Expert: Brad Sears - 2/12/2009

Question
I have a 1967 Ford F-250 4X4 truck.  I recently converted it to a 300-6 motor
from the horrible 352 V-8.  The 300-6 was one of the three engine options
that year, so it is stock. It had a lot of popping through the carburetor when I
got the new engine going.  I drove it about 500 miles before i found out that
the wire from the ignition to the coil that I replaced is supposed to be a
resistor wire.  I bought the correct wire and installed it with a new coil. Most
of the popping is now gone, but not all.  I understand that there is supposed
to be about 6 volts or so going into the coil, but I do not know the exact
amount.  1) How exact does it have to be ?  2) How do I measure the voltage
to be sure what it is getting ?  3) The coil says "use external resistor" on the
side.  Is there supposed to be anything in addition to the resistor wire ?

Answer
Do you have electronic ignition or points? If points,and you ran the engine without a resistor, I would suggest replacing the points as they have carried too much current without the resistor, and are burned. If electronic you should have not hurt anything. The voltage at the + side of the coil with the engine idling should be between 6 and 9 volts. However before I did anything i would do a compression test on the engine. I would assume that you have a supposedly good used engine. Popping through the carburetor especially under load and acceleration can be caused by a leaking intake valve and the compression test will show this up. If the compression test is ok then the problem is outside the engine. There is no second resistor in the this circuit.
Brad