AFR 165 heads - spark plug recommendations needed

[Capricious]

Gents,
I’m currently having a problem fouling some of my spark plugs on cylinders 1,4,5,6,and 8. I’m using the recommended Autolite 3924’s. I’ve gapped a set at .044, and another set at .036. Same problem.
My set up is: 306 block, AFR 165 heads, Holley 650 dubble pumper, Edelbrock Air Gap intake, BBK shorty headers. I’m running 91 octane non-ethanol pump gas. Have the air fuel mixture set at a lean-best condition. Timing set at 13 BTDC.
When I have a fresh set of plugs in, it runs great on the high end, has some mild stumbling at low RPM under load.
Im looking for some insight, and a spark plug recommendation. I’m considering going with NGK 6953 plugs.
Any input is appreciated, thanks.

[Ourobos]

Stock or hotter ignition? Blower? Set them to the factory .054"

[Capricious]

Stock ignition. All motor, no blower.

[slomont]

Is this a new issue?

Likely a dirty carb.

[Capricious]

Not a new issue. Pulled both bowls off, all jets were clean, floats set right.

[STL79Coupe]

My last set of AFR 165's I was running autolites, but that was back in the early 2000's.

[erratic50]

I tend to think its a spark delivery problem because of the stumbling you mentioned. It could be a coil, but I'd eliminate other obvious things for reliability purposes first.

Double check that your ignition switch inside is not separating. That can lead to all sorts of impossible to find problems.

Definitely worth checking all of your ignition system connections. And by check I mean CLEAN. Also anywhere that a wire is in a crimp, look to see how many wires are actually IN the crimp or better yet add some solder to them or redo them just to be extra double sure.

Ignition relay is a possibility.

The ground from the back of the motor to the firewall is a likely suspect too. I'd add one from the front of the motor to somewhere on the K member as its cheap insurance and very-well could be the problem.

If its EFI, worth checking the ECU grounds. And by checking I mean cut off the factory 4 lead to one that runs over to the fender, solder up a nice piece of heavy wire, and home run it to the battery and sandwich a loop between the existing ground nut and a new nut. Also worth checking the advance spout to make sure its A-OK. Or just verify the connector and replace the spout itself.

You didn't mention stuff like what fuel pump you're running, what type of return setup you have, etc. I've seen where a bad return causes fuel pressure to go sky-high and causes a rich issue that you just can't get rid of. Happens with both mechanical fuel pumps and also electric. Worth a double check.

If it has an electric fuel pump, make sure its not crazy big for a factory hanger. The biggest I'd go without messing with the return line is 190.

I would extra triple or quadruple check the coil wire. As in just replace it maybe. They tend to burn off after a while, especially with tight gaps on plugs.

You didn't mention what cap and what rotor you're running.

You did not mention what compression ratio you're at.

We had the exact same symptoms on 99% stock 86GT and it turned out that the cap and rotor. While they were new, they were junk. Look for any signs of the center electrode contact being chewed up. Any carbon. any gouges or scoring, etc. We put that mcparts new stuff in the trash and ordered all Motorcraft -- problem solved.

Good luck!!


Nice looking car, BTW. If you can't figure it out, you can always sell it to me.

[Walking-Tall]

Not a new issue. Pulled both bowls off, all jets were clean, floats set right.

Low rpm under load is idle/transition circuit territory within carburetors, not main circuit territory (jets). Check the idle jets too? If not, pull the metering blocks off too and check that there isn't crud in one or more idle feed restrictions (idle jets), and remove the idle mixture screws and blow everything clear with compressed air... as well as make sure all of the air bleeds up top are clean and clear.

... and with an air-gap intake, make sure that the engine is hotter than hot (with say a 10-15 minute drive that wouldn't hurt toward getting some heat into the intake and carburetor) up to engine operating coolant temperature prior to setting idle speed and idle mixture... can make a sizeable difference in more solid predictable function with those intakes. Is there an operating choke, and whereabouts are those idle mixture screws set at, turns out from lightly seated?...

[Hissing Cobra]

Stick with those 3924 spark plugs. It's what AFR recommends and it's what I used on my AFR 185 equipped dual quad 306 and I'm currently using on the 185's on my 347 stroker motor. I've never had a problem with them and they run fine. If you've got a 650 double pumper, do yourself a favor and buy a jet kit, bigger squirters, jet extensions for the rear bowl, and some power valves. Start experimenting! They're really easy to work on. Make sure that all orifices are blown out with compressed air and that the front squirters are squirting correctly when you smack the throttle. Make one change at a time and if it makes things worse, go back to where it was and go in a different direction. If the change makes things better, you're going in the right direction.

Also, what is your base timing at? Where is it "all in?" On my 306, initial was 16 and total was 34 (all in by 3,000 rpms). On my 347, initial is 14 and total is 32 (all in by 2,500 rpms).

It's time to experiment!

[78futura]

I have the same setup but a 600cfm carb. I went 2 colder than recondmended to see if helps with pinging(which has been my enemy all year) the recondmended plugs were perfect tan.
Depends how much idling you do and how rich it is running.
I have the stock ignition as well.
I would start by leaning out the mixture.

[Ourobos]

So why the tighter than stock gap? If it's a stock ignition, it likes a certain distance, as the Ford engineers spec'd.