Flathead V8 spark plug colour variation

[Mark Towers]

Hi,

I removed my spark plugs from my 1944 Ford CMP (V8 flathead) for inspection this morning and took note that some were a nice grey/tan colour .... and some were Jet sooty black !! (Not wet shiny black but dry Matt black)

I'm not sure what this could indicate? .... if my carburettor was too rich then would all Spark plugs be Jet black ?

Can anyone kindly help?

Kind regards

Mark

1944 CMP F60 LAAT
1942 L60 Canadian Bofors gun

[chris vickery]

I would think bad ignition to said spark plug. Could be plug, wires, or distributor. If the caps on the distributor have badly worn contacts you will get erratic ignition cycles.
It may also be from sticky or worn valves as well.

[Lynn Eades]

IF the engine was idling before you shut it down, and one idle mixture screw was on the rich side, it would have made a difference to the look of some plugs.
They are harder to read with modern fuel.

[Mark Towers]

Originally Posted by chris vickery

I would think bad ignition to said spark plug. Could be plug, wires, or distributor. If the caps on the distributor have badly worn contacts you will get erratic ignition cycles.
It may also be from sticky or worn valves as well.

Thanks Chris,

I appreciate your input.... I will investigate all of the above and see if I can remedy the issue.

I know I have cylinder missing but I have conducted a pressure test on all cylinders and observed a reading of 95psi in all 8 cylinders. I have a new distributor cap, rotor arm, new HT leads and spark plugs .... I've only driven 500 miles from fitting new pistons and rings .... the valves are original as they looked healthy.

So looks like it could be valves then ? .....

Many thanks

Mark

[Mark Towers]

Hi Lynn,

Yes she was only idling... I fired her up for 10 mins stationary and then shut her down ...
I'll take her for a little road run and see if that changes anything before investing further then...

Thank you for your input ����

[David Dunlop]

Mark. Did you plot which cylinders each plug came from? That information can be helpful to the analysis of engine performance as well.

Good plan to check again after a road trip when the engine has reached normal running temps and load for a while.

David

[Richard Farrant]

If the engine was only idling then the black on plugs would denote a rich idle mixture, which is adjustably, also choke might be partly on. When you do a plug check on racing engines, the engine is cut at running speed and knocked out of gear, brought to a halt, then plugs are checked. That way you get a true picture of the plug and can access the mixture or whether the plug needs changing to a hotter or colder type.

[Phil Waterman]

Hi Richard

How old is the gas, modern gas (at least the stuff in the US ) goes bad so quickly that I've given up trying to do any engine tuning without having system filled with fresh gas. Also find that it take an hour or so drive to clean out the engine.

Of course my CMPs have two less cylinders to start with.

Cheers Phil

[Mark Towers]

That's interesting Richard .... I will try not to let her idle long then when I get her back to the shed after the road run .... I'll switch her off as soon as I roll in.

And Philip the gas/petrol is from early start of the year ... so probably on the turn ! Good point !

Many thanks,

Mark

[Howard]

Originally Posted by Phil Waterman

Hi Richard
How old is the gas, modern gas (at least the stuff in the US ) goes bad so quickly that I've given up trying to do any engine tuning without having system filled with fresh gas. Also find that it take an hour or so drive to clean out the engine.
Of course my CMPs have two less cylinders to start with.
Cheers Phil

Phil, Lucas make a 'fuel stabiliser' (or stabiliZer in your version of English) that I can recommend. Prevents the fuel going sour, even over an extended period. Great for ski boats during winter storage etc.