Stock 1970 SBC spark plug question - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 15 (permalink) Old Nov 3rd, 18, 10:11 AM Thread Starter
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Stock 1970 SBC spark plug question

I am having a problem with my plugs loading up with carbon over time on my bone stock 70 Malibu with 350/300 HP L48. The plugs that came with the car when I bought it 2 years ago are Champion RJ12YC. I am now switching to a AC Delco and I see the Chilton's manual says to use AC Delco R44 which NAPA says is a hotter plug than the RJ12YC.

Would switching to the R44 plug help reduce carbon buildup and are there any other pros/cons of going with a hotter plug? i.e. car running hotter or pinging etc. -Thanks
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post #2 of 15 (permalink) Old Nov 3rd, 18, 10:53 AM
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Re: Stock 1970 SBC spark plug question

How bad is the buildup? How quickly does it occur. Wet? Oily? I'd look at the underlying causes first before changing heat range. Does this occur evenly on all spark plugs?

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post #3 of 15 (permalink) Old Nov 3rd, 18, 10:53 AM
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Re: Stock 1970 SBC spark plug question

If the plugs are black your motor is running to rich. I would not mess with hotter or colder plugs until you get the carb/distributor/timing dialed in first. then replace with the correct plugs for the motor.


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post #4 of 15 (permalink) Old Nov 3rd, 18, 11:45 AM
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Re: Stock 1970 SBC spark plug question

If is working fine and no smoke from exhaust try bringing up your octane rating,Poor combustion temperature with cause this to and run a bit more advance on your timing.recurve kit etc.

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post #5 of 15 (permalink) Old Nov 3rd, 18, 12:57 PM Thread Starter
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Re: Stock 1970 SBC spark plug question

Here are some pics of the plugs. I put about 200 miles on the car after the carb rebuilt by Cliff Ruggles, but I never adjusted the idle mixture screws. A friend told me that even though it's a new carb that's been tested at Cliff's, I still need to dial it in for my car- makes sense. Any guesses as to why the previous owner would have installed a colder plug than the one that's called out in the manual?
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post #6 of 15 (permalink) Old Nov 3rd, 18, 1:45 PM
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Re: Stock 1970 SBC spark plug question

That's only very slightly rich in my estimation. And I'd way, way rather be slightly rich than too lean. Lean = hot. I've seen melted piston tops from being too lean and in your case, I'd continue to monitor things and not change any hardware now.

I'd also change gasoline brand to a Top Tier fuel in a higher octane, as advised above. I'd also run only no ethanol fuel if you have it in your area. It's worth every penny of the 70-80 cents a gallon extra around here. My only problem with no ethanol fuel around here is that it is unbranded - I have no idea what, if any, cleansers or detergents it has in it. So I add some Chevron Techron from an auto parts store.

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post #7 of 15 (permalink) Old Nov 3rd, 18, 3:04 PM Thread Starter
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Re: Stock 1970 SBC spark plug question

Thanks Rick. Let me back up. The root of the problem is that I drove my car 25 miles to a friends house with no problem then on the way home, about a mile from my house sitting at a stop light, the car started to stumble and stalled- totally out of the blue. I had to feather the gas pedal and limp it off the main road into a parking lot. I ended up having it towed home. There was no prior indication that anything was wrong. The points, plugs, wires, cap and rotor were put on about 500 miles ago, and still look fine. The quadrajet was built by Cliff Ruggles back in June. I also tested the coil primary and secondary resistance with a multi meter and it checks good. If it's not a plug fouling issue what do you recommend?
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post #8 of 15 (permalink) Old Nov 3rd, 18, 3:55 PM
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Re: Stock 1970 SBC spark plug question

Has the car started since you had in towed home? Champion plugs suck, but they're not the culprit of the stumbling/stalling issue. That sounds like a distributor/ignition issue.

Get that figured out first & then for the $20 it costs, I would throw the R44's in there & see how it responds. I'm betting you'll be right where you want to be.

If you're still running rich, then you know you need to adjust your timing & your carb.

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post #9 of 15 (permalink) Old Nov 3rd, 18, 3:56 PM
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Re: Stock 1970 SBC spark plug question

Your 1st and last question are not even closely related.lol.Number 8 has a bad seal or guide.Carbon buildup takes 1000 of miles.Your asking about deposits,totally different in the short term.Check your vaccuum at your carb with a vaccuum guage for tuning ,lots of articles on line..Check for fuel pressure 5-7 psi.Check your timing as stated.

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post #10 of 15 (permalink) Old Nov 3rd, 18, 5:07 PM Thread Starter
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Re: Stock 1970 SBC spark plug question

Sorry for the confusion. The car starts but won't stay running. It didn't show any prior symptoms. Just bang- stumbling and stalling while sitting at a light. I will re check timing, vacuum and fuel pressure, etc. as mentioned..- thanks
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post #11 of 15 (permalink) Old Nov 4th, 18, 12:23 AM
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Re: Stock 1970 SBC spark plug question

You cannot test a coil unless it's up to operating temp, otherwise, it'll probably show it's OK. Good luck!

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post #12 of 15 (permalink) Old Nov 4th, 18, 4:52 AM
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Re: Stock 1970 SBC spark plug question

Does the engine still have points ign? If so, that is probably causing the no-fire. If you had HEI, it would blast right through those contaminate plug tips & the engine would run happily.
If you have converted to HEI, are you sure you are getting 12v to it? Resistance wire MUST be bypassed for HEI.

Correct heat range plug is NGK -5, which equates to a 12 in Champions, so I doubt it is a heat range problem.

Carb idle mixture should definitely be adjusted if that was not done.
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post #13 of 15 (permalink) Old Nov 4th, 18, 7:29 AM Thread Starter
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Re: Stock 1970 SBC spark plug question

I tested the coil cold since I can't keep it running long enough to get it hot. The car has points which look almost new. Maybe I'll swap the coil from my 65 GTO to see if it's a coil problem. Thanks for the additional tips.
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post #14 of 15 (permalink) Old Nov 5th, 18, 5:12 AM
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Re: Stock 1970 SBC spark plug question

Your symptoms are also indicative of closed up points gap. The gap closes as the points wear. Make sure you have points gap.
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post #15 of 15 (permalink) Old Nov 5th, 18, 9:27 AM Thread Starter
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Re: Stock 1970 SBC spark plug question

The car is running! It was a bad points condenser, which only had about 300 miles on it. Thanks for all of your help.
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