Spark Plug Reading


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By Nat Santamaria - 16 Years Ago
Hi guys. I pulled my spark plugs on my 312. I found that cylinder #’s 1,2,5,6 (the 2 front cylinders on either sided closest to the rad) looked pretty normal. Firing end and ground strap were clean and porcelain and ranged from a light tan colour to slightly darker. The last 4 cylinders 3, 4, 7 and 8 were considerably cleaner. The porcelain was considerably whiter and there was a small trace of white powdery crusty deposit. Is it running too lean? I mentioned last week under hard acceleration it pings slightly. Under normal driving conditions and moderate acceleration it does not ping. I am running a FlameThrower coil, Pertronix Ignition and plugs gapped at 0.40.



Thanks
By crenwelge - 16 Years Ago
I don't like to hear them ping. I would either give it a little more octane or little less advance. I would get the pinging to stop and then look at the plugs again.
By simplyconnected - 16 Years Ago
"Just to be sure" I would go around the intake manifold and carb base with an UN-lit propane torch and check for any intake leaks.  If your engine rpm raises, it's time to seal the leak.

It's a very easy test, done at the highest vaccuum (idle speed).  If your rpms go up, you get one of those, "Ah, Haaa" moments.

Hope this helps, because all of your plugs should look like sisters if they're getting the same mixture.

By charliemccraney - 16 Years Ago
I agree with the above.



The front 4 cylinders are fed by both sides of the carb so it is unlikely that one side is running leaner than the other. The pattern is kinda odd to be a vacuum leak but it is easy to check and eliminate a vacuum leak as the cause.



I remember that you had to advance your timing to get satisfactory performance. It is likely that it now need some additional fuel. Or maybe some colder plugs.



I have seen similar results on 2 engines now without finding any obvious reason. I just make sure the plugs have at least the proper tan color. It's better to be a little rich than a little lean.
By Ted - 16 Years Ago

I’m assuming the same spark plugs in all eight cylinders?  Had to ask.

 

A compression check on all eight cylinders would be in order just to insure that the front cylinders are not lower in compression than the rear cylinders.  What Dave suggested with the propane is easy enough to do in order to check for an intake manifold gasket leak but is considerably more dangerous to perform than some of the other methods being used to check for vacuum leaks.  But if insisting on using unlit propane, then be extremely careful with it around the distributor and any plug wires.  And run a wrench over the intake manifold bolts anyhow to insure that they are also tight.

 

You didn’t say which intake manifold you’re dealing with but if it is one with the vacuum port at the back of the intake on the drivers side then this port also a probability so check that out closely as well as any vacuum lines that hook to it.  The vacuum port being completely open is also a possibility as it’s out of sight and easily overlooked.  Temporarily plugging the port at the back of the intake can also point you to a problem if it indeed originates from this area.  You didn’t say if you have vacuum wipers but if they don’t work or work well, then the hose to the wipers or the wipers themselves could be the source for a vacuum leak and especially if they connect to the vacuum port at the back of the intake manifold.

 

These are just more suggestions to add to the mix.

By oldcarmark - 16 Years Ago
I have a suggestion.Instead of propane there is a product called quickstart which is I believe Ether in a spray can.Spray around intake manifold using the "straw" and see if revs increase.Safer than Propane I think
By MikeLongIsland - 16 Years Ago
Plain old carb cleaner will work as well for an Intake Leak
By Moz - 16 Years Ago
so will wd40 thats what i use never had a fire
By 63 Red Stake Bed - 16 Years Ago
Just curious if your pcv runs to the rear intake thread boss that Ted was questioning. With a carb that is a touch on the lean side to begin with, & all the extra timing that would surely run those rear cyls. Lean. I had to use a carb with the port at the front between the two mixture screws to make it right & run nothing to that rear port.