Re: Rough running 20 hp Mercury
Please post your serial number. If this indeed a 1974, you will find it has no points, so don't be in a big hurry to find them! The ignition on this motor has a CDI box on the side of the motor with a bunch of wires. If you see that, you have CDI ignition.
The spark comes from red coils mounted above the spark plugs on the back of the motor. Under the flywheel you have a stator which supplies current to the CDI box and a trigger that is actuated by a magnet on the flywheel and tells the CDI box when to fire. You could have any one of a number of problems, but it does sound like it could be ignition. Go step by step. I bought a cheap inline spark tester at Harbor Frieght ($5.99) which is very helpful in diagnosis. You unplug the plug wire from the plug, and fasten this in between. It has a bulb that lights when the wire fires, and the spark still fires, so you can see it with the engine running. You may need to shade it with your hat on a sunny day to see the bulb. This will help you see if the problem is isolated to one cylinder or if it crops up on both. If it is just on one, swap coils and see if it moves to the other cylinder. If it goes with the coil, coil is bad. If it stays with the same cylinder when you swap coils, the switchbox is suspect.
A few other possible causes of low end problems: 1. The ignition stator has two coils, low-speed and high-speed. But if the low speed coil was shot, seems like the motor wouldn't even start. But maybe it's just weak. 2. The trigger moves when you advance the throttle, and the wire from it flexes. If the wire is frayed you may be losing the trigger signal at certain throttle positions. These motors were notorious for the insulation on the wires rotting and falling apart. Check all the wires. 3. The wire from the terminal block to the kill switch is always suspect--disconnect it and see if your problems go away. You can kill the motor with the choke, then replace the wire. 4. You could have a sheared flywheel key, causing the motor to be out of time. 5. You could have bad crankshaft seals, messing up the mix at low RPMs. 6. Your low speed adjustment on the carb could be off. The high speed jet is fixed.
Grubbs has all the ignition parts, or watch eBay, although I have bought two NOS ignition coils for mine on eBay, at different times, that turned out to be bad. The ignition parts are expensive, so try to diagnose as much as possible before you start just throwing parts at it. If you have to buy them, as a mechanic told me when I bought a Volvo years ago, prepare your pocketbook!
Keep us posted on your progress. These are good motors, and the ignition parts are easy to replace so the mechanical work is not rocket science if you can figure out what the problem is.
