The Stovebolt Site Forums The Shop Area Engine Shop Back two plugs sooty - what to check

Hi,

The back two plugs of the '58 235 in my '51 4400 are sooty and I was wondering if you all could point me in the right direction. I changed the plugs out and drove it a couple of miles and #5 & #6 are already showing soot.

The plugs, wires, cap, rotor, are new, and I'm running electronic ignition (Duraspark II module with Chrysler pickup in the stock distributor) with plugs opened up to .44. Vacuum and mechanical advance are functioning. The other four look great. Compression ranged from 111 to 120. No visible smoke from the tailpipe. It's got the stock carb and manifolds, btw.

Overall the engine seems weak and underpowered (inevitable in a big bolt, I'm sure) and maybe needs more timing, but this doesn't explain the difference in plugs. Where do I go from here to chase this down? Thanks!

I would have to assume what you're seeing is not soot, as left by excess fuel, but oil , probably from a worn valve guide.

The way these engines are designed, if any cylinders get too much fuel, it will be the center ones. That's what is making me think you're seeing an oil problem.

You've obviously done a compression test, your numbers are even, so it should not be in the rings. That leaves valve guides.

Devin

I don't know... the plugs are dry, not oily. The fouling looks like, well, soot. Like you put the end of the plug in the flame of a candle.

I would try another set of plugs. If it does it again, you can start trouble shooting then.

If they are still firing just switch the front two for the back two and recheck after some miles on it.
Also what is your driving habits, slow around town, road where speeds get up some? Intake gaaket OK? Maybe try a different range plug in those?
Just making suggestions but I to would "lean" toward a valve guide...pull the valve cover and check the seals. New seals may help out.

With The fuel we get today it takes just a little bit rich to get that light soot coating on the plugs. Check your heat risor valve maybe stuck open. That would get less heat to the end cyls. & cause it to be rich. Check the float level in the carb. Try leaning down the idle mixture screw. Make sure the vacuum outlet in the bottom of the carb has a passage to the intake manifold vacuum. You may have to go 1 or 2 thousands smaller on the main metering jet.

The heat riser is something that I haven't thought of checking. Good idea and thanks a bunch for your suggestions.

I replaced the plugs last weekend and drove it hard for a couple of miles. That was enough to show significant signs of carbon on 5 & 6 with it present on the insulator and the center electrode (but mostly on one side), as well as on the face of the body of the plug.

I've been really hesitant to crack open the Rochester B on it as it barely leaks at all and I was hoping to keep it that way. I'll probably have to break down and do it if I can't figure this out.

Just for grins and giggles, if all of the previous suggestions fail, try a hotter plug in those two cylinders. The plug seats may not be cut as deep into the head on those two.

I am leaning toward crossed plug wires my self maybe a cracked cap. Then valve stem seals or worn guides.
If it is black soot it is fuel if it is gray ash it is oil.

If it runs good & doesn't act too rich I am with you. Just leave it alone. Do you use extended tip plugs?

Yep, they're extended. I used Autolite 85's as they were the only appropriate ones my auto parts place had in stock. They're supposedly equivalent to Champion RJ12YC or AC R44S.

Hopefully I can find some time to tinker with it this weekend.

Yes, double check wires, may have swapped the back two and plugs would look black if in wrong order. Wires could be just junk, swap with others to make sure.

Well, I re-tested compression with a new tester I bought as I kind of suspected my old one was reading low. The new test showed every hole reading 140 to 147 psi dry (kinda high??), so that confirms the compression rings at least are OK. All ignition components are brand new (except for the coil and ignition module) and I double checked the wires for correct order. The heat riser is free with intact thermostatic spring, too. I even checked the intake gasket seal with propane and no rpm change was found.

I guess the valve seal guys must be right.

I'm a V8 guy so I could be wrong but wouldn't swapping the rear two wires make it run like dog shet?