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Spark plugs black

3171 Views 114 Replies 9 Participants Last post by  ericnova72
Hello all,
Can you take a look at these plugs and tell me what’s going on? I just installed them and have only idled this freshly rebuilt and already broken in cammed 383 stroker. The strap is shinny black. The center electrode is flat black. These are brand new Autolutes 3924. I was fighting a very rich stinky idle with this AED Holley 750 DP, but fixed it and trashed the old fouled plugs. I’m running MSD 6A and a lot of timing at idle. 36 degrees.
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I looked up the FR5 and they are exactly what AFR calls for.
A good place to start. keep in mind that a lot of these heat range recommendations are for racing where there is a lot of heat. So don’t be surprised if the recommended plug is a bit cold on the street. This is just a cut and try exercise to dial it in. I have a large grungy spark plug collection, I keep ‘em for protecting the spark plug threads when painting heads.

Bogie
 

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Discussion Starter · #23 ·
I just hope it’s not this AED 750 carb. I found a few complaints online about them idling pig rich. Not many but they are there. Something about too many emulsion holes.
 

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I just hope it’s not this AED 750 carb. I found a few complaints online about them idling pig rich. Not many but they are there. Something about too many emulsion holes.
Must be some other issue....main emulsion holes have nothing to do with idle characteristics.

If idle is too rich...idle feed restriction too big, idle air bleed too small, or transfer slot is too wide or too low in the base casting. Float level will have a minor effect too.

Have you got a link to the "emulsion holes" info? I'd like to see what the "genius" said.
 

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Discussion Starter · #25 ·
Good point. Still learning here. IFR are .033 and IAB are .075. I’ll try to find that forum where they said that.

I have the primarily curb idle stop screw set to exactly 3/4 turn. This leaves the primary T-slot to .020. Primary blades have 1/8 holes drilled in them. Secondaries have the higher T-slots on this unit. To maintain 900 RPM, I have to open them to 3/4 turn. All 4 idle mixture screws happy and sensitive at 1-1/4 turn. Ignition locked out at 36. Idle is clean here. The problem is a lean backfire through the carb on hard acceleration. So, when I close the secondaries to 1/2 and open the primaries to 1 full turn exposing .030 T-slot, the lean carb backfire goes away, it hauls ass, except the idle stinks, burns my eyes, and blackens plugs. Not to mention the primary idle mixture screws go numb. They work a little, but closing them won’t stall the engine.

Should I get a hotter plug than the FR5?
 

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I have the AFR195 heads and had same experience with the AFR recommended Autolite 3924s covered with soot. I ended up with NGK FR4s, but you might want to start with FR5s. Run a 180 thermostat if you're on the street.

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Discussion Starter · #29 ·
I found this on yellow bullet…..


“keep it up to at least 2200 when you first start it up, and run the HOTTEST spark plugs you can find. If the carb is right , these little things should really help out.Never let it idle slow when it is cold. Plug fouling is almost always from improper startup proceedures.
JOE SHERMAN RACING”

Any truth to this? I have an HP body so no choke. I hold the idle to around 1500 to warm up.
 

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Joe knows his stuff.....but this is WAY out of context.
DO NOT TUNE the hesitation out of it with the IAB's and IFR's. That's the wrong way of doing it.
AED makes good carbs, at least the ones Ive seen work well but the holes in the throttle plates is a good sign the carb was messed with after is left AED. If AED sent it that way, well then, .......I'd be looking elsewhere.
 

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Discussion Starter · #31 · (Edited)
Great info. I’m open to all advice because I’m still learning. This unit came from AED with drilled primaries. The hole size is not specified. Neither is the IFR size. All other specs are detailed in the carb card.

The backfire through the carb on hard acceleration only happens with the secondaries open to 3/4 turn. This clean up my idle due to being able to close the primaries to where the primary T slot is only .020. Clean idle but stumble and back fire.

When I close the secondaries to 1/2 or less, acceleration is awesome! But now I have to open the primaries to where the t-slot is at .030. Then the idle gets fat again. Stinks and blackens plugs.

This is all to maintain a 900 rpm idle in park / neutral. Around 800 in gear. Could I be chasing too high of an idle? Lowering the idle to 800 in neutral would theoretically give me the best of both worlds. However, will that be an issue with the cam?
 

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Sounds like maybe it also needs bypass holes in the secondary blades.
This would allow you to close the secondary to the point it works correct and not have to open the primary's further to get your idle airflow just because you closed the secondary.

Throttle plates aren't expensive, buy a spare pair and drill holes in the current set....if it doesn't reach the desired effect you can always put the spare undrilled set in to get back to stock.

I'll ask again, have you done any squirter size tuning?? Or just been trying to make it work with the supplied squirters??


From AED's web site, last step of basic carburetor set-up adjustments on the "Tuning Tips" page:...

"The accelerator pump nozzles or “squirters” are sized according to the size of the carburetor. Different intake and cylinder head combinations may require more or less pump shot. If the engine seems to have an off-idle stumble, changing the size of the squirter itself can fix this issue. The first thing to do is determine whether it’s a lean or a rich stumble."

  • "If the engine seems to take a deep breath and then accelerate quickly (almost like the ignition cuts off for a split second), it’s generally a lean stumble. Check the size of the squirter on the primary side and install the next largest size. If that seems to help, but not completely eliminate it, try increasing the size of the secondary squirter.
  • If the engine seems to have a “blubber” or is lazy to accelerate then cleans up as it builds RPM, it’s generally a rich stumble. Check the size of the squirter on the primary side and install the next smallest size. If that seems to help, but not completely eliminate it, try decreasing the size of the secondary squirter."
 

