Misfire at idol

[Rudolph]

I have a 42 Mercury with a pertronic points replacement and new leads and NGK plugs, runs really well but after a long drive the other day it developed a misfire at idol like someone is switching the ignition off and back on, if i am in gear and let the the clutch out till the engine get some load and then push the clutch to the floor seems to trigger it, it dose not miss a beat under throttle,

I did notice one of the plugs has a burn mark down the side of the insulator on the outside, I tried a different cap and rotor, I gave the pilot mixture screws a twiddle,

I was wondering if my vacuum brake on the dizzy witch doesn't seem to work was sometimes letting the ignition retard then advancing ?

[51 MERC-CT]

" NEW LEADS?" Unless you are using 'Spiro-wound resistor plug wires' there could be interference that may cause skipping.

[AnthonyG]

I did notice one of the plugs has a burn mark down the side of the insulator on the outside, I tried a different cap and rotor, I gave the pilot mixture screws a twiddle,

Sounds like u found the issue! Intermittent arcing @ that plug. Plug wire to plug insulation allowing a short from there to some nearby grounded point.

[tubman]

Some times, a new high power ignition source is not the best to be used when the rest of the system is stock. It was just not designed to carry the high voltages that these systems can produce. I had a 276 Merc in a '36 three window about 25 years ago, and installed a complete MSD setup, distributor, 6A Box, and all. That things would burn through the distributor cap coil tower in less than 1000 miles. If the engine started missing, it was time for a distributor cap replacement. I used stock GM replacement caps from NAPA (which fit the MSD distributor at less than half the price), which may have been part of the problem.

I am not saying this is your problem, just stating that one should strive for all systems to be kept in balance. Trying to push that much voltage through an otherwise stock ignition system can cause problems, much like installing a 454 Chevy against the stock transmission using an adapter plate; something's gotta give and it won't be the high tch new replacement.

[drolston]

If you are using the super high voltage Pertronix flamethrower coil, that could be just too much. Try switching to the blue Bosch coil. Cheap fix.

[Rudolph]

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Yes I got the distrubtor module and coil as a kit and thought the coil was too much, it burned thrugh one cap already but that was mostly because the rotor clipped the cap and bent the end,

I am running resistor leads because they say normal leads can create noise that interferes with the module,

Would any 1.5 Ohm internal ballast coil be ok to use?

[Rudolph]

I found the problem, i had put fiber washers under the banjo for the vacuum line to the distrubtor at the manifold end and it was loose, I put copper washers with a dab of locktite on the threads,

I need to adjust the vacuum brake for the new motor as it has a lot more vacuum

[Tinker]

I prefer idle to idol's.

Glad it worked out.