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repairing ferrite/magnets of old ignition coils

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peter1965

Member
I have noticed that my blue-band 4 cylinder 50 hp (1979) motor misses at wot, starts well,planes but after a few minutes starts to miss. started the motor at home on muffs at night and after about 3 minutes noticed that the 2nd cylinder ignition coil is arcing between its center and the metal board that the coils are held to. I removed the coil and tested the primary and secondary with a voltmeter, ohms are in specification, however the ferrite magnets appear cracked on both sides. The other 3 coils are also withinn spec but have intact magnets. These coils are very hard to find i have tracked down an outboard wrecker that has a few but their condition is questionable. I guess the questions i have are
1) would a cracked magnet make the coil defective, and cause the miss at wot.
2) can i repair the magnets (if i cant find other ones)
3) can i use the new black coils as a substitute (spoke to the mercury dealer and he wasn't sure as they are for 1980 and newer motors.

regards Peter
 
Yes, no and no.

Do yourself a HUGE favor and buy a new Sierra coil from this website.

I've run into a whole bunch of these old Merc coils that have gone bad of late. It appears that the wire insulation inside is breaking down after decades of use. Also, you might need to replace more than one.

Jeff
 
Agree with Jeff.

In answering your question "can it", the answer is yes. The iron core in the coil of wire has a tremendous effect on the size of the magnetic field, hence the energy you can produce and get to your plugs for firing. The physical constraints of the coil are factored into the "magnetic field" requirement. Any alteration of any gap, number of laminations, material structure change or magnetic properties will (usually...you can bet on it) decrease the ability of the coil to produce as required. High rpms means higher temps and the magnetic material changes characteristics as a function of temp for the worse as temp increases. So if it was marginal when cool, due to the separation, it very well may not supply the required amount of energy when hot.

Lot of blab but it keeps my mind and typing skills active "beins" I'm retired.

Mark
 
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