"Merc has/had 4 different vers
"Merc has/had 4 different versions of the Thunderbolt ignition.
The earliest one T1 was also known as a Phelon ignition which was just a Magneto system with points/condenser.
The T2 was the one where they started putting "Thunderbolt" decals on the motor. This was also called a "Phasemaker" ignition and it still had points (but they renamed the condenser a capacitor - yipee)
The T3 was a modular CD ignition, so that the charge and ignition coils were mounted on a common base, but were still individual. And the system used a trigger, early switchbox with an external SCR (which took the pulse from the trigger and told the switchbox which plug coil should get the power).
The T4 started seeing service in the late 1970's and stayed as Merc's premier ignition system up until they moved to computer controlled ignitions (and is still used on the 50 and 90 horse two strokes sold outside of the US).
This ignition is the modern Stator (single component), Trigger, Switchbox and spark coils and basically after the late 70's, because it was near perfect, it was never changed.
Merc also horsed around with a CDM ignition system (mostly on 3 and 4 cylinder models) for a while, but it never really took hold.
If you are looking for an excellent motor in the 100 horse class you should set your eyes on a 3 cylinder 90 horse Merc, 1985'ish or newer.
The older 100 horse used a different rating system, so by todays standards only put out about 90 horses.
The earlier 90 models put out close to 100 (until the EPA made the mfgs tighten up their emmissions), and the best part is, as I stated above, the 2 stroke 90 is still in production - here in Canada I can walk into a Merc dealer and buy a "brand new" model - so parts support is 100%.
Merc has a few models that were very prolific and tended to be fairly trouble free (with regular maintenance) - in the size you are looking at, the 90 3 cyl and the 135 V6 come to mind immediately.
I would stay away from the 115/125 inline four's that have the funky 2+2 CDM ignition, not because they are particularly bad, but they are a "quirky" motor that never (seem) to run "quite right"
And if you don't limit your options just to Merc's, there are others out there in the higher horsepower that were very decent engines. I'm pretty much a Merc guy, but my biggest motor is V4 OMC Looper (140 horse) - why? Couldn't find a 135 Merc for what I wanted to spend and considered that "the next best thing in that size"
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