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Old 08-18-2010, 01:29 AM   #1
Join Date: Aug 2010
Posts: 2
Colder Spark Plugs for 4.2


I have an '87 Wrangler with a 4.2 straight six, 170,000 miles. I have done an MC2150 conversion (1.08), ignition computer bypass, ford large dizzy cap upgrade, new plug wires, and have installed an MSD 6A with MSD Blaster SS Coil. My spark plug gap is .45 with stock Champion RFN14LY plugs.

When the Jeep reaches operating temperature (oil at operating temp), I experience pinging especially in fourth gear under high throttle. I have set timing to the factory spec, but there is still pinging. I have read that when using the MSD box, it is recommended to run a one or two step colder spark plug. Does anyone know which Champion part number would achieve this? I was told to use the RC12LYC but I am not sure that is correct.

Any help would be greatly appreciated. P.S. What timing specification should I use for a setup like this?

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Old 08-18-2010, 10:28 AM   #2
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Location: pinole ca.
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I think the spark plug # is rc9lyc, try running higher octane, change timing by 2 deg.

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Old 08-18-2010, 02:26 PM   #3
Join Date: Aug 2010
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Alright...I've already retarded the timing and the more I push it the more power I lose. Even though it pinged in fourth it seemed to accelerate better. Now it's worse. But I will try pulling it 2 more degrees.
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Old 03-15-2012, 11:06 AM   #4
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Location: Raleigh, NC
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Have you modified your distributor mechanical advance? They are factory-limited to about 1/2 the advance needed w/out the ignition computer hooked they have very slow advance springs.

It's a bit hard to get to, since you have to take most of the distributor apart to get to them....but 5 dollar springs (Mr. Gasket #925D) & elongating the advance slot made a HUGE difference to my 87 (same mods/carb as yours). Took me 1.5 hours total.

Having the proper advance CURVE is what you are shooting for.....not just initial. Mine works great w/ 12 degrees initial, about 34 total & all in by 2200-2500RPM (w/ vacuum advance temporarily pulled & plugged).

Without the lighter springs & full advance capability, it will run like a slug & you'll be tempted to over-advance your initial timing to compensate.....which makes it ping under load.
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Old 03-15-2012, 08:04 PM   #5
Join Date: Jan 2012
Location: Okanagan Valley BC
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Did you use both the MR.Gasket 925D springs or just one spring only?
I have seen advise on this both ways...
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Old 03-16-2012, 08:34 AM   #6
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Location: Raleigh, NC
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I used both, since the stock set up had one lighter & one heavier spring (common in factory setups)...the lighter being about the same as the new springs. So basically I just ditched the heavy spring. It's the combo of springs/weights, even the factory stuff changed depending on suppliers, etc....I'd err on the side of caution. If you go in & there's TWO heavy springs...just switch out one.

To get full effect, you have to provide for more mechanical advance too....the springs just give it quicker. There's a limiting post on the base plate that restricts the total movement of the advance plate. You either have to grind/drill out more room for travel.....or modify the post. I removed the post entirely & fabbed a little bushing that slips around the adjacent spring post & acts as a stop for the advance plate.

BTW, I pulled plugs after a good highway run yesterday to check how the ignition mods are behaving........perfect, light tan color across all 6 & no signs of pre-ignition/etc.

And it's got WAY better acceleration than the stock setup.

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