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Looking for suggestions. Uneven spark plug coloring. Some are super dark, some look good?


Well-Known Member
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4:19 PM
Jul 29, 2015
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Cincinnati, OH
A bit of background to start:

440, .030 over, 10.5:1 compression 440 source aluminum heads. Performer RPM intake, Edelbrock 1413 (800 cfm, mechanical secondaries, e-choke), spark plugs are NGK BKR5E and probably have < 600 miles on them. I haven't touched the carb since I got the car, so I haven't gotten in to look at the jets and don't know what they are. I've never tuned a carb and currently trying to figure out how to read plugs.

A while ago the mechanical fuel pump failed, I got a new Edelbrock unit and it was putting out crazy high pressure ( ~12 psi or so). I installed a fuel pressure regulator and had it dialed for about 6 psi, which I thought was reasonable and between the 4-7 recommended in the paperwork and ran it that way for a while. Still seemed to run rich, so I did a bit of asking around and was told that 6 psi it's on the high side for Edelbrocks, and I should really be targeting closer to 4 psi. About 50 miles ago I set the pressure to 4-4.5 psi and the fuel smell does seem to be less.

Yesterday as I was parking in the garage I shut it off and it "coughed" once, maybe twice. It sounded like dieseling/auto-ignition, so today I pulled the plugs to see if I could see anything obvious. Maybe things had gotten carboned up from running too rich from high fuel pressure?

Here are the plugs:


1 definitely looks the worst/darkest, 4 and 3 are close behind.

Which of these (if any) is closest to a 'good' looking plug?

I'm looking for suggestions as to my next move.

- New plugs and run for X miles, see what they look like?
- Reinstall these plugs and run some Berryman or SeaFoam through to try and clean them up?
- Get into the carb and see what jets are in there?
- Something else?
Thanks in advance :thumbsup:
They all have a black soot to them none of them look "good" as they should have a tan color not a black color.. Did you drive it with that fuel pressure or find it quickly and put the regulator on it? Looks very rich and maybe sucking some oil in there too. Being that rich it could even have a "fat" miss driving down the highway. Maybe pull the intake and see if any of the runners are sucking oil into the chambers..
Your suspicion may be correct. Clean the plugs FIRST then re-install them. A dab of anti-seize on the threads. You may have fixed the issue with the excessive fuel pressure. See how car runs, and check plugs after a few hundred miles.
[1] Have you adjusted the idle mixture screws?
[2] Does the engine have a performance cam?
[3] What is the idle rpm [ in gear if auto ]?
[4] Smooth idle?
[5] Your fuel pressure is ok & despite what you read/heard, these carbs will accept 10 psi of fuel pressure. What THAT does is raise the fuel level, which requires re-setting the float level.
[6] For your current fuel pressure, the correct float level setting is the factory level: 7/16", & this should be checked. You are in luck because unlike the brand H garbage [ & clones ], no gaskets are reqd if you are careful. Take pics/drawings etc of linkages. Edel book can be found online.
[7] Before you do anything, check that the choke is 'off' at operating temp. The choke blade should be vertical, & unable to flop around.
Aside from the unevenness in the plugs, what type of driving are you doing? Mostly short trips around town, lots of idling are likely to foul the plugs. If you're doing 50, 100 mile trips on the open road they should read differently i.e. cleaner
Thanks all, for the pointers.

The car ran for 300-400 miles with the higher fuel pressure. It does have a pretty aggressive cam: 241/247 @ 0.050, 0.545 lift and 110 LSA. As a result, idle is not very smooth, but I have it set to around 800-850. Trips are most commonly 15-30 miles one way, usually county roads through some towns, so I'd say 75% cruising and 25% stop-and-go. I don't typically do much freeway driving. RPM at cruise is usually between 2500-2800.

