Firstly Welcome to ribnet!
And welcome to the "ancient Mercs" dept!
This engine is the generation before / overlapping mine, which means you have a few possible "lack of spark" potentials.......
This one uses 4 individual coils for the high voltage side - I found my HT leads (spark plug leads) had disconnected themselves from the push - in connectors. Pul lthem all out and check the state of the ends. - one of mine left the connector i nthe coil's socket!
An easy fix to replace them - take them to your local auto factors (to compare the length) - it was a £7 repair for my 3 cyl!
There is a black box with a million (slight exaggeration) screw terminals - the wires form it should go to the coils, and also dissappear undr the flywheel. Check all those connections are clean & tight.
Spark plugs - just change them. The ceramic absorbs water and stops insulating.
Now, rather than cutting (I hope you just pulled the bullet connector out!) the stop circuit there are three possible places that could be shorted o nthat stop circuit:
1) the deadman switch
2) the ignition on- off switch (it should short the deadman wire to earth when you put it in "off" position)
3) the mercury(as in the liquid metal - not the make!) "tilt protecton" switch. -Theory was if you tilted the engine high enough to lift the water intake out the water it would switch off to protect itself from overheating. I removed mine as it was causing problems.
Absolute worst case the 3 cyl Clamshells were reknown for the coils under the flywheel just suddenly failing - I hope yours isn't suffering something similar.
So, what I would do:
1) check the Spark plug leads (the thick ones) - pul lthem all out and check the connectors are making good contact with the wire itself!
2) check & clean all the connections to the "switch box" (the one with a million screw posts) - obviously only do one at a time or you will have some entrtainment putting it back together!)
3) replace the spark plugs. (or if you have a lawnmower ror similar use a known
working plug to check for sparks)
If none of that works, we're into multimeter territory - but don't panic-you can buy cheap ones for under a tenner.
So, if none of that works, post back and between the ribnet massive if you post pics we can talk you through what to prod with your meter....
Does your wiring look abit like this?