The Ford Barn

John Stone · 07-20-2010, 08:12 AM

I worked on a friends Model A all day yesterday. It runs fine when cool but when it heats up, it cuts out, back fires and no power. Pulling the choke out helps. The car was converted to 12v negative ground. Here is what was changed to known good stuff. First a condenser and then a complete distributer, carburetor, some MMO to the gas tank, gas cap in case it wasn't venting, spark plugs and coil wire. He said he had the coil checked and it was good and I checked the spark and it was good. Every change we made seemed to work for 2 miles and then the old problem started again. Finally put on a new coil and external resistor and it fixed it. Drove the car for 8 miles and it was fine. Since it was 102 here yesterday it was a good day for a test. The coil fix ruled out vapor lock. He called last evening and it started again after driving about 2 miles as he and his wife went to dinner. After dinner it ran fine going home but the temperature had gone down.

I am a 6v guy so enlighten me on the conversion to 12v. How much resistance should there be from the coil resistor?

Should the voltage drop to 6v at the other end?

I believe he is getting 12v to the coil causing them to fail. He is bringing the car back this morning so we can start over again. I am going to check the wire very carefully. I haven't pulled the dash panel to look at the wiring in there.

John

07-20-2010, 08:12 AM	#1
John Stone Senior Member Join Date: May 2010 Location: Wichita, Kansas Posts: 710	Can't seem to find the problem I worked on a friends Model A all day yesterday. It runs fine when cool but when it heats up, it cuts out, back fires and no power. Pulling the choke out helps. The car was converted to 12v negative ground. Here is what was changed to known good stuff. First a condenser and then a complete distributer, carburetor, some MMO to the gas tank, gas cap in case it wasn't venting, spark plugs and coil wire. He said he had the coil checked and it was good and I checked the spark and it was good. Every change we made seemed to work for 2 miles and then the old problem started again. Finally put on a new coil and external resistor and it fixed it. Drove the car for 8 miles and it was fine. Since it was 102 here yesterday it was a good day for a test. The coil fix ruled out vapor lock. He called last evening and it started again after driving about 2 miles as he and his wife went to dinner. After dinner it ran fine going home but the temperature had gone down. I am a 6v guy so enlighten me on the conversion to 12v. How much resistance should there be from the coil resistor? Should the voltage drop to 6v at the other end? I believe he is getting 12v to the coil causing them to fail. He is bringing the car back this morning so we can start over again. I am going to check the wire very carefully. I haven't pulled the dash panel to look at the wiring in there. John