Running Rich

M

Maurer1929

Member
I noticed from my plugs that the engine is running rich, they are completely covered in light black soot (as shown in Les Andrews book). I am using the Autolite 3076 plugs, have GAV open 1/4 turn or less and timing and idle is set properly. I switched to Champion W18 plugs (supposed to be hotter) to see if it cleared up the soot but it did not, still a fine black soot. Any suggestions on why is it still running rich?

Just bought a set of Motorcraft TT10 plugs to try.
 
Check the float level in the carb, try running the GAV all the way closed. If that doesn't work you'll have to resize the main jet in the carb. I'm assuming it is a zenith carb.
 
You might want to check the GAV where it attaches to the carb. Mine was loose, so turning the GAV had no effect on the mixture! Made quite a difference once things were properly attached.
 
How much spark advance are you using while cruising? Too little will foul plugs and overheat the exhaust.
 
I haven't had much luck with Zenith carbs. I have rebuilt a couple and both have run too rich. I want to go back into one of the carbs and look closely at the jets that were in the rebuild kit. I think it is best to use original jets and clean them carefully. If you have oversized jets, solder them up and redrill using the correct drill size. Also, lap in the dash adjustable jet to be sure it shuts off tightly. If all else fails, I have had great luck with Tillotson Carbs. With mine, the car runs much leaner (nice light gray deposits), good mileage, idles nicely and plenty of power on hills. Ed
 
Specs for all the jets can be found on http://www.model-a.org, including the drill sizes for the orifices and the acceptable range of flow rates under a column of water. The 3hr or so that I spent building a simple water column and testing the jets was well worth it. In my case three of the four were too big and had to be soldered down to get the flow rates in spec. The carb now adjusts very nicely.
 
The carb was professionally rebuilt and runs fine but just, by the plug with the black soot, is running rich. I will check the GAV seat. Timing is set per Les Andrews and the spark advance I run at 3/4 down. Engine has not been rebuilt for over 20 years but runs solid otherwise. Don't run an air cleaner.
 
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