Mid 1980's 70hp Running Issues

[tpal988]

So I purchased a 1990 Sunbird from a guy who "just put $900 into freshening up the engine". Well I didn't notice the little hint he added saying "We always just puttsed around in it, never really got on it," until after it quit running the first time out.

So here's whats going on. It starts, hard, and barely runs. It sounds like one of the three cylinders is missing bad. I replaced 2 of the coils after seeing that the iron core was all busted up in two of them (replaced with Sierra instead of Mercury). Didn't make much of a difference. So I replaced the wires and boots on the plugs to make sure they weren't grounding out (they were a bit rough looking), no change. I used an inductance tachometer to check that there was juice going through each wire, which there was. But one wire seemed to read a much higher RPM (firing more often) than the others?

This makes me think its a timing issue. Not sure where to go from here. Any assistance?

About 126 psi in each cylinder, one fresh coil, one of the iron cores cracked during installation of second new coil (fixed with a copper wire to ensure continuity), and an older Mercury coil that still looked good.

Is there a way to test these coils? They are open loop from plug end to pos/neg and closed (no resistance) from pos to neg. Also, I believe the iron core just needs to provide continuity from top to bottom all the way around the coil, correct?

Thanks in advance for any help!

TJ

 

[orbanp]

Re: Mid 1980's 70hp Running Issues

Hi TJ,

Welcome to iboats!

It would help in the future if you gave the exact model number and year of the motor, it helps to nail down variations!

In any event, the ignition coils should test as follows: the low tension wiring (between + and -) should read 0.02 - 0.04 Ohm, it would read close to zero for you for all practical purposes.
The high tension coil should read around 9 - 12 KOhm between the plug connection and ground. There is no electrical connection between the high and low tension windings (infinite resistance readings).
Ignition coils usually do not have closed iron cores, like a mains transformer in a radio or any other electronic equipment, just a core in the middle of the windings. There is not much to go wrong with them, it could be rusty and the coil still operational.
What you should be concerned with is the condition of the outside insulation of the ignition coils, cracks in them usually render the coil useless.

You could also check the trigger coil windings, those are the wires going to the CDI unit (switch box, as per Mercury nomenclature).
There are three trigger coils for you, one end of those is common, it is the white/black wire, and goes to the CDI box. The individual trigger coils are the brown, white, and violet wires going to the CDI box. The resistance of the individual trigger coils should read about 1.1 - 1.4 KOhm according to the Seloc manual.
Disconnect the wires when you make the resistance measurements!

I also understand that older Mercurys have real crappy wiring harnesses, the insulations and connections could deteriorate quite badly. Check out all the wires and connections!
Also, try to find a factory service manual, it would be a worthwhile investment if you keep your motor for any length of time!

Good luck, Peter

 

[CharlieB]

Re: Mid 1980's 70hp Running Issues

The stator generates all the electricity, the switchbox stores it, the trigger tells the switchbox when and which coil to fire.

The stator has three sets of windings, lo speed ignition, high speed ignition, and battery charging.

Test spark on each plug wire using a 7/16 inch gap to ground. The ignition must be capable of arcing a crisp blue arc across this gap or the motor can be hard to start.

If it does generate this nice fat arc then your problem is likely to be dirty misadjusted carbs.

 

[quicktach]

Re: Mid 1980's 70hp Running Issues

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