Re: IH 666 Weak Spark When Cranking

Posted by Steve@Advance on September 11, 2016 at 14:09:25 from (107.203.134.67):

In Reply to: IH 666 Weak Spark When Cranking posted by in-too-deep on September 11, 2016 at 07:49:06:

What you are describing is a symptom of weak spark. There are basically 2 reasons for this, either there is not enough voltage at the coil during cranking, or there is weak spark due to an ignition problem.

If everything is working correctly, on a low compression, 6 cylinder engine, there will be plenty of power to run both the starter and ignition system. Sometimes several things can add up to cause the problem.

A good place to begin is to check all the electrical connections, battery cables, grounds (especially where the ground cable connects to the engine block), starter cable, even under the dash at the amp meter and back of the ignition switch. A bad connection will show up as being corroded, loose, discolored from heat.

The starter is a common cause of weak spark during cranking. If the bushings are worn, the armature will drag on the field magnets, causing the starter to draw too much current. Typical symptoms are slow cranking when the engine is hot, or cold weather-cold engine start up. Also the battery cables will tend to heat up. The Delco starter is field rebuildable. New brushes, bushings, and starter drive will usually make it good as new for minimal cost.

As for the ignition system, it could simply be time for a tune up. Worn plugs will do exactly what you are describing. Anything that weakens the spark (bad points/condenser, points out of adjustment, low voltage to the coil primary side, weak coil) can contribute. Then there is spark delivery, plug wires, cap, and rotor. Resistor wires can break down internally, wire core wires can leak through old insulation. Look under the distributor cap, be sure the carbon contact is in place where the rotor contacts the cap.

Another clue, a weak ignition system will generally be more troublesome in damp weather, and can cause misfire under load.

As for the electronic conversion kits, I've never had a problem with one. They are a great solution for a distributor with worn bushings, and with the questionable quality of today's points and condensers, the kit is a real problem solver.

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