What are my plugs (and burnt up rotor) telling me?

[SadbutTrue]

Today I pulled the old distributor and installed the MSD, with the goal of also changing out the wires, coil and replacing/cleaning the plugs.

At my dyno, where my engine failed miserably, the sniffer said i was running fairly lean (pretty steady 14:1 ratio after 3000 rpm). When I posted about it on here, several people chimed in that the 670 Holley Street Avenger (which I installed this summer) generally comes lean from the factory, and said those sniffer readings made sense.

Today I pulled the plugs expecting to see the plugs being slightly white, which (as far as I know) indicates being lean. However, the two plugs I've pulled so far are both black and look burnt, if anything (pics below).

Also, when we pulled the old (stock) distributor, the rotor contact was completely gone... kinda looked like it was burned off. Not sure if it was related but figured I'd post a pic and throw it out there.

So yeah... what are these plugs telling me? Am I lean as I had thought?

 

[SadbutTrue]

Other things to consider before telling me what these pics mean:
My engine does burn a little oil, mostly at high rpm.

I'm using the Champion RC9YC plugs that Trick Flow recommended with the Twisted Wedge heads.

My spark plug wires (the old ones) had a lot of corrosion.

My timing is off. Not sure where it is (another thing we're working on).

The carb is set pretty rich at idle, but still has the stock jets which are (allegedly) very lean. I had been planning to re-jet it considerably richer but am obviously waiting to see what you guys tell me.

My carb's choke is not working right. Its another thing I'm going to address, but the butterfly valve doesn't has been staying too far shut, which has been causing some rough starting etc. Only reason I mention it is because one of the random webpages with info on fouled plugs mentioned that an improperly adjusted choke can do lead to the black colored plug.


Thanks!!

 

[rbohm]

your engine is running rich. you can tell by the sooty deposits on the plugs. fix the choke, lean out the idle mixture, and perhaps replace the power valve as well.

as for the dist issue, that is normal for an engine that has been run a long time on the same rotor.

 

[2nd Mustang]

The plugs do look like it's burning rich, but not oily like mine. Two of my cylinders are burning oil and the plugs are dark and oily when I pull them, but it doesn't seem to affect performance very much. I guess because I gap them at .045, it still fires well. In fact, I've been using two sets of plugs for the last several years. I just clean them and check the gap and reinstall them.

As for the rotor, I can tell you not to use cheap stuff. I've literally had a distributor cap crack the top off when dumping the clutch in first gear when I had a cheap cap.

 

[ForceFed70]

Keep in mind that reading the plugs typically only gives you a picture of how the last few miles/minutes have been. So leading up to those pics you've been running a little rich... did you just start the engine from cold before pulling the plugs?

Best time to read the plugs is after the engine has been warmed up and you've gone for a drive.

That rotor is definately screwy... looks like a mechanical problem there.

 

[Arron Johnson]

Your plugs are bound to be black if you are idling rich and you burn a little oil. If you want to use the plugs as an indicator, like Force Fed 70 said, you need to drive it at RPM for a while, get it up to temperature (such as take a 20 mile drive down the freeway), and then come back, kill the engine and check the plugs after it cools down. You may be lean at 3,000 RPM and above, but how often are you really cruising around at 3,000 RPM and above as compared to putting around town and idling most of the time?

As I said though, if you are idling rich and buring a little oil, you WILL have black plugs.

As far as your rotor goes, that can be caused by several factors.

1) It was old and on there for a long time and needed replacing anyway.
2) It had a thin spot and couldn't carry the current, so it burned.
3) Too much current going through your ignition system (but this is HIGHLY unlikely).

Imagine a set of jumper cables made out of 12 gauge wire. The instant you put any current (current = amps, not "Volts") through those wires, they would glow cherry red and melt into pieces. The metal parts of your rotor "can" fall into this same scenario IF your ignition system has a lot of current going through it, or IF, past the rotor someplace, you have too much resistance in the current flow path.

HEI ignitions on GM vehicles are known for burning through rotors because of the high current they produce. This is especially true with "cheap" rotors that have thin metal to carrying that current.

Lean down your idle mixture by screwing-in (clockwise) your idle mixture screws about a quarter to a half turn, and see how that affects your rich idling problem. You can also tighten-up the gap on your plugs a little (.032 - .033") to make a little more intense of a spark and to take away some of the resistance in the path of current flow going through your rotor. Remember, electricity takes the path of least resistance. Add resistance and you need to increase the current flow. Your coil has reserve power just for that reason, so you may be "asking" your coil to put out more energy and the resistance in the path is asking for more "current" (miliamps), which will cause things like rotors to burn through.

I hope that helped

 

[rbohm]

As far as your rotor goes, that can be caused by several factors.

1) It was old and on there for a long time and needed replacing anyway.
2) It had a thin spot and couldn't carry the current, so it burned.
3) Too much current going through your ignition system (but this is HIGHLY unlikely).

