“I try to start it and it won’t turn over, sounds like the battery is going dead and if I continue to try to start there’s a burning smell (maybe or maybe not related?).”
If that means that the starter motor is not rotating the engine or that the engine is rotating very slowly (some people are rather loose with what ‘turns over’ means), then I line up with the other people who have responded that this problem is likely in the starter motor system which could include:
- the battery
- the electrical connections at the battery
- the chassis ground connection on the battery – terminal
- the cables connecting the battery to the starter motor and chassis ground
- the starter motor and its solenoid
-the ignition switch
Last item first. If the starter motor does not turn at all the ignition switch on a 1971 would be a likely candidate. Ignition switches are notorious for becoming intermittent before they fail completely. However, if the starter motor is turning over; but, slowly it is likely not your ignition switch. As an observation, an ignition switch on a 1971 will be nearing the end of its life and exact replacements for the up to 1971 are not available. It would be prudent to remove and disassemble the ignition switch so that you can clean and lubricate the contacts to extend its useful life.
First item on the list. How did your mechanics test the battery? Did they use one of those little hand held electronic load testers? If so, they are notoriously unreliable. The most reliable indication of battery condition is one of the old school load testers that operates like a big toaster oven. If your starter motor sometimes spins the engine fast then the battery is probably not the problem.
Next three items. Cheap to address because it involves a physical inspection of all the connections and the cables. Deteriorated battery cable clamps are notorious for becoming corroded and creating poor electrical connections causing starting problems.
Starter motors can operate intermittently and the problem is usually the contacts on the starter motor solenoid. What did your mechanics do when they ‘checked’ the starter motor? Did they remove it and examine the solenoid contacts and the starter brushes and commutator for damage? As noted by Eng Tech the starter motors are rebuildable. However, in this case I would be inclined to replace it with a later Bosch SR 437X permanent magnet starter motor which all Volvo red blocks engines had from about 1985 on. The PM starter is smaller, draws less current and will spin the motor faster under the same conditions than then old style starter. The later starter is pretty much a drop in replacement.
“and the first time I did that (I started the car about 3 hours after doing the usual errand run scenerio), there was a “pop” almost like a back fire but in the engine not in the tail pipe and the car started the next morning.”
People confuse back fire and after fire. If the ‘pop’ comes out the tail pipe that is an after fire and is the more common event. Back fires occur when fuel ignites in the intake manifold and the ‘pop’ comes out the carburettors. A really good back fire can blow the air filter housing off. True back fires are really rare. In order to get a back fire the valve timing has to be really off or the ignition timing has to be really off. Generally if the conditions for back fire are correct the engine will not be capable of running or at best it will run really badly. If the car drives reasonably well then the conditions for back fire are not present. However, if something is loose in the distributor the ignition timing might be moving around creating the temporary conditions for backfire. However, distributor problems will not cause the starter motor to turn over slowly or not at all. Don’t waste time inspecting the distributor until you get the starter motor to consistently spin the engine at a good speed.