Carbon on plugs

[donm]

I've been running Bosch wr7dc plugs on my '79 with a Weber 32/36 and they always have a carbon coating on them when I check. The car seems to run fine but I'm thinking maybe I should run a hotter plug. Does this make sense? Any recommendations? I've seen NGK plugs recommended here but which ones should I use?

[18Fiatsandcounting]

NGK BP6ES seems a good plug to use for Fiats. Or BPR6ES (the resistor plug version).

But, if your plugs are routinely sooty, your carb may be set up to run too rich.

-Bryan

[donm]

Thanks, Bryan. Does adjusting the carb involve changing the Jets? You're not talking about the idle mixture screw, are you? If it's the jets, what would you recommend?

[18Fiatsandcounting]

Does adjusting the carb involve changing the Jets? You're not talking about the idle mixture screw, are you? If it's the jets, what would you recommend?

Both jets and idle mixture screw, actually. Changing main jets is a bit of an art, and it can be done by a variety of methods including timed acceleration runs on a flat (and deserted) stretch of road all the way up to sophisticated measurement of the air/fuel ratio on a dynometer.

First thing I would do is find out what main jets are in your carb currently. There are very tiny numbers stamped on the sides of them, like 140 or 135 or the like, and this is the jet diameter in millimeters (140 = 1.40mm). I'm guessing your jets should be in the 130 to 140 range for both the primary and secondary barrels.

For the idle mixture jet (not the mixture screw but the one up near the float cover), that is usually 0.45 to 0.55mm. These are fairly easy to get the size right, and here's how I do it: Warm the engine up. Turn the idle mixture screw all the way in, and the engine should stall (don't force the screw tight, just barely snug). The best (highest, smoothest) idle should be when this mixture screw is backed out 1.5 to 2 turns from its seated position. If it takes fewer turns out for best idle, your idle jet is too large. If you have to unscrew the mixture screw 3 or 4 turns to get best idle, your idle jet is too small. In either case, go one size up or down until you feel it is right. So, if you currently have a 0.50 idle jet and it's too large, go down to 0.45.

Both idle and main jets are in increments of 0.05mm, by the way. There are also the air corrector jets, but this is for fine tuning the A/F mixture across the rpm range, and requires much more expertise than I can offer. And don't even get me started on emulsion tubes as this involves waving chicken bones over a bonfire at midnight while muttering incantations.

Also, if by chance you live at any altitude greater than 4000 feet, you will generally need to lean out the mixture anyway. Less oxygen molecules per volume at higher altitudes, so you need less gas for the right mixture.

Finally, don't judge the plug color after the car has been idling for a while. Idling runs inherently rich, and it's not a good indication of the running mixture. Best way to judge plug color: Drive for 30 minutes on the highway at speed, shut off the engine and coast to a stop (in a safe place), then pull a plug or two and check the color.

-Bryan

[Nut124]

Does adjusting the carb involve changing the Jets? You're not talking about the idle mixture screw, are you? If it's the jets, what would you recommend?

...
Finally, don't judge the plug color after the car has been idling for a while. Idling runs inherently rich, and it's not a good indication of the running mixture. Best way to judge plug color: Drive for 30 minutes on the highway at speed, shut off the engine and coast to a stop (in a safe place), then pull a plug or two and check the color.

-Bryan

Bryan has some good advice there. Sooty plugs is not necessarily a problem unless the plugs actually foul and misfire. Just about all carburated engines run on the rich side at idle and low speed and turn the plugs black.

The Bosch 7 should be very similar to NGK 6. If after sustained moderate to high load on the highway (the test Bryan described) the center electrode is black on a Bosch 7, then it may indeed be too rich. Or bad rings. Again, not a major problem unless you are anal about MPG or the plugs foul, misfire requiring frequent replacement. The outer, ground ring is just about always black with carbon.

I would try turning down idle mixture screw a bit and see what happens. If you turn it too far, driveability will suffer and the engine will hesitate and spit back thru the carb.

I think a little rich runs good and adds safety margin against detonation as long as you on occasion step on it and burn off the excess carbon.