Jump to content

Help reading my spark plugs


Recommended Posts

I finally got around to removing all my spark plugs today in order to see what they may tell me about my engine's operation. I could use some of the expertise on this board to help me determine what the plugs may be telling me. Let me share with you some background:

* Engine is original, unopened, cared-for '64 425 dual quad factory setup with 133,700 miles. I've owned it for a year and half.

* Engine runs well with no major leaks

* The plugs appear to be incorrect. The manual calls for AC 44S.

* I checked the gap and they were pretty close. I made minor corrections for the ones that were off. (I gapped them at 0.035).

* No engine run-on when I turn off the key. Even after a long drive on a hot day.

* These plugs were in the car when I purchased it. The previous owner's maintenance records indicate these plugs were put into service in 2006, which corresponds to 10,000 miles.

Photos are below: The first two photos are labeled properly. Passenger side are here:


I forgot to number the second set of photos (Driver side), but the plugs are in order (2, 4, 6, 8). Higher numbers are nearest the firewall.


Here is a close up of plugs 6 and 8


Here are my observations:

1. There was a good amount of oil on the #1 plug, but not on the tip. I checked the value cover above the plug and it seemed free of oil leaks.

2. The #8 plug seemed to be the most beaten up. However, the #7 plug didn't seem to be similarly beaten up. I expected them to be about the same. (I expected them to show more wear because on the dual quad setup the first carb does all the work below 50% throttle, so I think the cylinders nearest the firewall may run richer due to their proximity to carb #1).

3. When I removed plug #3 I was surprised how little torque was required. I think it was a bit loose in the hole.

4. The other plugs seem fine

If you can provide any insights as to the story the plugs are telling me about my engine it would be very much appreciated. You can see that my reading was not particularly insightful.

Thank you.



Link to comment
Share on other sites


The plugs look fine. What would be more typical, especially in a dual quad car, is a more sooty appearance as most cars do not have the choke setup properly and are operated in a fashion that encourages rich fouling. To add to that situation, someone has installed cold plugs which was most likely a poor choice for the manner in which the car is being used (making the assumption you are not engaging in "motorsports'). The 44`s have been unavailable for quite a few years but there are other choices beside AC and in the proper heat range. I use NGK and have had good service from them. Put in a new set of plugs and you will most probably notice an improvement in starting and idle quality.

Tom Mooney

Link to comment
Share on other sites

They all look normal to me for that vintage car with 10,000 miles. Just replace them and drive on!

As Tom Said, the AC 44S plugs are no longer made new. I run across them every now and then at swap meets, so keep your eyes open and ask around. They are out there.

You can go with the colder 43 plugs again if you want and it won't hurt a thing, or you can go up to the hotter AC R45S plug, which is available.

The hotter plug will help keep off some of that black deposit you see on the tip now on #7 and #8.

The main risk you run with a 45 plug is possible pinging at really high load (wide open throttle) when hot. You need to listen for that and back off on the gas if it does it. Other than that, a 45 plug is a good choice, especially with the high miles. Some oil is bound to be coming up by the rings. What weight oil do you use?

The different amount of black seen on the plugs is less about the dual 4-bbl carbs and more about having a carb in general. You do not get a uniform distribution of the mixture to all cylinders. Some get more, some get less. That is one reason Detroit went to fuel injection to help reduce air pollution. Too much fuel in a cylinder gives you unburned hydrocarbons, and too little gives you poor performance from the cylinder. Fuel injection can meter it more precisely and evenly between cylinders.

As Tom mentioned, make sure your choke is adjusted and working correctly, is free to move, and I suggest setting it a tad on the "lean side".

Hope that helps!

Link to comment
Share on other sites

I would go with the R45S, I run them in every Nailhead I build including my own 64 Dual Quad Riv, unless it is a built engine with corrected/increased compression. Most driving in these cars is low speed with lots of idling and cold starts, the 45S is little hotter as Tom mentioned and will last a bit longer. Overall, your plugs look great. Tom, a note on the NGK's. I love them, they are my favorite plugs by far, however... I checked an NGK in a 401 head I had on the bench to see if indexing was feasible and found out the NGK's do not fully protrude into the chamber as the AC plugs do. I was shocked, but check the thread length against an AC and you will see the difference. They may have a crossreference that is a closer match, but would require some experimentation vs. their recommended replacement.

Link to comment
Share on other sites

Thanks everyone for your replies. Here is an update:

I went with the R45S and installed them this morning. I drove it listening carefully for any pinging or rattle and there was none. I'll probably pull them out in a month or so to have a look at them, especially #7 and #8, which had some deposits on the colder plugs.


Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Create New...