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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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How do you know you fixed it, as far as the over-rich idle mixture??

Still looks rich, and maybe a heat range too cold but with no WOT run time to verify it the heat range issue can't really be decided yet.

Aluminum heads?
 

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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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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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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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Could the fuel gotten sucked into the intake some other way?
Not to my knowledge.
If the vacuum passage isn't open through the main body to the baseplate there is no other exit.
I have heard of Methanol users actually leaving the power valve plug out and using the cavity as an expanded bowl capacity area.
 

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Check simple stuff first.
Float levels still good?
Float bowl screws tight?

Any chance you could try another carb?? Any carb that would bolt to the manifold, regardless of the CFM rating....just to tell if it is carb or ignition.

Your 6AL a digital, screwpot adjusters?? or an older analog, chip style box?
 

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One additional item that came to mind, I recall reading something on aftermarket metering blocks used with an incorrect metering block-to-main body gasket on the primary side.
It left an exposed channel that leaked fuel on the passenger primary side.
Maybe you've got a similar problem since you say the front left mixture screw gets unresponsive....that would be the side that could get the same open channel leak path. Plus it ties in with the general wetness you are finding.
IIRC it leaks to either exterior or the power valve vacuum cavity, or both...?

Something to look for, anyway...
 

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Once plugs get gas fouled or oil fouled, it is very hard to bring them back to good. Especially if you are still chasing the tune.
Once it is running well, you may be able to swap fouled plugs in and get them to clean.
Right now your only choice is replace them.

On the accelerator pump, if you rip the diaphragm it gives an instant leak out of the pump housing. Leaks all over the manifold, you'll know if its ripped just as soon as fuel is in the float bowl.

A hesitation rev like you described is typically lack of enough pump shot....if the pump shot seems weak, examine the pump check valve.
Pump check valve is either a suspended ball in the pump cavity, or it will be an umbrella check valve in the pump cavity....check to make sure the ball moves free/or the rubber check umbrella fits flat to the inside of the pump cavity on the bowl.
There is also another pump shot check valve, a needle check valve, just below each squirter....make sure they are there.

What color pump cams are on it??
What hole are they set in??
Absolutely zero slack in the accelerator pump linkage at idle?

Ring break-in will be longer if the block wasn't honed with a torque plate bolted on it....and ring break-in really needs to have a load put on them to get it done.

EDIT:
Idle feed restriction seems big to me if it is 35.....streetable 750 carbs will typically be in the range of .028-.031"
 

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also, call me crazy. I know this goes against the carburator gospel, but I swear that switching the primaries from 77 jets to 75 helped clean up my idle.
If you study the fluid route through the metering block, you can see how jet changes will have no effect on idle.

What is more likely is the removal and retightening of the float bowl and bowl screws created a better seal at the metering block-to-main body gasket and sealed up a cross leak.

I've got no issues with the different primary and secondary mixture settings. If you've got the idle cleaned up leaner to where you are happy with it, now tune the stumble away with pump cam and pump squirter changes.
 

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i'd do the pump cam first, just in case it may manage to be enough, but would also expect to need a squirter change too.
Orange then 35 sounds good....I've taken a lifetime's of left-over 25's and re-drilled them to in-between sizes Holley doesn't offer.....made several 33's.Pin vise and #61-80 drill set.
Holley offers a 32 but you don't typically see them around.

Trial and error is about the only way to find the sweet spot.
 

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Yeah, fresh plugs first and foremost.

If needed...
Pump cam and squirter changes on the just the primary first, since you said no stumble if the throttle is opened enough to bring the secondary's in.
 
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