Misfire at low RPM


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By Nat Santamaria - 16 Years Ago
Hello fellow Y blockers.

I have a 57 Tbird with a 312 that has developed a misfire at idle when in drive. It does in park as well but it is not as severe. When I pull up to a light the car is idling smoothly and then gradually it will start missing. When I pull away it stutters a bit but once I gather a bit of speed it is fine. No high RPM misfire. My plugs, wires are 1 year old. Distributor cap contacts look good - no pitting etc, rotor looks good. I have the Pertonix ignitor and Flamethrower coil. My plugs are gapped at .046. Any suggestions? On another note does anyone recommend the Autolite Platinum spark plugs If I need to replace them. Any advantages? Thanks
By MikeLongIsland - 16 Years Ago
Sounds more like your carb might be loading up.
By Ted - 16 Years Ago
I’m assuming at this point you’ve double checked that the ignition timing is set to spec?  If you’re still retaining the ’57 and up carb/distributor combination, then having the distributor hooked to direct vacuum rather than ported vacuum will force too much ignition timing at low rpms if the ignition timing was initially set with the vacuum line disconnected.  You can test this by just unhooking and plugging the vacuum line to the distributor and if the misfire goes away, then look hard at the vacuum routing you’re using.  I’ll add that even a ported vacuum signal can be inappropriate with a mismatched carburetor or the initial timing too far retarded to begin with.
By GREENBIRD56 - 16 Years Ago
Nat - I've got an idea that Ted is closing in on the right culprit - this sounds to me like a "lean miss" at idle. Which can be had by over-advance at idle speed.

If this engine is timed to spec and has the correct vacuum scheme in place - try raising the primary float level just a hair. Also - you might want to reduce the spark plug gap a bit to say .038 to .040. 

The Pertronix will handle a "hot" coil down to the point of 1.5 OHMs minimum resistance. Are you running the Ford coil and ballast resistor? 

I'vve been running the Autolite AP46's in mine - have a set of 45's (slightly colder) waiting for the right day to go in next.

By rick55 - 16 Years Ago
Whenever I get funny little misses of the nature you describe, I always go back to basics.

Start your engine in a darkened area with the lights off and check under the bonnet to see if you have any sparks jumping from the leads anywhere.

The new silicon leads we use nowadays are great at finding a place to short across as they have a high internal resistance.

The way our engines have the spark plug leads run around the exhaust manifold it is easy for them to touch and short out and the symptoms will generally be exactly as you describe.

If you don't have any spark jumping dig deeper.

Regards
By Nat Santamaria - 16 Years Ago
Hey guys. Thanks for all your input. I will look at some of the suggestions over the winter - Time to put put the old girl away. But in the meantime I reduced the spark plug gap down .034 and it is better already. The car barely misfires and the acceleration is better and the hesitation is nearly non existent.



Many thanks