have newly rebuilt 396 in 68 ss, new heads, new carb. new distributer , plugs burn black not tan . Engine builder said run it get it hot , that helped but still burns black. anyone have any ideas? bumped up to a NKG 6 heat range plug still black.
Thanks Steve

 

Generally means too much fuel.

 

Yup.
What carb and intake do you have and what heads and cam?
What exhaust are you running?
Are you sure your choke is coming off properly?
​​​​​​​What ignition system?

 

Hotter plugs means hotter fire, this raise temp of the burning gases which can cause the plug to look cleaner but you are just hiding the symptom of what is wrong. Hotter plug can cause preignition and raised cylinder temps. Heat = wear. The ticket is to get everything working right and then run the coolest plug you can along with the least amount of timing your car will run without giving up too much HP.

I would suggest taking the car to a dyno tune shop to get the carb set. Of course you would need to find one that knows carbs. I have never heard someone say " that was a waste of money" In most cases I hear "why didn't do this sooner" . A good tuner will ask you questions like what are you trying to accomplish. All out speed or engine life or even fuel economy (LOL). Most guys will first say they want max power, but when they find out that will be at a heavy cost on the life of the engine then tend to change their mind. That means back off the timing go to a cooler plug keep the engine a little fat not so fat it washes the cylinder walls of oil. The next best thing to a dyno is a portable oscilloscope you can view while you drive but I do not think many guys to that anymore.

The issue with carburetor is that when you adjust stuff it is a balancing act. You add in timing and now you have a whole bunch of variables. On the power side of the carb you may have to give some high end RPMs to gain lower end power. Also everything could be working perfectly and just your Idle circuit is not right and the black is caused strictly at idle. The opposite is true in that the idle circuit can clean away the hints of what is going on during driving.

If I really want to tune this right yourself and did not have access to a dyno I would install OS sensors in each header and get a pair of O2 gages in the car. Heck it may be worth while to add them even if you get the car Dyno tuned it would be nice to have the O2 sensors just to make sure things do not change over time. Then you would need to do research, a lot of it so you know how to tune the car.