I am hoping that it isn’t cam timing.

An ignition scope (oscilloscope) would sure be very useful in this situation, the patterns will show defects in plugs, wiring, coil, points, condenser, worn bushings etc, and isolate a bad cylinder.

Is the -C cam the high lift cam?  If so, then 15in hg is in the ball park..


Lawrenceville, GA

I don’t have the specs with me right now but it is the high lift cam

You  mentioned you have a teapot.  What distributor are you using, the blower distributor or some other?  Is the teapot a genuine blower teapot, or some other application?  The reason I'm asking these questions if because of a mismatch between the distributor and carburettor could cause excessive spark advance under certain conditions causing rough running.  Try unhooking the vacuum advance and see if that changes the running.


John - "The Hoosier Hurricane"

I have the reproduction that Mummert produced about 2 decades ago.  My vacuum has been as low as 14 and as high as 17.  The difference was a combination of carburetor, ignition, and intake changes.  Not necessarily anything wrong, just changes that had different results.


Lawrenceville, GA