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Discussion Starter · #33 ·
i have not touched anything other than primary and secondary idle snd 4 mixture screws. The squirters are both #31. I don’t want to screw things up worse.

the front blades are drilled 1/8. It came that way. Should I drill the secondaries 1/8 or like 3/32 to start?

I stuck with the idle circuit as the culprit due to the black plugs and how I can get rid of the lean carb backfire by giving it more transition.

I think you are on to something…. Leave the primaries to .020 transition where the 4 idle mixture screws like are become sensitive. Then drill the secondaries to close them to get rid of the lean backfire. Then add more squirter if needed.
 

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I think you are on to something…. Leave the primaries to .020 transition where the 4 idle mixture screws like are become sensitive. Then drill the secondaries to close them to get rid of the lean backfire. Then add more squirter if needed.
This is along the lines of what I would be looking at.....just hard to decide the choice of drilling holes first or making squirter changes first.
I've got squirter assortment on hand, so I would head there first before actually drilling holes, as a larger pump shot may be all it needs even with the secondary's opened a bit (your backfire mode).
May need nothing more than larger pump shot to kill the lean backfire.

I'd start when drilling holes with a 1/16"....you can always go bigger if needed. Hard to drill a hole smaller if you start too big, lol.

No, 900 rpm is not to high for idle speed.
 

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Really need to know cam being used that will establish the idle speed by virtue of what RPM is needed to keep the engine taching over.

Stall speed on tge converter is good info as will but only a 100 rpm loss from free idle to in gear sounds like some sort of higher stall converter.

Since you have a 6AL box I don’t expect you have misfires, capacitive discharge usually fires the plug through a lot of gunk on the insulator.

You are running one hell of a lot of advance, having it all in at idle indicates a big time racing cam. Is this what’s in there? Lots of base advance makes an engine hard to start. I separate the ignition on these full race engines from cranking so turning the engine over doesn’t turn on the ignition at the same time. So I put the ignition on a separate switch so tge starter can get some spin on the crank before dumping extremely advanced ignition on it. Another trick is vacuum advance with a more conservative base so the high amount of idle advance doesn’t kick in till the engine is running. This takes having an adjustable vacuum can on the advance when a big cam that doesn’t generate a lot of idle vacuum. A problem you might get into with such a combination is the vacuum usually spikes rather high for a moment as the throttle just tips in raising the RPMs at a fast rate which pops the vacuum advance up suddenly and temporarily.

I like to set the primary side idle throttle blades just into the range that exposes .020 to .030 inch of the transition on the primary side. The rear only opened enough to get enough air to achieve the idle speed the engine is looking for with none to very little fuel. I make no attempt to equalize the primary and secondary settings. I saw an Uncle Tony’s Garage vid the other day where he has picked up on a trick a used for a long time of using aquarium air bleed adjusters to feed extra idle air while trimming the fuel only on the primary side.

Big cams at idle or low speed cruise are given to high amounts of reversion. The problem with this is a carb will add fuel to flows going out the air horn as well as flows coming in so it’s easy to get way fat mixtures at idle through midrange RPM’s till the port velocity gains enough inertia to quell the reversion.

Idle and putting around with a big cam is just a PIA and that’s just the way it is. Port or direct injection is a big help for this condition but big cams go beyond what self learning systems can do, so like everything else there are limits to simplicity where that ends custom tuning comes in and that quickly gets expensive.

Bogie
 

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Discussion Starter · #37 ·
Starting with the squirter does make sense. They are easy to get.

the cam is a Comp Cams XR282HR 230/236 @50 110 lsa retro roller. 3200 stall 9.5 inch converter.

The engine would not even start under 20 degrees initial. 20-30 and it ran like ass. I called my engine builder and he says that recipe he built calls for locked out timing. It didn’t even start to idle ok until 30 degrees. I figured what’s the point of adding 6 degrees of advance. 36 runs good.
 

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Something's not right if it wont start on 20 degrees initial. Trying to remember if you are the one that bought the all electronic advance distributor (both vacuum and centrifugal). Your builder is making idiotic statements.

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Discussion Starter · #39 ·
Yeah that’s me. It’s a full electronic distributor. It has start retard and full lock out feature option. It cleaned up the smell but not the black plugs.

I checked my balancer and TDC like ten times. It’s dead on.
 

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Right now I wouldn't sweat the black plugs, it is not likely they will ever be anything but black ever again. What I’d want to see is for them to dry out and the over rich smell to lessen but with that much cam will probably will not go away entirely.

As long as you don’t get misfires and with a 6AL you probably won’t I wouldn’t worry the insulator color they’re dirty carbon color and will probably stay that way. Obviously you won’t be able to read those plugs in the future but until your sure it isn’t running dirty rich which you can smell I just wouldn’t worry about reading plugs just yet.

I forgot that you had a distributor that electronically resets the timing when cranking so if it takes 20 degrees out I could see it having a hard time getting going if you back out 20 or so degrees from the static setting.

I set things up for myself mostly the old fashion way as after 54 years in aerospace I‘ve seen enough of the combination of electronic failures and asleep at the stick air crews to know that when something electronic goes out or temporarily fixes a problem the crew doesn’t know about such that the operator is situationally unaware things go to poopoo mighty quick when the automatic system either dies, glitches, or runs out of its limits of authority.

Bogie
 
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