Black spot on spark plug- causes?

[Sonny]

I recently replaced my 25 spark plugs with cooler 23s to eliminate spark knock. Helped some, but I have these isolated black spots after driving 500 miles. What causes that?
440 with .5 lift cam, Keith Black raised pistons, MSD ready to run dizzy, rpm air gap and Brawler 650 carb, vac advance on.

 

[MoparMur]

View attachment 758959 View attachment 758960 I recently replaced my 25 spark plugs with cooler 23s to eliminate spark knock. Helped some, but I have these isolated black spots after driving 500 miles. What causes that?
440 with .5 lift cam, Keith Black raised pistons, MSD ready to run dizzy, rpm air gap and Brawler 650 carb, vac advance on.

First I'd like to know what your original (hotter) plugs looked like? If they were burning clean all around, and if you've made no other changes, then what I'm thinking is changing to cooler plugs has exposed a basic weakness in your engine combo. What I mean by that is a hotter plug may have been just enough to mask an overly rich condition or an inefficient burn. Did the engine run fine before? What is your compression ratio and cam specs other than lift? I'm not familiar with the ready to run MSD distributors other than I know they don't need a stand-alone control box, but if they produce a hot spark like say the 6A does and your plugs wires are good (no misfiring), I don't think your ignition system is at fault. What were the conditions as to when the engine would knock? What's your base timing, mechanical advance curve, and total timing? What octane fuel are you using? May be premature to suggest this, but perhaps a better timing curve may be the better remedy?

 

[steve340]

Other than there being another tuning issue, which it sounds like there is - looking at that earth strap that plug is to cold.

 

[Bill Monk]

http://4secondsflat.com/Spark_plug_reading.html

 

[Sonny]

First I'd like to know what your original (hotter) plugs looked like? If they were burning clean all around, and if you've made no other changes, then what I'm thinking is changing to cooler plugs has exposed a basic weakness in your engine combo. What I mean by that is a hotter plug may have been just enough to mask an overly rich condition or an inefficient burn. Did the engine run fine before? What is your compression ratio and cam specs other than lift? I'm not familiar with the ready to run MSD distributors other than I know they don't need a stand-alone control box, but if they produce a hot spark like say the 6A does and your plugs wires are good (no misfiring), I don't think your ignition system is at fault. What were the conditions as to when the engine would knock? What's your base timing, mechanical advance curve, and total timing? What octane fuel are you using? May be premature to suggest this, but perhaps a better timing curve may be the better remedy?

Here are the hotter 25s. They look better but have a brown spot not black.

 

[Sonny]

First I'd like to know what your original (hotter) plugs looked like? If they were burning clean all around, and if you've made no other changes, then what I'm thinking is changing to cooler plugs has exposed a basic weakness in your engine combo. What I mean by that is a hotter plug may have been just enough to mask an overly rich condition or an inefficient burn. Did the engine run fine before? What is your compression ratio and cam specs other than lift? I'm not familiar with the ready to run MSD distributors other than I know they don't need a stand-alone control box, but if they produce a hot spark like say the 6A does and your plugs wires are good (no misfiring), I don't think your ignition system is at fault. What were the conditions as to when the engine would knock? What's your base timing, mechanical advance curve, and total timing? What octane fuel are you using? May be premature to suggest this, but perhaps a better timing curve may be the better remedy?

Im about 9.5/1, initial 18 total 36, 93 octane, would knock doing 75 mph when I started to lightly accelerate. Could’ve been my vac advance cause if I floored it there was no knock?

 

[Bill Monk]

Im about 9.5/1, initial 18 total 36, 93 octane, would knock doing 75 mph when I started to lightly accelerate. Could’ve been my vac advance cause if I floored it there was no knock?

Is your vac advance connected to manifold or ported?

 

[Sonny]

Is your vac advance connected to manifold or ported?

Ported.

 

[MoparMur]

Wow Bill that is an incredible article on reading plugs. Sonny I think I'd go back to your original combo but try it without the vacuum connected to the vacuum advance. I've seen some vacuum canisters add a huge amount of additional timing on even a small vacuum signal, so fine-tuning the vacuum can might be all you need. I like how your original plugs look. Might want to have the engine at idle with a timing light on it, then add a outside vacuum source to the distributor vacuum can and see how much timing it's adding under various amounts of vacuum. Also, hook a vacuum gauge to your ported source and see what the reading is at the point of throttle you're using where it knocks. You might be surprised to find it adding 20 or more degrees at that point.

 

[Don Frelier]

Perhaps it's just the picture, but the gap looks small on the half black plug.
I'm not a big Autolite fan either...

 

[lewtot184]

dirty burn. what heads? I always go thru this with iron heads.

 

[Bill Monk]

Ported.

Try manifold vacuum, manifold vacuum falls off with acceleration, ported increases with throttle. Try it! Here's another article on it.
http://4secondsflat.com/Ported Vs Constant Manifold Vacuum.htm

 

[Sonny]

dirty burn. what heads? I always go thru this with iron heads.

Rebuilt 452 heads from Aerohead Racing.

 

[Sonny]

Try manifold vacuum, manifold vacuum falls off with acceleration, ported increases with throttle. Try it! Here's another article on it.
http://4secondsflat.com/Ported Vs Constant Manifold Vacuum.htm

Thanks. I will try that. Interesting article.

 

[steve340]

For engines that are totally stock I would leave the vacuum advance on.
Most vacuum advance units have way to much advance in them for modern fuel and fast burning combustion chambers.
Detonation will kill your engine faster than you can say I wasted my money.
As soon as you put in a hot cam ,high rise manifold etc I disconnect it and run with centrifugal advance only.

 

[lewtot184]

Rebuilt 452 heads from Aerohead Racing.

why the tapered seat spark plugs? what valve seals?

 

[Sonny]

why the tapered seat spark plugs? what valve seals?

This is a 1978 rv 440. It has tapered seats and additional cooling passages around the plugs.

 

[Sonny]

For engines that are totally stock I would leave the vacuum advance on.
Most vacuum advance units have way to much advance in them for modern fuel and fast burning combustion chambers.
Detonation will kill your engine faster than you can say I wasted my money.
As soon as you put in a hot cam ,high rise manifold etc I disconnect it and run with centrifugal advance only.

Is the argument that not running vac advance is bad for cylinder walls (fuel washes the oil off the walls) not valid?

 

[steve340]

All vacuum advance does is advances the ignition timing up while the engine is on light load and cruising.
The argument of fuel washing is probably a tiny factor but a badly tuned carb is going to be much worse. I believe that argument holds little water if the engine is modified.
Trying to tune it so you avoid detonation is not worth the hassle. If the engine has high compression, hot cam it is so far from what the unit was designed to work with it is not worth the risk of detonation.
In saying that you can get it to work but you would need to be pretty clued up on tuning and modifying the distributor.
I have heard the "you must have it" brigade but I never bother and have not had any problems so far.

 

[Sonny]

I mainly do short trips to work occasionally and local car shows. I think I’ll plug it for a while and stick to distributor timing. I’ll put the 25s back in to. I don’t race it either. Thanks for all the tips!