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Random Misfire

int77345

Member
my scout has developed a random misfire while accelerating or when the engine is loaded as in climbing a hill. misfire is not consistant, it might not happen for 10min or it might happen for 10 min straight

1977 scout II 345 edelbrock carb prestolight distributor with igniter

over the weekend i changed plugs and wires, this didnt have any effect on the misfire. it even feels a little more noticeable.

next step-- i have a distributor cap and rotor coming. its been 20 years give or take sense i changed them i dont remember. can a cap or rotor cause a random misfire ? both cap and rotor look really worn, all terminals in the cap were blackish and the rotor was to. i cleaned them got them shinny, no effect, still misfires.

coil--- what are the symptoms of a failing coil ? are there any ? my coil was last changed in 1997 i remember doing that one.

igniter--- what are the symptoms of a pertronix igniter failing if there are any ?

any other ideas on a misfire ?

thank yall for any insight !
 

Robert Kenney

Super Moderator
Can you describe the miss fire?
Is it like one cylinder is missing for some period of time or totally random?

Coil can fail a number of ways. The secondary (high voltage winding) dielectric (usually oil) can start to fail from high resistance plug wires. That is usually some what sensitive to temperature, worse when hot and random in miss profile. Look for oil being pushed out at the top crimp. Even an open in the secondary won't last long due to the fine wire used in the winding.
The primary winding is far more robust and usually fails open or short to ground. Not so much come and go but a short fail period as the current is high and the open becomes quickly total fail.

The Pertronix ignitor is a transistor switch, will usually be temperature sensitive. Occurring more so after coming up to temp.

Now coil power supply, and dizzy ground are. Not remembering if the Ignitor has it's own ground that connects outside the dizzy. Are you running a coil resistance?.
Test your loaded coil voltage. Remove the pertronix from the Neg coil terminal. Ground the coil Neg terminal and with the ignition on, read the voltage on the positive terminal. Don't leave the power on for more than 10 seconds to be safe.
 
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int77345

Member
the occurrence of the misfire is random.

the actual misfire feels like one of two cylinder skipping.

no resistor to the coil that i know of. no ceramic resistor any way. and the scout has been re wired so i dont think it has a resistor wire in the wiring.
 
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Hondo

Member
Unless you've installed a new one recently, might want to check your fuel filter as well, especially since it is happening under load. Also the first ignitors tend to die if you leave the ignition on with the engine not running.
 

int77345

Member
Roger on the igniter. Iv never left the key on.

Fuel filter is a clear one. And looks clean No big junk in it But while I’m at it. I’ll change it to
 

int77345

Member
----update---

parts came in a day early. cap, rotor, and coil.

installing these three cured the misfire. i suspect the cap and rotor were to blame. maybe it was longer then 20 years sense i did the cap and rotor.??? comparing the old cap to the new cap was night and day. all the contact parts in the old cap were severely worn down, especially the one in the center that feeds the rotor. there as almost nothing left compared to the new cap.

oh-- my distributor was 180 degrees off, or the reference chart i used was 180 degrees off either way, i adjusted the position of the wires accordingly

back to running good, as good as an original 40 year old engine will run.

side note--- does any one else scout like a lot of ACCEL pump. mine seems to like the richest setting on the carb.
 
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