Imagine a set of jumper cables made out of 12 gauge wire. The instant you put any current (current = amps, not "Volts") through those wires, they would glow cherry red and melt into pieces. The metal parts of your rotor "can" fall into this same scenario IF your ignition system has a lot of current going through it, or IF, past the rotor someplace, you have too much resistance in the current flow path.

HEI ignitions on GM vehicles are known for burning through rotors because of the high current they produce. This is especially true with "cheap" rotors that have thin metal to carrying that current.

one other thing that causes the rotor tips to burn is the larger gap between the rotor tip and the terminal on the cap. this was done to increase spark energy at the plug, but causes excessive wear and burning at the rotor. some after market rotors have narrower gaps.

 

[SadbutTrue]

Your plugs are bound to be black if you are idling rich and you burn a little oil. If you want to use the plugs as an indicator, like Force Fed 70 said, you need to drive it at RPM for a while, get it up to temperature (such as take a 20 mile drive down the freeway), and then come back, kill the engine and check the plugs after it cools down. You may be lean at 3,000 RPM and above, but how often are you really cruising around at 3,000 RPM and above as compared to putting around town and idling most of the time?

As I said though, if you are idling rich and buring a little oil, you WILL have black plugs.

As far as your rotor goes, that can be caused by several factors.

1) It was old and on there for a long time and needed replacing anyway.
2) It had a thin spot and couldn't carry the current, so it burned.
3) Too much current going through your ignition system (but this is HIGHLY unlikely).

Imagine a set of jumper cables made out of 12 gauge wire. The instant you put any current (current = amps, not "Volts") through those wires, they would glow cherry red and melt into pieces. The metal parts of your rotor "can" fall into this same scenario IF your ignition system has a lot of current going through it, or IF, past the rotor someplace, you have too much resistance in the current flow path.

HEI ignitions on GM vehicles are known for burning through rotors because of the high current they produce. This is especially true with "cheap" rotors that have thin metal to carrying that current.

Lean down your idle mixture by screwing-in (clockwise) your idle mixture screws about a quarter to a half turn, and see how that affects your rich idling problem. You can also tighten-up the gap on your plugs a little (.032 - .033") to make a little more intense of a spark and to take away some of the resistance in the path of current flow going through your rotor. Remember, electricity takes the path of least resistance. Add resistance and you need to increase the current flow. Your coil has reserve power just for that reason, so you may be "asking" your coil to put out more energy and the resistance in the path is asking for more "current" (miliamps), which will cause things like rotors to burn through.

I hope that helped

Thanks man, very helpful. Yeah, was planning on leaning the idle mixture and re-jetting it richer so that it'll be more steady. One other thing... the power valve. Another forum suggested it as a potential culprit re: the plugs. Only reason I mention it is because the dyno shop owner also mentioned I should take a look at it, because of an oscillation in my A/F ratio between 2,000 and 3,000 rpm. As I said, it starts rich and ends up lean, but in between (don't have the sheet to give very specifc numbers), it spiked up and down a few times pretty far after 2k and up to 3k before settling at just over 14:1 the rest of the way.

I'm not that familiar with Holley carbs (this is my first) and I have no experience with power valves, but is it something I should look at?

Thanks again!

 

[Edster]

Power valves are used to enrichen an other wise lean mixture at higher rpms . Tere are different "sizes" (like jets). Also sometimes they fail as they have a diaphragm and a spring. Usually the diaphragm will rupture or get a small hole in it. They open based on a given vacuum level when the throttle is opened to a certain degree. I would find out what size yours is and replace it or get the next step larger/less vacuum.

 

[SadbutTrue]

Power valves are used to enrichen an other wise lean mixture at higher rpms . Tere are different "sizes" (like jets). Also sometimes they fail as they have a diaphragm and a spring. Usually the diaphragm will rupture or get a small hole in it. They open based on a given vacuum level when the throttle is opened to a certain degree. I would find out what size yours is and replace it or get the next step larger/less vacuum.

Well, my carb is essentially brand-new... bought in this summer, has maybe 2,000 miles on it but most of that is because of a single trip up to the Bay Area and back. I don't think the power valve has failed... after doing some research the car doesn't seem to have the symptoms. Is it worthwhile to try to enrich the WOT mixture by increasing the size of the power valve or should I not bother and just do it with bigger jets?

 

[D.Hearne]

The Powervalves job is to enrichen the intermediate circuits more than the WOT running. If you want more fuel at WOT, go to larger jets. I seriously doubt the powervalve is ruptured. These things don't rupture anywhere near often as some are led to believe. When they do, it's because the carb has sat on a shelf for years at a time then bolted on a motor and then they go south from a dried up diaphram. That's the only time I've ever had one go bad.

 

[Decurion]

The easiest way to check a bad powervalve is while the engine is warmed up and idling, turn the idle mixture screws in until the engine dies. If it keeps on running just fine with both mixture screws all the way in, theres about a 99% chance the power valve is blown. Im not sure if youre new carb came with powervalve protection (a small check valve in the base plate most likely), but if not theyre pretty easy to blow. All it takes is one good backfire, Ive blown plenty of em to know.