Try to turn the motor over by hand if you can , with positive battery post disconnected for safety it should turn some , battery hooked up correctly ? Positive should go to one side of solenoid ground - should go to frame or motor base .
Charge the battery , 12.4 is basically stone dead . 2 amps or less 12 hours and see what you have then . Right off the charger it should be about 13-13.2 a 1/2 hour later should be 12.8-13.2 .If you can have it load tested .most auto parts stores can do this for you .
I have used business visions for years and very satisfied . did everything I needed then and more as I am not an accountant but he had an easy time at year end . I was after inventory control and location and it did that well . requires you to input product as it arrives every day but that was...
Make sure you have the throttle wide open while doing the test as well as all the other spark plugs out to reduce rolling resistance . Put the plugs in the caps and lay them on the heads while doing it . Jmho
I could not tell a customer its not worth fixing till it is apart and the cause found . The fact that it has no external damage and it still runs is suspect . Obviously the crank is not broken or the cam shaft either . Not a lot of room in the cases hard to think it broke a rod and did not break...
Was it something you forgot to include in the quote ? Can t imagine you have been in business that long and don t have a disposal service already . What have you been doing till now ?
Looking at the pic the muffler heat sheild is cracked from both top mounting corners as well . Not likely to contribute to noisy but it will likely rattle and break off with time . While your there weld it up ?
Start by doing a compression test and then adjust the valves and replace the plug and see what your compression test reads after to see if there is any improvement .
If you step on the snap on truck the calibration device is usually on the wall behind the driver. Beam type and several klicker types and a new digital beeper one , checked every year . And likely the only thing free on that truck !!