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By randyh - 7 Years Ago
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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
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By Daniel Jessup - 7 Years Ago
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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.
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By randyh - 7 Years Ago
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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.
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By Ted - 7 Years Ago
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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
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By randyh - 7 Years Ago
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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.
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By Tedster - 7 Years Ago
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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.
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By charliemccraney - 7 Years Ago
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Is the -C cam the high lift cam? If so, then 15in hg is in the ball park..
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By randyh - 7 Years Ago
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I don’t have the specs with me right now but it is the high lift cam
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By Hoosier Hurricane - 7 Years Ago
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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.
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By charliemccraney - 7 Years Ago
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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.
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By randyh - 7 Years Ago
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John, I have the original blower carburetor and dual point blower distributor. At idle there doesn’t seem be much difference with the vacuum line hooked up or off. 15 seemed low to me, but if that is not that far off for this cam, the miss is probably ignition and not leak or cam timing.
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By Gene Purser - 7 Years Ago
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The way the plug wires run through the insulators it is awfully easy to get wires crossed.
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By randyh - 7 Years Ago
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Rechecked spark plug wires for firing order all looks correct, checked dwell on both sets of points, both are at 35 degrees. Will try to check compression tonight.
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By charliemccraney - 7 Years Ago
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That cam has a slight lope and with my combo, (naturally aspirated) doesn't start to perform until about 1800rpm. So idle to 1800rpm (mostly closer to idle speed) it does "misfire" but that's just the nature of the beast - it's how cams tend to work as they get bigger. This is more pronounced when the vacuum advance is not used, which I think is the case with a supercharged engine, is that right? Make sure you're not confusing something like that as a problem.
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By randyh - 7 Years Ago
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The distributor used with blower does use a vacuum advance together with centrifugal advance.
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By KULTULZ - 7 Years Ago
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"The distributor used with blower does use a vacuum advance together with centrifugal advance."
... hmmpf ...
Interesting. So it uses a H4000 with a 57 dual advance distributor? Is the advance signal ported or manifold vacuum?
Did any F-CODE variation use only a mechanical advance distributor (HOLLEY - MALLORY)? Was there a STREET VERSION and a RACE VERSION?
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By randyh - 7 Years Ago
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I only know of the factory distributors. The tubing in your picture running from front of carb to distributor with the large loop is correct.
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By randyh - 7 Years Ago
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Issue is not compression, cylinders read, 130, 132, 135, 135, 140, 133, 130, 135 in order 1-8. With the blower heads the compression should be 120-130.
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By Sandbird - 7 Years Ago
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Could be rotor phasing is off in the distributor, I've heard talk that people fabricate the dual point breaker plates from the stock single point ones. That could be pretty tricky to do that accurately. I would check it dynamically using a timing light with the hole in the cap method.
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By 57RancheroJim - 7 Years Ago
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I may be off base here but 35 on each set seem high. I've only run factory dual points on my FE and set at 28 each and then the total comes out about 32. Too much dwell may not be letting the coil build up enough and causing a week spark?
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By randyh - 7 Years Ago
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Jim thanks for post, been too long since I worked on a dual point distributor, need to go back and check these. The spec says 0.014-.016 gap and both are to be set same. The total dwell, both points should be 33-36 degrees. There is no dwell spec for primary point set, at least in documentation that I have.
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By 57RancheroJim - 7 Years Ago
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Sorry, I thought you meant each set was at 35. Your 33-36 total should be good.
Just an after thought but I had a Mallory dual point at one time and the engine started missing, after isolating one set of points it smoothed right out, never found out why that happened or what was wrong with the second set of points..
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By scicala - 7 Years Ago
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The specs on the original B7A-C cam are .447" lift (intake and exhaust) and about 290 degrees of advertised duration (intake and exhaust).
So 15" of vacuum at idle sounds logical to me.
The "C" cam is the optional cam for the dual quad and blower engines. So a lot more radical than the factory installed dual quad and blower cam (B7A-B).
Pretty sure the B7A-B cam is the same one in the 312 single 4 barrel engine too (245 HP).
Sal
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