Remove any good film. If you have an old roll (out of date), you can load it. Otherwise do the test without film. It will not make much, if any, difference. This testing assumes the camera is in serviceable condition and ready to shoot film. Any problems you might find may not be with the motor but may be with servicing, i.e., lubrication, dirt, etc. Don't be alarmed if the camera "fails" the test. It may just need a service and not a motor. Or it may need both.
Bolex tests camera motors with a specialized piece of equipment. This process detailed below will show you how to check the motor without any equipment by listening to the sound made by the motor's gears and related mechanisms. The following procedure works on any spring wound motor.
1) After removing any good film, make sure the pressure plate gate lever is in the forward or "run position", i.e, against the film or gate area. Close the film door and lock it.
2) Set the FPS selector to 24 FPS.
3) Fully wind the camera.
4) Run the camera and listen to the sound of the motor. If you hold the spring motor close to your ear or press the camera body against the side of your head near your ear, you can hear the motor speed/ sound much better.
5) The motor sound should be consistent from beginning to end. If the motor begins to "slow down" (sound will change) start counting in seconds. One thousand one, one thousand two, one thousand three, etc. If your counting, remember the number of seconds.
While the motor is running, it should run smooth. That is, it should not make any "bumping" or grinding noises, as though there is something "wrong" with the spring in the motor. If the motor went from the beginning of the test (full wind) to the end (stopped) and you cannot determine, or hear if it is slowing down, then the camera motor is strong. A full wind will run approximately 21 seconds at full wind and at 24 FPS.
If the camera's motor slows down within 2-5 seconds of the end of the wind, the camera motor is a little weak but still serviceable. Bolex recommends fully winding the motor after each shot. A typical shot may be 3-10 seconds or longer - up to the 21 second limit.
Remember this is film, not video. Film shooters plan their shots in advance and accurately estimate the length of the clip. They break up longer shots into less than 20 seconds. No wasted footage or camera stopping during a planned shot. Videographers tend to shot everything and sort/cut in post production. If you know the camera is weak in the last 3-5 seconds, limit your shots to 14-15 seconds or so.
If the motor begins to slow down, and your count is higher than 5 seconds, the motor is considered weak. This could be from lack of lubrication, the speed governor assembly could be dirty, or some other problem. Service is indicated.
New replacement motors are not available from Bolex for the 8mm "Double Run" series. The 8mm series of Bolex "Double Run" cameras have been out of production for approximately 50+ years. However, there is an ample supply of used cameras for sale on eBay. If you need another spring motor, you can buy a complete camera for parts. Look for a camera based on cost. Typically the motor will be ok.
Try to purchase a camera with of the same model number and a serial number near yours. If that can't be done, try for a close year. If you have the serial number, you can look up the year of manufacture here:
Spring motors can be damaged from overwinding, overuse, or long term storage. Bolex says you cannot overwind - but you can. If the camera was stored fully wound, the flat spring in the motor to take a permanent "set". In this case, it will make a "bumping noise" when running. You WILL hear the "bumping" as well as feel it.
The are at least 2 different spring motors for the 8mm double-run Bolex cameras. The difference is the diameter of the (1) spindle drive gear located at the top of the spring motor. It engages both the lower spindle drive gear and the footage counter drive gear. Both these gear are located on the underside of the top plate. There are (2) spring motor drive gear diameters: early - .648" and late - .683".
If you get another camera (late) as a donor and your camera (early) has the other motor, it may still be usable. You can switch top plates with the cameras and the take-up spindle and counter drive gears will mesh correctly.
No guarantees here. Some top plates are different. The P series (Reflex) top plates are not compatible with non-reflex series top plates. The reflex series plate is missing the "notch" cut out of the back of the plate where the redesigned case now covers the redesigned footage counter mechanism.
Bolex parts are mostly interchangeable, however. When newer models are released sometimes older parts are used. It is NOT uncommon, for example, to find a D8LA camera with a serial number for a D8L.