Hi. I have an old (early 70s) Mercury Thunderbolt 650 S/N 33353051 outboard motor that just stop working. It cranks fine and fuel is getting to the cylinder but won't start. Troubleshooting reveals no spark at all. What else should I be looking at? The switch box? The ignition coil? Stator? Cap/Rotors? Plug wires? Any help would be appreciated. Thanks.

I honestly don't remember that system anymore.
I haven't seen one in at least 20 years.

If you look into CDI electronics they should have a troubleshooting guide for your motor.

Thanks for the reply. I downloaded the troubleshooting guide and will try to figure out why the engine stop working. Maybe it's time to let it RIP..

Check if there is an small wire running from the coil under the flywheel to the block for ground or to an electrical connection for power. I think that vintage engine ignition design rotated the coil to change ignition timing (via connection to throttle linkage). That small wire can break from years of movement and the conductor can be broken even if the insulation is intact. I may be confusing your engine with the mid 80's versions so sorry if I have given bad info.

is your 650 a 3 or 4 cylinder??

It's a 3 cylinder. Engine works perfectly before the no spark problem.

With my 65 hp Thunderbolt, I had a bad connection to the control module box. take all the wires off one at a time and clean with 500 grit emery and reinstall. Is your timing belt good?