I appreciate all the help! I'll start by cleaning the plugs, verifying choke is off at temp, and check float level, then run it for a couple weeks and see if anything changes.
try a .110 jet in the primaries. looks fat. you may need a better timing curve too. step-up springs may need a change to yellows.
Quick mentioned oil

I agree

You can see oil on the threads and gaskets

Pulling the carburetor, can you see oil inside any of the intake manifold runners

Is the car using oil

Spark plugs look way way rich and burning off oil in some cylinders
I would also change the oil as with 12lbs of fuel pressure and a lot of unburned fuel washing down the rings it may have thinned the oil… 400-500 miles is a lot especially if this is early on a new engine…
Congrats on buying an EXCELLENT carb.
The problem is the big cam & new plugs will NOT fix it.
I am sure if you remove the carb you will find more than 0.060" of transfer slot exposed. That is because the higher idle speed, reqd because of the cam, needs more air; hence pri blades need to further open. This upsets the delicate idle-to-main cct transition, causes rich running [ black plugs ] .
Engine really needs 35-40* [ actual amount not critical ] of timing AT IDLE.
5 min with a 9/16" spanner turning the dist will show this, rpm will increase & idle will be smoother.

I suggest you try this FIRST & report the results. Then a permanent fix can be detailed.

Don't worry, ign timing at idle is very misunderstood, even with professional engine builders. See David Vizard link, which is in a carb book. Carb & ign, hmm....

Gotcha, thanks. Will pull carb first, check the idle settings and get a peek at the jets. Will also look for any oil in the intake.

@Geoff 2 I want to make sure I understand your comments about timing (I'm a noob). Timing is currently set at 15 degrees initial with 35 total. It sounds like you're suggesting 35-40 deg at idle and if that works, then I'd want to lock out the mechanical advance so there was no additional timing?

No, not quite. With a street driven car, vacuum advance is a must. For this application, it MUST be connected to manifold vacuum [MVA ]. For reasons known only by Chrysd, Chrys FAILED to use MVA & used the useless PVA. MVA does what PVA does...& more!
[ See link below by DV & also this one, post #6: www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html ].
What you will need is a dist that has an adjustable VA unit. Here is how it works. The VA unit provides about 30* of timing. So your init timing stays the same, as does the internal curve of the dist. VA is load dependent, so as engine load is increased & vacuum is decreased, the VA drops away.

At this stage, try the 5 min test. Turn the dist to advance timing, engine idling until the highest rpm is reached...& note what the timing is. There is more to be done, but this is the starting point, so report back with the results.

Ok, Here's what I have done so far (Vacuum advance is not connected):

1) Cleaned and reinstalled the plugs
2) Removed carb, gaskets look good, verified jets, rods and springs. Everything appears as stock. Idle speed screw was all the way in (fast). Idle enrichment screws were 1.5 turns out.
3) Verified timing. Initially it was set for 15 deg initial. Manifold vacuum was around 7.5"
4) Adjusted timing until RPM smoothed out. Highest RPM idle was between 35-42 degrees of advance and manifold vacuum was 12.5"
5) I had a valve cover leak on the driver's side, so I removed both valve covers and checked the gaskets. Gaskets seemed ok, but neither side was very tight. Tightened both down per Moroso's instructions (80-100 in-lb).

Edit: I did look in the manifold and I didn't see any oil in there, but it was hard to see all the way down to the ports. I'll see if I can borrow one of those borescope cameras and get all the way down there.

