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I have a 1968 Chevelle with a 1974 350 chevy with HEI, and a 600 single feed holley carb. I ran through more spark plugs in that 350 than my father runs through fuel for his 307. In the last 5 years, I replaced dozens of spark plugs. I have to tell you first about how often this car has been run, before you trying to tell me how to troubleshoot it. The car hasn't seen the road in 10 years and is run at least once a week mostly idling. Does excessive idling with a holley carb cause spark plugs to be fouled out? When the most of the spark plugs foul out, the carb and exhaust backfires loudly. NOW heres the kicker......when I install new AC delco R44T gapped at 0.35 (I have that memorized now), The 350 purrs like a kitten and runs great. Over the past 5 years that I owned it, the engine recieved a great, but used 350hp 350cuin cam with matching lifters and push rods. I know it can't be something inside the engine due to when I change spark plugs, it runs fine. I have a feeling its in the holley carb. Does excessive idling on a rebuilt holley cause fouling of spark plugs?
 

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When you say fouling is it black and sooty {too much fuel} or black and and wet{ too much oil}.Holleys will idle as well as any carb "when set right".For a low speed engine you may have to go to a hotter plug,an R44T is a truck plug I would try R46 with the extended tip...FRED
 

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interesting, I had this weird problem with my 355, which was it kept fouling the #1 and #2 cylinders. After i did a leakdown test I concluded the cylinder sealing was fine, so I gave up on AC plugs and tried autolites. After switching to autolite 25s haven't had a problem since. My conclusion is AC plugs are crap.
 

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Steve I had a 78 GMC pick up with 300,000 miles on it. It ran so well I forgot to change plugs,don't remember how long they were in there.One day on a hard pull with my 28' trailer on one of the plugs blew apart it had been in the engine so long the metal that holds the porcelain rusted away.R44Ts, come to think of it I better change them in my 90 its been about 3 yrs also R44Ts.Heat range is the way to choose plugs not manufacturer...FRED

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hmmm, my friend who owns a camaro had the same problem with ac plugs as I did, so he swiched to autolites, and it cleared up his problems, however; if the truck plugs work better than standard ac plugs more power to yah. http://fastlanenitrous.com/wwwboard/ also seem to favor autolite plugs over ac plugs. They listed NKG plugs second best.
 

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Thats my point they work too good, but that doesn't stop them from rusting.Maybe if I switch to Auto Lites they will last longer than 4yrs but will they rust?...FRED

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I'll have to disagree. If you go with a plug thats too hot you could cause detenation, pre-ignition problems, melt your pistons, and or burn a valve. If the truck plug is the correct heat range and it works better why worry if it's not in a truck. Same thing about one plug manufacture vs. another. If plug X beats plug Y because of better manufacturing with closer tolerences and better material use the superior plug.
 

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Well the plugs are black, sooty and smell of raw fuel. No oil at this point. Think its the adjustment on the carb? I tried adjusting it before, but its hard to determine what is correct.

HELP!!!! I am tired of wasting R44T's
 

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Forgot to mention....they are also wet from fuel I assume
 

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Sounds like it's too rich. Turn the idle mixture screws clockwise until it runs poorly, and then turn the screw a quarter turn counter clock wise.
 

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A little info on R44 and R44Ts the heat range is the same, R is tapered seat, 44 heat range ,T means it has larger centre electrode and ground,which add to the life of the plug.I would have to agree, could be flooding a hotter plug in an engine that is idling a lot as stated will not harm it.I have question why is this engine idled so much...FRED

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