I am just starting out learning on how to fix up old cars, but i’m having problems with the carburetor but now it barely cranks over and the battery is full and all the fuses work. It’d be great if someone could help a guy out.
When you say "but now it barely cranks over", does that mean it was cranking normally and then it suddenly began cranking slowly?
Anyway, the starter motor electrical circuitry is fairly straight forward. It consists of:
1. The battery
2. Two battery cables; one runs from the battery's positive terminal to the ("B") terminal on the solenoid and another that connects the battery's negative terminal to the engine block.
3. The starter cranking motor and an attached solenoid
4. The control wiring - runs from the SOL contacts in the ignition switch to the ("S" terminal) on the solenoid (always a purple wire)
5. The engine block
The battery is fully charged - so it can probably be eliminated.
The solenoid can be assumed good because:
A. It is able to engage the starter motor drive pinion with the ring gear on the flex plate/flywheel and
B. It is able to close the internal contacts - which supply power from the solenoid's "B" terminal to the starter motor windings.
It follows that because the solenoid is functioning, the ignition switch and the purple wire that runs to the solenoid "S" terminal are okay as well.
The engine block just acts a conductor that provides a ground path from the starter/solenoid windings back to the battery negative cable. The starter/solenoid windings are grounded to the engine block by the starter mounting bolts. To complete the ground circuit, one end of the battery negative cable is bolted to the engine block (usually at the alternator bracket) and the other end is clamped to the negative battery post.
What cannot yet be positively eliminated:
1. The condition of the battery cables and that they are properly connected at their terminal ends
2. The starter motor itself
The current flow for the cranking motor windings is like this:
Starting at the battery positive, power is always available to the solenoid's "B" terminal. When the ignition switch is moved to the START position, power is supplied to the "S" terminal on the purple wire. At that point two things happen:
1. The solenoid plunger pulls a lever which engages the starter pinion gear with the teeth on the ring gear of the flex plate.
2. It also connects a large set of contacts which then begin to supply full battery voltage from the "B" terminal to the cranking motor windings.
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You are concerned with number 2 (and the negative battery cable) - number 1 appears to be working as designed.
I would begin by checking/replacing the battery cables and confirming that they have clean/tight connections at both ends. If cranking speed doesn't improve with new cables, you may be looking at a new starter.
