71 1210 spark disappears when engine in hot

[71ih1210]

I have a '71 1210 with a 304 sv engine. It starts runs drives great up until a certain point. What used to be an occasional inability to start after running for more than approximately 30 minutes or so has turned into occasionally dying while idling once the engine is hot. So my problem seems to be getting worse. I have replaced the wires, points, coil, condenser, cap, rotor, basically everything I can suspect something wrong with better products. The symptoms seem as though the truck is overheating despite my dash temperature gauge (knowingly not a diagnostic tool) staying ho hum middle left. The engine is getting fuel (a beautiful mist) and the points have a spark always, but the spark doesn't make it to the spark plug when this failure occurs. For some reason taking a sparkplug out and grounding it on something helps me start the engine. Then I pop it back in and most generally go on my merry way. I was thinking about replacing my Holley points setup with an electronic conversion. Any help or advice would be very much appreciated. I read about someone else having a similar problem somewhere on the web with maybe a 345, but nobody responded. Thanks,
-will

 

[Michael Mayben]

Two things are normally root cause here....but do not perform any conversion until it's running perfectly, changing parts out now will not correct any issue, only make it worse. Once it's back on track, then make any mods...one item at a time and verifying after completing each change.

First and easiest check...remove the bulkhead connector which has those heavier 10 gauge wires going in and out. You will most likely find the male/female contacts to be real dirty and some will be charred from overheating due to poor contact over the years. Clean those contacts well, in many cases they need to be eliminated and the wires spiced to insure continuity and low/no resistance in the circuits/wire runs.

As the connector heats from underhood heat source along with high resistance in the connections, your primary voltage is being reduced to the point the coil can't fire a plug when under compression (but will fire in the open atmosphere).

Then go for a test drive and see if that corrected the issue.

If not, then test the primary resistance of the coil (all wires disconnected), it should be in the range of 1.4>2.0ohms when cold. Then when the issue occurs, quickly test the primary resistance again, if it's higher than say 3.5 ohms, then the coil is failing under heat/load and should be replaced with a service part....very common occurrence.

The bulkhead connector under discussion also contains the ignition feed "resistance" wire which is fed from the ignition switch when it's placed in the "run" position. That wire could be failing inside the harness it's wrapped in and will eventually either burn into an "open" condition or create issues with burning/melting adjacent wire insulation resulting in a "short in those circuits and an "open" in the resistor wire. So while the bulkhead connector is separated, make a continuity test in that wire run (circuit #18) which is the whitish wire wrapped with a braided fiberglass insulation which runs from the connector to the coil positive terminal, the nominal resistance you are looking for in that wire is 1.8ohms when it's hot (yes, that wire is supposed to get hot when the engine is running, that is why it's wrapped inna fiberglas braid).

 

[71ih1210]

Thanks for responding back so quick. My 9 month old new lap top crapped out on me and is now in taiwan or wherever they fix under warranty so I'm using my roommates antique model. I replaced the points again before I read your response and I'm back to where I was initially- not dying while driving, but still difficult to start while hot. I'll check that bulkhead connection and and resistance in the next couple days. The wiring looks moderately sketchy under the hood and I do recall seeing the white fiberglass insulated wire running to the coil. I have tried running a jumper wire from the battery to the positive terminal on the coil. Didn't really help. I'll keep you posted on the readings and diagnoses. Thanks for for being apart of a great site in a field with such few resources.
-will

 

[71ih1210]

I pulled the bulkhead connector today and inspected the connections. They didn't look terrible, but definitely looked their age. I cleaned those up and got readings from three of the wires of 2.2 ohms, 20-22 ohms, and 2.0 ohms- these three wires were on the upper part of the connection. The other three wires (two on the bottom part of the connection) showed no continuity.
I started it up and it ran fine as it usually does, but I didn't take it on a problem facilitative extensive journey. The high resistance reading of the one wire makes me think it's touching something its not supposed to or it is a bad connection. I need to unravel the bulk of wires on a day I have more time and follow some trails.

 

[71ih1210]

Boiled this down to the points condenser needs to be replaced every couple months. Don't know what's causing this premature wearing out, but hopefully somebody else will be able to use information.
-will

 

[Michael Mayben]

boiled this down to the points condenser needs to be replaced every couple months. Don't know what's causing this premature wearing out, but hopefully somebody else will be able to use information.
-will

You are describing the condenser/capacitor inside the distributor which has it's pigtail connected to the insulated side of the breaker point and coil negative terminal???

If so, what you are describing is unheard of! If so, then the breaker points should be overheating and heavily "arced" (carboned) and beyond use also. There are millions of similar capacitors in use daily inside distributors of all kinds that are more than 40 years old and have never been replaced. I never replace 'em when I service a breaker point system unless they are bad.

Are you sure you are not dealing with the capacitor/condenser that is mounted to the ignition coil clamp? The only function of that item is rfi attentuation for the am radio "crackle" noise. It can be tossed and is not needed for any engine function whatsoever.

 

[71ih1210]

Since I've owned this truck maintaing spark has been an issue. This was the largest problem I've had with maintaining the reliability of the vehicle. I replaced everything I could think of as being a problem for dwindling spark. Replacing the capacitor inside the distributor was one of the last things I did. When I did that the vehicle started literally everytime hot, cold whatever. After about 5 months it would crank an extra time or two and then fire. Until it didn't. I replaced the capacitor inside the distributor and reset the points which looked gapped and alighned decentely before. The problem went away. I'm nearly satisfied spending $8 a couple times a year to make my truck reliable. I could totally believe I am treating a symptom for a problem such as points overheating. How do points overheat and how can that be corrected? The actual starting problem did occur after I replaced the points.
-will

 

[Michael Mayben]

Breaker points are "overheated" (their current carrying capacity is exceeded) by two things...

1) the ignition coil has the wrong primary resistance value. For the sv system, the coil should have a primary resistance value between 1.4 and 2.0 ohms.

2) the "resistor" wire has been bypassed or eliminated. The wire which carries current to the coil positive terminal when the key switch is in the "run" position.

An sv engine must have those two conditions met in order for the breaker points to survive a typical 12>15k mile life cycle before replacement.

Put an ammeter in series with the coil negative terminal, and the wire going to the points/distributor and crank the engine. We want a current reading of between 2.5 and 3.5a, any less, you don't have adequate "coil saturation" for engine performance above an idle. Any greater and the breaker point life is drastically shortened to maybe only a few hundred miles.

"overheated" breaker points will show evidence of heavy arcing at the contacts, a "blue" section in the insulated contact arm just behind the tungsten contact patch, and another "blue" area where the insulated contact arm is riveted to the spring. The gap will close very quickly also since the insulated arm is overheated to the point it is easily bent from spring pressure.

"dwell" determines proper ignition coil coil saturation time, primary current is controlled by coil primary resistance value and the correct voltage "feed" value to the coil primary. Dwell and current control add up to proper coil saturation and strength of the magnetic field in the coil system.

Proper condenser/capacitor value aids in creating the collapse of the magnetic field in the coil which in turn induces the high voltage needed to ionize the spark plug gap. And with an incorrect/faulty capacitor, the breaker points arc with great force when making and breaking, which in turns leads to drastically-shortened contact life but not overheating of the breaker point set itself.

So...measure your "resistance wire" resistance, that wire is 68">72" long and should measure a nominal value of 1.8ohms. Measure the coil primary resistance and see if it's within the spec mentioned above.

The distributor breaker plate system must be sterile internally and the ground lead intact. When new points are installed, we want "point resistance" to be as low as physically possible, about 0.3ohms is as low as can be achieved witha sterile distributor that is properly mounted witha clean ground path.