I bought a brand new Holley Street Avenger 670cfm carb a couple months ago to replace the 600cfm Edelbrock that came on this 390fe. I also bought new spark plugs and ignition. Pertronix billet dizzy with Ignitor II unit and appropriate coil. Also bought new plugs to help me tune the carb. New AC Delco recommended units gapped to .045.

This motor was rebuilt less than 10k miles ago. Has roller cam and rockers, poor mans CJ heads with larger valves and Edelbrock Performer 390 intake with stock exhaust manifolds (I know, its kinda silly and I have plans to upgrade this latter) I have the dizzy attached to manifold vac, idle timing is set to 15 before with 20 deg of adv plus vacuum adv.

My issue is started with the truck feeling like it needed more fuel, was surging at highway speeds and seemed a little weak. Pulled the plugs after running them for about 2 tanks of gas, no change. Started going up in jet size two at time and checking every couple tanks. Still no plug change. The surging went away though. I kept going up in jet size until I was 10 sizes above the stock setting and the plugs were black.

I cleaned the plugs off and went to 4 over stock on the jet size. Its been about 500mi on this setting and I still cant get any change in appearance on the plugs. The bottom two threads are slightly wet looking when pulled and there is a slight bit of carbon on the top of the threaded portion. (See pic)

I am missing something here?
Thanks in advance

 

Gaps at .045 work well on EFI engines and not so well on Carburated from my experience, try going back to .035. I never use AC Delco plugs in any of my Fords or my lawnmower

 

I've had more problems with AC plugs than I care to mention. That includes in my 428CJ and a 350 Chevy I used to have. Ran like crap right out of the box. Changed to Motor Craft and problem solved. Champions also worked OK. I installed a 770 street avenger, vacuum secondaries on my 428 and basically only had to adjust the idle. It was dead on out of the box.

 

"I have the dizzy attached to manifold vac"

Why? If the dizzy is a normal "Ford style", the vacuum advance should be on "ported" vacuum, not manifold vacuum.

Manifold vacuum can be a band-aid for really lopey cams to try to get it to idle better, but with a roller cam, you shouldn't be having idle vacuum problems to begin with.

 

Maybe, how are you checking the plugs, when are you pulling them?

If you let the engine idle before you pull the plugs then all you're checking is idle mixture. If you want to check plugs to get a read on highway cruise mixture then you need to stop the engine in that condition, then pull the plugs. That is get going at a highway cruise or whatever condition your concerned with and without slowing or even lifting your foot off the throttle click off the ignition, pull to the side of the road and then pull a plug. You have no idea how many plugs I've pulled on the side of the highway.

The key here is that it's running better, sure, you can never tune too much, but what really counts is results.

An FYI that may or may not be an issue. A strong ignition will make plugs read leaner/mask a rich condition.


I completely disagree with this. There's only three things to consider with gap choice, the strength/condition of the ignition system, compression, and how long you want them to last. Any decent ignition can fire a 45 gap just fine, and a DSII with a decent aftermarket coil(45KV or better) and decent wires certainly counts. Higher compression ratios need smaller gaps, but a FE on pump gas isn't going to have a high enough ratio to care. A wider gap will have issues sooner as the plugs wear but .045 is no problem at all, I run .065 in my 460.



I disagree with this as well and have written about it a lot in this forum, but I'm disagreeing with a mod so I'll probably be banned tomorrow.

 

Correction on plugs, they are motorcraft not AC Delco. I had AC Delco in before these ones. I am running on manifold vac because I do have a hotter cam then stock and on ported, when the idle is raised to proper level, I get run on.

The coil is a new MSD 45k volt low resistance type and the compression on the truck is around 9.5:1.

Brute Ford, I have driven the truck at high rpms and then shut it off immediately without idling and checked the plugs. Seems to make no difference. I'll try running it ont he freeway for a couple miles and pull them out there. Haven't tried that yet. I'll get back with results of that soon.

 

Everything I have read by some of the best engine builders would totally disagree with you. The OP isn't running a DSII and coil voltage is marketing BS. Plugs fire when the coil only reaches about 10,000 volts..

 

Well let's hear it from them, till then I'll stick with my years of experience actually tuning performance engines for a living.


Says who, he certainly doesn't.


Again, says who, you? Coil voltage when talking coils is potential voltage, actual voltage is determined by resistance, basic electricity 101. If you're only reaching 10,000V then you got some tuning issues.


You tell me you're theory then, you make an assertion with NOTHING to back it up. Why would an engine with a carb need a smaller plug gap? Why would it effect plug reading? That is what this thread is about after all.

 

That's interesting, have you ever investigated it further. The relationship is clear, the engine will run cooler at idle and tip in with the better timing provided by manifold vacuum. But the base problem may be too hot of a plug.


I think you get this but just so it's said, RPM alone is a minor consideration. It's about what fuel circuits in the carb are dominant and how much they are flowing. High RPM with little to no load may still mostly be fueled off the idle/transfer circuit thus largely unaffected by jetting changes.


Taking a closer look at that plug, something I admit I didn't do well enough before, it's clear it is was overly rich recently. You can still see a layer of soot at the end of the threads. I have no way of knowing what that's from but it is there. EDIT... LOL, you even mentioned it, woops.

 

Open your eyes and read what the OP posted "Pertronix billet dizzy with Ignitor II unit and appropriate coil"

 

OK missed that, but so what, same thing still applies.

Is that all you got?

 

I'm only going by my own experience, I had the same exact set up as the OP in my 390 and it wouldn't fire the plugs worth a crap at .045, at .035 it runs great..

 

Sure if it's missing under load, I know I miss things but I didn't see him say anything about that.

 

He only said it was surging at hiway speed, didn't say load. I miss things too but I have never seen anywhere on Pertronix systems do they recommend increasing the plug gaps.

 

Sorry for the BS reverhardt.


Direct from Pertonix

"SPARK PLUG GAP
In stock applications, the manufacturer’s recommended spark plug, and spark plug gap will work best. For performance applications, the spark plug gap may be increased up to 0.010” from manufacturer’s specifications to take advantage of the extra energy, produced by the Flame-Thrower coil. Since PerTronix cannot test every configuration, the end user must determine what spark plug gap works best for their application."

http://www.pertronix.com/docs/instru...eets/40001.pdf