Thanks for the help :thumbsup:
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IMO......as there are several carb "experts" contributing opinions, I'll just add my comments.
FIRST.....the carb is too big. As fuel mixture is a distribution function, large capacity carbs will result in low mixture velocity; low mixture velocities will result in uneven fuel distribution, which is also influenced ny manifold runner configuration (length and shape). The plenum, space under the carb, will cause a pressure drop and subsequent velocity drop of the fuel mixture.
True, timing and cam specs will influence mixture velocity as long overlap cams promote more exhaust gas left in the combustion chamber resulting in dilution of the incoming fuel charge. Once the idle mixture has been established, one possible solution to the mixture distribution issue MAY be to stagger jet fuel metering, either by a combination of jets, step up rods, depending on manifold runner length. There is NOTHING AS IN ANY HARD AND FAST RULE that prohibits stagger netering. Fuel pressure and volume applied to the carb as well as float levels will effect operation....stay within manufacturers guidelines......in this case, the "my buddy's rules" do not apply.
The problem is complex and likely that a complex solution will result.....nothing as simple as a few degrees of timing or an adjustment of delivered fuel pressure will fix. The spark plug coloration is a result of fixing mixture issues and MAY require a slight heat range revision of your "favorite" brand.
The first thing I would do, is to go to a 650 CFM carb....my preference is Holley, but it's your vehicle, and see if the mixture distribution issue (velocity) is resolved....then proceed .....and with DOCUMENTED changes of each change or revision........just my opinion of course....
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Carb is not too big. It is actually smaller than the 850 TQs that Chry fitted to emission 440s & Inter trucks that had smaller engines....Plus the engine has a dual plane intake that divides the airflow. There would be little sense in adding higher flowing heads & intake, big cam, all of which move the power into higher rpm ranges where more carb airflow is needed.....& then choking it with a small carb.
Good work!
The timing numbers are about what I expected. Just timed an engine recently, it idled best with 40*. My GTO has 48* at idle.

Where to from here. If you want the best performance from this engine, & I include idle quality & tip in response, then it needs 35+* of timing at idle. You can buy programmable boxes that do this, but I have no experience with these. A very simple & reliable method is using Vac Adv hooked to Manifold vac [ MVA ]. You will need an adj VA unit, & if your dist does have one or the capability to add one, it would be worth buying one. Elec dists cheaply available from Summit with adj VA; turn the Allen Key fully CW, softest spring setting. Most adj VA units add 25-30* of timing. You can keep the 15* init timing & add the remaining 20-25* with the VA unit. You may need to fabricate a stop for the plunger arm on the VA to limit travel. VA is load sensitive, so at WOT it goes to zero; that is the beauty of it, automatically load compensating. As for vac supply to the VA unit, any port on the carb that is below the t/blades is man vac & can be used. You can also Tee into any vac line such as a brake booster, except for the PVC.
With this extra timing, you will be able to re-adjust idle mixture & idle speed. The engine will run cooler, idle smoother & have better tip in response....because the extra timing has produced more HP & made the engine more efficient.
For reasons known only by Chrys, they did not use MVA on production cars. GM did & millions left showroom floors idling at 24-26*. My GTO had 26*, high comp engine with small cam.

Setting this up takes time & patience, but the results are worth it. It always amazes me that people will buggarise around with emulsion jets & air bleeds optimising a carb, but spend no time optimising the ign.
Thanks, everyone, for the pointers. I've learned a lot in this discussion.

I currently have an MSD Pro Billet 8387 and according to the documentation (https://documents.holley.com/8387.pdf) it can do 10 degrees of vacuum advance at 15 in of vacuum, so best case I'm looking at 8-9 degrees at the 12.5 in that my engine produces. Sounds like that's better than nothing, but won't get me near 40 deg of timing at idle without way too much initial.

Right now the distributor has the 21 deg mech. advance bushing in. I could change to the 17 deg bushing and move initial timing to 20 deg, so I would have 20 deg initial, 28 at idle (probably less, since it won't make max vacuum), and 37 total.

Does anyone know if there is an adjustable vacuum canister that would work with the MSD? Something like this:

Seems if I want to get closer to 40 deg at idle that (or a new distributor) are my current options.

Also, from reading I've done, more advance may help with engine heat issues at idle? That would be great as it really heats up, especially as the weather gets hotter.

How hot does it get at idle? You may have air in the cooling system.

From a cold start it warms up pretty quickly, and once warm, if left idling or in stop/go traffic it will get to the fan trigger temp (195) rather quickly. The fan moves enough air to bring it back down, but when it's hot outside the fan is running almost constantly.

The top rad hose/fill cap is at the highest point in the cooling system, and whenever I drain/fill it, I jack up the front of the car to try and make extra sure to get all the air out.

I even had a friend with one of those coolant vacuum fill kits to see if that would make a difference, so I'm pretty confident there's no air in there.