1925 Chevrolet Superior 171ci 4 Cyl Run issues

[SBlomquist]

The car starts and runs great when it is at ambient temperature. I can start it several times in a row with no issues. After about 5 minutes or so of running it begins to run rough, lacks power, and acceleration. If I turn the car off and try to start it again, it will turn over fine but never start. I can let it sit for an hour or so and it starts fine. Recently, I have gone through the carburetor, distributor, generator, starting motor (was cranking slow), set the ignition timing, has new spark plugs, plug wires, no vacuum leaks that I am aware of, and the compression is good. The coil primary and secondary circuits are 1.5 ohms and 3.8k ohms.

Looking for some insight on the ignition system. The coil is of the original design with a removable resistor mounted on top at the "BAT" terminal. It is an old exposed "coiled wire" type resistor and gets hot (will burn your fingers) after a minute or so of the ignition being turned on. Is this a normal temperature? If not, what can I replace it with? Is the resistor in the circuit to lengthen the point life?

[Alfa]

Hello

 

Sorry that I do not have specific experience of the 1925 Chevrolet, but the resistor that you mention, might it be a ballast resistor? Is there also a direct (not through the resistor) feed to the coil from the starter switch for instance? If it is a ballast resistor it will get too hot to touch for certain. Alternatively, it might be a type of interference suppression. Can you measure the voltage between live and earth (with ignition on, engine off, but points open) before and after the resistor to see what effect it is having?

 

One other thought, from your description of the problem, is the fuel tank vented? You might try running it with the fuel cap removed to check that.

 

Hope this helps

Adam.. 

[Old buicks 2]

My 2 cents worth,   is the secondary winding in the coil breaking down when it gets hot?   I had a coil several years ago that did exactly the same thing you are referring to.    The secondary winding had a small break in it and would start and run fine until it got warm and then would expand and open up the gap in the fine wire just enough to stop the engine.  

 

Tom

[rsb]

In addition to what has been mentioned, I would try substituting a coil from a newer vehicle such as a 1954 Chevrolet with a condenser on the negative lead and no resistor. The original coils are known to be unreliable. They have the condenser built-in and unless it has been taken apart, as in having the tar melted out and condenser replaced with a modern equivalent, it's definitely bad by now.

 

Another question, do you have the carburetor intake hooked up to the pre-heater on the exhaust? These motors run best with warm air.

 

[AlanT]

Is it a 6 volt car? It probably was, originally. If so, I doubt that you should be running a ballast resistor. Ballast resistors were introduced to limit current to the ignition coil when manufacturers switched to 12 volt systems. The ballast will increase resistance as it heats up. That's what its designed to do. That could explain your symptoms, unless the car has been converted to 12 volts.

[rsb]

The original coil had a resistor on top by design. If you replace the original coil with a newer 6V coil and condenser, you do not need a ballast resistor.

[SBlomquist]

Alfa, the resister mounted on the coil is an original design and everything I have seen indicates this. I do not think it is a ballast resister as they weren't used until the later years. Their isn't a wire direct from the power terminal of the starter switch to the coil. Need to do the before and after voltage measurement. The fuel is gravity feed and vented.

rsb, the carburetor intake is connected to the stove exhaust.

AlanT, The car remains a 6 volt system.

I have a new coil ordered that is setup with the correct value resistor attached. The company has done some research on it and has sold many of them to Chevrolet owners. I think this might solve my issues. I'll post if I still have issues or the issue is resolved.

[Dave39MD]

SBlomquist, What company is selling the coil you described?

 

Thanks

 

Dave

[26-25Buick]

I agree the resistor is not there as a ballast resistor. If it’s the one I think it is it is on low voltage side of the coil to protect the windings of the coil if the ignition is left on without the car running. The wire heats up and the resistance increases and protects the coil by lowering the current. So yes it will get hot if the car isn’t running. 

Another quick thing to try is swap out the  condenser for a new one from NAPA.

[SBlomquist]

Dave39MD, Hagen's Auto Parts, Puyallup, WA. Ph # 253-845-7020

26-25Buick What condenser? The distributor does not have a provision for a condenser inside or outside. I adapted one for a 6 volt on the outside to try with the present setup and no change in the performance.

[Vintageben]

I had a similar problem with my 27 Chev a few years ago, it ran fine while cool and it was in winter which really isn’t that cold but is bit cool in the morning anyway ran fine until mid morning and I hit a really big hill and the car ran terrible. Then it ran well for an hour after lunch then the same thing missing and carrying on. By this stage I had replaced the coil and condenser and didn’t know what else to try plus I was a couple of hours from home so eventually through trial and error I traced the problem to faulty spark plugs, it was all very frustrating. So if the problem persists don’t forget plugs and leads 

[SBlomquist]

I received the coil last weekend and installed it along with a condenser for a 1942-1954 case VA series four cylinder 6 volt tractor. I have it purring like a kitten now! Sweet!