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Old 02-05-2007, 11:14 AM
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Spark plug selection

I have a 383 stroker, 461x iron heads, mechanical dizzy, holly 3310 mechanical secondaries, light cam (13"hg at 750rpm idle) and I am running Autolite 85 plugs. I do a fair amount of driving on city srteets, not too much hot rodding and been thinking maybe I should go up in temp on the plugs. What do you think?

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Old 02-05-2007, 04:18 PM
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Hi jsdailey,
Are you having problems with spark plugs fouling?
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Old 02-05-2007, 05:19 PM
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Yep, they're loading up with casual driving, not bad, but enough that I'm thinking about it.
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Old 02-05-2007, 05:55 PM
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maybe try another brand? ive had good luck with accell u grooves and ac delco on hp engines.
you could also have too much carburetor
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Old 02-05-2007, 06:04 PM
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run141, pretty sure my carbs right on. Maybe another brand, don't know yet.
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Old 02-05-2007, 07:09 PM
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Try a NGK medium range non resistor (racing) plug.
If your plug now is a medium they also make a medium hot.
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Old 02-05-2007, 08:30 PM
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I use Autolite plugs exclusively with very good results.

You donít have many choices with a resistor type power tip plugs like you
are running. The power tip plug wasnít invented when those heads were
made. Autolite does make an 86, one number hotter. I donít see a problem
running them when crusing around, the plugs donít get very hot with light
throttle. If you do decide to run a few WOT blast, you could pop a set of
colder plugs in for safety.

Another thing you can try is to recurve the distributor adding a little more
initial and less centrifugal advance. That alone might clean up the plugs.

What type of ignition, are you getting a hot enough spark?
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Old 02-06-2007, 05:51 AM
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AC Delco MR43LTS.

Vince
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Old 02-06-2007, 09:33 AM
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another option is Auto lite .... AR25 gap .035 - .050, I use them, run a 383 stroker, radical cam, bouncing 6-7 hg @950. You just got to love the 383 stroker motors, they are fun and fast

Last edited by pepi; 02-06-2007 at 09:39 AM.
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Old 02-06-2007, 10:19 AM
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what are the present plugs gapped at and what is the KV rating on the coil
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Old 02-06-2007, 10:42 AM
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Present gap is .45 and I'm running a Mallory Unilite (with ballist resistor) with their Universal treet & Strip ignition coil.

Universal Street and Strip #29440
Primary Resistance 0.6 ohms
Secondary Resistance 12.3K ohms
Maximum Voltage 55,000 volts
Inductance 7.3 mH
Turns Ratio 105:1
Peak Current 100 mA
Spark Duration 500 uS

Does this help?
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Old 02-06-2007, 11:09 AM
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due to the resistor, you don't have enough volts to the coil for a good spark at .045 at idle...... you are at approx 9V at idle which would call for a approx max gap of .040

the coil can handle the .045+

try .042 as a compromise

a hotter plug at .045 will still be a weak yellow spark, due to the 9V

best(bang for the buck?) cure is put a HEI dist in it, 14V to the coil at idle with a .045 gap is roughly a 15-18,000V "blue" spark

work on getting the idle Hg higher, carbs want 15Hg to atomize the fuel, less than 14 is definitely in the "possible puddle" zone at idle

Last edited by red65mustang; 02-06-2007 at 11:35 AM.
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Old 02-06-2007, 11:22 AM
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Hi jsdailey,
If youíre not running a capacitive discharge box like the
MSD 6AL, get one and your problems will likely go away.

pepi mentioned autolite AR25 plugs, they have a taper
seat, these won't work with the early iron heads.
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Old 02-06-2007, 02:49 PM
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jsdailey,

bear with me.....to many irons in the fire......edited my post twice above

next question: what is the CR and/or comp test psi and what is the base timing set at
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Old 02-06-2007, 04:53 PM
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Besides being fouled wet, what is the color of the plug?

I'd probably try the hotter first.

Even at 9 volts 41,000 volts is more than enough to fire a spark plug at 4,000 volts. If you have a poor looking spark there is some other problem.

You could also try some spark plug tricks to make the spark sharper.

Like the grooved tip electrode, and filing the outside wire back to 1/2 the distance across the center electrode and then rounding the end. This ushrouds the gap and makes the fire sharper. A lot like splitfire type plugs.

Nothing will help as much as an MSD-6 multi-spark box. It will make life much nicer below 3000 rpm.
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