What Spark plug
#1
Huh   I am back and came across something that I am trying to figure out just what spark plugs to put in my 71 Mach 1, 351C.  When I bought it 7 years ago and changed the plugs, it had 25's in it so that is what I put back in it.  Being parked most of last winter, was getting ready to get a bunch of work done and went out to get it started and that was a no go.  The next weekend, tried again and finally pulled the plugs out and they were all black, didn't look good so I decided to put in a hotter plug and went to the 26's.  Put them in and the car started right up. Is blubbering at times but stills run pretty good so I decide maybe change the plugs again so went and got another set of 26's, but was looking in my Chilton's book to make sure I had the gap right and found out in the spec chart that it said this 351C used AFR-32's. So now I am wondering what plug to put in and also the brand.  In the book, it also said that if you do a lot of highway driving a colder plug would be the way to go and if you do mostly city driving, go to a hotter plug, and I do mostly city driving but not afraid to get it out on the highway..  My 351C is the 300 hp motor, I have headers on it and put a Holley 650 Avenger dual feed carb on it and as far as I know, that is all been done to it.  So anyone have a clue what plug/brand/ and gap to put in the motor?
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#2
If you have the stock ignition system, leave the gap at the factory recommendation, I believe that is 0.035".  I like either Autolite or NGK spark plugs.

I would use the stock heat range for your car, increasing the heat range will increase the chances of pinging (preignition) which is bad for any motor.

Since your plugs were carbon fouled, the motor is either running too rich or something in your ignition system is too weak, i.e. bad ignition coil, or more common the resistance in the spark plug wires is too high.  Cars of this era didn't have today's powerful ignition systems and spark plug fouling was much more common where as today it is unheard of.  Ignition timing too retarded will also cause a motor to run too rich.

One thing I would do is freshen the carb.  Most people with Holley carbs will freshen them up once a year.  Something as simple as an air bleed being plugged or partially plugged, or leaking power valve gasket will cause it to run too rich.


After you know your ignition system is okay, and the carb is in good condition and properly adjusted, if it still runs too rich, then we can look at carb rejetting.
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#3
(08-31-2019, 02:30 AM)1969_Mach1 Wrote: If you have the stock ignition system, leave the gap at the factory recommendation, I believe that is 0.035".  I like either Autolite or NGK spark plugs.

I would use the stock heat range for your car, increasing the heat range will increase the chances of pinging (preignition) which is bad for any motor.

Since your plugs were carbon fouled, the motor is either running too rich or something in your ignition system is too weak, i.e. bad ignition coil, or more common the resistance in the spark plug wires is too high.  Cars of this era didn't have today's powerful ignition systems and spark plug fouling was much more common where as today it is unheard of.  Ignition timing too retarded will also cause a motor to run too rich.

One thing I would do is freshen the carb.  Most people with Holley carbs will freshen them up once a year.  Something as simple as an air bleed being plugged or partially plugged, or leaking power valve gasket will cause it to run too rich.


After you know your ignition system is okay, and the carb is in good condition and properly adjusted, if it still runs too rich, then we can look at carb rejetting.
Thanks for the info!  I did put in NGK after some more research and actually, made a big difference.  Still think the car is running a little rich and will check on the jetting and see if rejetting will help.
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