The Stovebolt Site Forums The Shop Area Engine Shop SPARK PLUG SOOTING

I have been trying to determine the cause of my spark plugs getting black soot on them after only a 20 mile run. The black soot does not feel oily but has no gas smell. The engine idles well....here are some things I have done.

I have leaned the carburetor to only one turn of the mixture needle outward.
I have overhauled the carburetor.
i have made sure the hole on the under side of the carburetor is not blocked by the base gasket.
I have double checked the exhaust heat riser to make sure it operates properly.
The engine appears to be properly timed and runs well without hesitation.
I have performed a compresion test and all cylinders are within 10% of each other.The cylinders vary from 90# to 105 #.
I have adjusted the valve lash.
The engine is a 1955 235 with a model B Rochester carb

I am not sure if this is fuel related or an oil problem....not sure what to check next.
The only thing I was thinking of doing now was to lower the carb float slightly to see if that might help.
If it does not help then i have to assume it is an engine problem or am I missing something else "fuel wise " that I could check ???
The truck does smoke a little and when I first start it there is a slight metalic pinging or popping for maybe 2 or3 seconds and then idles great with no issues..
Would timing cause an issue like plug fouling ?

Soot is due to an over-rich condition. Oil fouling is black and wet. It sounds like the power piston in the carb is stuck in the down position, which keeps the power valve open all the time. That allows fuel to bypass the main jet and the mixture stays in the full-rich range. The power piston should move freely up and down, and as soon as the engine starts, the piston will lift from manifold vacuum and allow the ball check valve to close and route the fuel through the jet only.
Jerry

Will check that out...thanks...

Also make sure your choke is fully open when it's supposed to be. It's one of those easy checks that oft times gets overlooked.

Good luck

The usual recommended setting for the idle mixture screw is 1 1/2 turns out to begin with. Only one turn out would be rich not lean. As Jerry said it's probably a fuel issue in the carb. JMO

The best way to adjust the idle mixture is to slowly turn the screw in till the engine starts slowing down. Then turn it back till it smooths out. The amount of turns can vary from carb. to carb. Without using the choke the engine should hesitate & bog till it warms up. If it takes right off without hesitation it is running too rich. That is a poor mans exhaust analyzer. Did you notice the size of the mainjet when you had the carb apart. You may want to use a smaller jet. A 235 usually has around a .058.
The size is stamped on the jet.

Thanks to all for trying to help me solve my sooted plug problem...
I found the problem when I took the top off the carb again to check the power valve as suggested by "hot rod lincoln". The float was rubbing on the gasket and causing the overrich condition.....I adjusted the float and now the problem is solved.

There is one thing I am confussed on and that is the idle mixture screw. When the screw is turned "in" does that lean out the carb or enrich it ????

We're glad you got it solved. Turning the idle jet in (CW) leans the mixture. Turning it out (CCW) richens it.