Low RPM Misfire


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By Nat Santamaria - 16 Years Ago
Hi guys. I am experiencing a misfire at idle. The car idle smoothly in drive waiting at a light and then slowly it will start to idle a bit more uneven and then it starts to misfire. I have narrowed down to #5 cylinder. I checked all the plugs and this one is black and sooty. All others are clean. Ignition wires are new, I replaced the spark plug and distributor cap. Can the rotor be an issue? Any comments or suggestions?



Thanks
By oldcarmark - 16 Years Ago
Nat,I don't think your rotor is the problem.Was that plug getting oily before you changed things?If there was a problem with that cylinder before you may have to go hotter plug in that cylinder.Do you still have a wire from the set you removed that you can put on the #5 cylinder temporarily and try it?Just because wires are new doesn't mean you can't get a bad wire.Is #5 wire seated into the cap?
By simplyconnected - 16 Years Ago
What engine, and how many miles are on it?  Is the same plug fouling or just misfiring?  An oil-burning engine will foul plugs faster than anything.  You can buy spark plug extenders for $2-3/ea., but that is a temporary fix.  They work, but they also lower your compression in that cylinder.

Low voltage produces misfire, too.  How long are you idling before the misfires?  Does it straighten itself out after running high rpm's for a while?  We need more info.  - Dave

By Nat Santamaria - 16 Years Ago
It is a 312. The car had frame off resto in 1994 and has low miles on the rebuild. Compression is 150 in all cylinders. The plug is not oil fouling but not getting a full burn on the fuel. The #5 Plug is the only one that has a sooty deposit. The problem is intermittent. Once I get up to speed it seems fine.

Accelerating from a dead stop it is a bit sluggish but once it pick-ups a bit of speed it is better. It has new plugs, cap wires.
By charliemccraney - 16 Years Ago
Nat Santamaria (4/15/2009)
It has new plugs, cap wires.




Before or after you noticed the problem?
By Brodie - 16 Years Ago
Nat:

What plugs are you running? I had a similar issue when I picked up tune-up parts at the local national chain parts house based on application. Evidently the Champions they sold me were the incorrect heat range and would soot up in no time flat. Swapped them out for some hotter Autolites and problem disappeared.

By Nat Santamaria - 16 Years Ago
Hey guys. I disconnected the the one suspect wire and re-connected it. The car ran very well. I know the connections were secure - maybe there is a break in the wire or connection - I ordered the #5 wire from Casco to be sure. On another note are there any advantages to using the Autolite Platinum 46 plugs over the standard Autolite 46 plug? I have a Flame thrower coil and Ignitor in my ignition.



Thanks