After pulling the K&N air filter out and realizing it was so clogged you couldn’t even see daylight through it, I think I may have found a possible cause for the cars crappy running state of lurching under load, misfiring, and cutting out when trying to take gently take off during test drives around the block.  The filter has been replaced but as the car still wants to sputter and lurch occasionally, I’m theorizing that the plugs are fouled up pretty badly.

My cat has a 351W with a 383 stroker kit and Edelbrock Victor Jr. heads. I’m currently running Autolite 3923 plugs and I pulled one plug to check it out. It’s blackened pretty uniformly. Should I stay with the 3923’s or move up to a slightly hotter 3924? I am not using forced induction, just basic multi port EFI. Also, what would be the recommended gap? I’m also assuming that one should not apply excessive torque when reinstalling plugs into aluminum heads. Any suggestions on installation techniques?

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i've always German torque, good&tight. tight enough that i don't want them to fall out, not tight enough to break anything, and always use a proper spark plug socket with some kind of rubber protector inside it to prevent cracking the plugs.

here's a page that goes over it, i'm sure there are many more for looking


they actually make a specific torque wrench specifically for spark plugs! if you're not familiar, it's not hard to reach 20 ft-lbs with a normal 3/8 drive ratchet, no gorilla tools needed. most guys use a dab of anti-seize on the threads to prevent galling on the next service.


how / why would an EFI system run rich?

if the 02 is reading lean due to an exhaust leak and keeps telling the injectors the engine needs more fuel?

or the 02(s) is past it's prime or fouled and not sending good info


@4vandproud The exhaust on the car is not original. At some point a set of Speedforce Racing 180 degree stainless headers were installed in the car along with a completely customized exhaust that doesn’t use the original exits. The O2 sensor was simply removed and never put back in the car so the EFI had absolutely no idea what the O2 levels in the exhaust have been. In reading the configuration of the ECU, it appears they just stuck it in Open Loop mode and left it.

This last weekend, I had the appropriate bung welded back into the exhaust and wired up the appropriate sensor for the car, and re-enabled Closes Loop mode in the ECU. However, I have no idea how long the car has been run between the time the sensor was removed and when I put it back into the car. What I do know is that prior to installing the sensor and enabling closed loop mode, the exhaust had probably enough unburnt fuel in it to ignite the exhaust at the tailpipes.

With the brand new NGK 24300 O2 sensor back in the car and closed loop enabled , I’m seeing it run much better at idle and it doesn’t smell as badly of gas as it did before. But I’m pretty sure the damage is done to the plugs as I still get a fuel smell in the exhaust and the car wants to misfire and lurch under light load. I have no idea why someone would remove the O2 sensor  from an EFI system.

I have owned the car for three weeks now, so everything currently wrong with the car is basically how I bought it. I even had to level out a large dent in the floor pan on the drivers side that caused the seat to tilt towards the center of the car at a pretty steep angle because someone thought that the floor pan was a great lift point for a floor jack.

Yeah that could be a problem

good luck with your repairs!

as far as heat range, i'd stick with stock until it proves to need to be changed. being a long reach gasket type plug, i just wonder who determined the Autolite 3923 to be correct to begin with?

i use this reverse engineering tool to find alternate plugs for various cars,

they also have heat range charts on the page. IIRC they say Autolite doesn't use a standard heat range code so it's hard to cross reference different types of plugs?

@4vandproud I looked up specifications on the Edelbrock Victor Jr. aluminum heads and they call for a 14mm gasket seat plug with a 3/4” reach. From what I gather on Autolite’s heat range, the 3923 has a heat rating of 3, and the 3924 has a heat range of 4 so it’s hotter. I’m just  naturally aspirated EFI and the 3924 is a mostly “stock” style plug from what I’m reading. 

I might put a call into Edelbrock for their input as it’s their heads on the car.

the Edelbrock page is messed up after their last remodel, here's a link to a complete catalog that works


'stock' heat range should act a little on the cold side if alloy heads really dissipate heat that much better? Summit can also be a good reference page, find your heads and open the instructions link. calling is good too, more people should pick up the phone!

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