Misfire

[41fastback]

Just rebuilt '76 500. Before rebuild it had what sounded like a misfire at
the exhaust pipes. Engine had and still has Edelbrock Performer manifold
and 750 carb. Rebuild has MT10 cam. Misfire is still there. Spark plugs
have carbon on them. I adjusted carb screws, still no change. Is it
the carb or maybe stock original distributor that causing this? or something else?
Forgot. The rpms at idle will fluctuate slightly on tachometer.
Thanks, PS. Are Holley carbs better to go with than Edelbrock?
Tony

[DOC]

Do you have a vacuum guage? Manifold vacuum at idle should be about 17 (4200 ft elevation). Rev the engine and max vacuum on deceleration should be about 28, both readings steady. Slow timing will be low idle vacuum, fast timing will be high vacuum with a very shaky indicator. A pulsating indicator in rythem with miss indicates a intake valve problem. You can also adjust idle mixture with the guage. I don't like the AFB style edelbrock carb, but the QJ style is a great carb, they just come from from the to fat for this altitude.

DOC

[curtis73]

Doc has really good advice. The combination of the words "misfire" and "fluctuating idle" always signal an ignition avance thing in my head. Pull the cap off and check for proper operation of the mechanical advance; make sure both springs are there and that the weights move freely. Do a test on the vacuum canister to make sure its not dead. With the cap off, reach in and push the vacuum advance rod out into the canister. Plug the vacuum nipple with your finger, a vac cap, or a piece of tape and let the rod go. It should stay put or move a very slowly. If it goes back to its original position, replace it.

If that all checks out, trace all of your vacuum lines for leaks. I doubt its a vacuum leak at any of the gaskets since the problem is the same both before and after, but just for giggles check for leaks by spraying carb cleaner, brake cleaner, or starting fluid at the gasket mating surfaces while the engine is running. If the idle changes, rises, or smooths out, you've found your leak. I guess its possible that the carb base or manifold has a crack, but the spray test will diagnose that too.

In my opinion, Holley carbs are great for all out WOT power. They run fine and dandy on the street, too, but they give very little time or technology to drivability and mileage. They are sometimes a little tougher to tune and they have gaskets below the level of fuel. I personally used one on a Ford 460 that I liked, but I switched to an Edelbrock; I got tired of draining fuel bowls, poor mileage, difficulty tuning and just general problems. BG's new Demon carbs based on Holley designs are getting much better, but for the street, my absolute first choice is the Qjet. No other carb is as good at linear metering. They also boost torque with their tiny primaries and don't usually give up any HP with the huge secondaries. I have them on every GM V8 I own and I find them to be incredible. My second choice would be Edelbrock. They don't quite as well with transition metering, but they are incredibly simple and easy to tune for the DIY tuner.... and don't let any advertisement fool you, no carb runs great "out of the box." I have my Qjets custom tuned by Jet, and they need some tweaking after they're on the engine since every engine is a little different.

[Al from MTS]

I've had issues with low RPM misfires with brand new junk HEI module's. Had one in a commercial rebuilt distributor that exploded a muffler! It would drop cylinders so bad below 1700 that the muffler filled with unburnt fuel. Above 1700 it would backfire hard as the cylinders came back to life.
That was the worst case I saw. Had many milder versions of this over the years. In these cases I was able to clear it up with a Delco module. Each case had an off brand generic module that was at issue.
All the other info in the thread is great info. This is just one more thing to look at.
Hook a timing light to each plug wire one at a time. Just watch for steady pulses from each cylinder as you rev the engine up and down.
If you are having the module issue, some cylinders will skip a few beats as you watch the light. A dead giveaway to this problem.

[41fastback]

Thanks for all GREAT ideas so far. Started by pulling distr cap and rotor.
Weights move freely but what's this rust colored dust on weights and springs? It's also on underside of rotor. I've seen this before and just
cleaned it off, but it comes back. Any ideas? Next is vacuum gage and
timing light as suggested.
Thanks Again,
Tony