[many birds]

My engine shuts off if the idle RPM drops to about 650 RPM. It shuts off like as if the key is turned to off. Only seems to happen on when the engine is fully warmed up. Sometimes, I have noticed a misfire, like as if it cutout for just a fraction of a second, but not long enough for the engine to shutoff

With the help of Cliff's book and other members here, I think I've pretty much got my carb's idle and primary side tuned about best it can be.

I'm starting to think my ignition is the problem. The previous owner changed from points type distributor to a large cap HEI with Pertronix FlameThrower coil and module. I noticed that the shiny round capacitor is replaced with cube-looking thing. I figured it was probably a newer type of capacitor.

The timing is stable at idle. I have played with the weights, springs, ported vs. manifold vac, limited the vac can to about 12 degrees. I checked the dwell the using an old SunPro tach/dwell meter from the early eighties. The dwell is not varying with engine speed, it is fixed at 30 some degrees from idle on up. Could this module be the problem? Is there any reliable way to test it or just replace it with another module?

[TAQuest]

Originally Posted by many birds

My engine shuts off if the idle RPM drops to about 650 RPM. It shuts off like as if the key is turned to off.

Maybe it's the ignition switch? Just starting to go out. Where does the power for the HEI come from?

[Bruce Meyer]

Originally Posted by many birds

My engine shuts off if the idle RPM drops to about 650 RPM. It shuts off like as if the key is turned to off. Only seems to happen on when the engine is fully warmed up. Sometimes, I have noticed a misfire, like as if it cutout for just a fraction of a second, but not long enough for the engine to shutoff

With the help of Cliff's book and other members here, I think I've pretty much got my carb's idle and primary side tuned about best it can be.

I'm starting to think my ignition is the problem. The previous owner changed from points type distributor to a large cap HEI with Pertronix FlameThrower coil and module. I noticed that the shiny round capacitor is replaced with cube-looking thing. I figured it was probably a newer type of capacitor.

The timing is stable at idle. I have played with the weights, springs, ported vs. manifold vac, limited the vac can to about 12 degrees. I checked the dwell the using an old SunPro tach/dwell meter from the early eighties. The dwell is not varying with engine speed, it is fixed at 30 some degrees from idle on up. Could this module be the problem? Is there any reliable way to test it or just replace it with another module?

I had the exact same problem once. Turns out I wasn't getting full voltage to the Distributor. There was resistance in the dash harness. Had to bypass it to get full voltage.

[many birds]

Originally Posted by TAQuest

Maybe it's the ignition switch? Just starting to go out. Where does the power for the HEI come from?

Doubt it’s the ignition switch since it only happens at low rpm, and when hot. The wire going to the distributor is the black/double pink stripe. Voltage is full battery. No ballast resistor.

[Scott65]

Maybe check the voltage with a multimeter to be sure. Because that wire is supposed to be full voltage(an assumption on my part) doesn't mean there's not an issue.

[many birds]

Just checked. 12.35V at battery terminals and 12.04V at the distributor.

[Scott65]

Was it hot, under the conditions that have been giving you trouble? Running, or just static?

[many birds]

Worse on hot. Worse when under hood temps start climbing from idling—like after I’ve been trying to tune the idle mixture or timing, etc. or stuck in traffic. Coolant Temps have been well controlled since I went back to the stock flex fan and shroud setup—Stays between 180 and 190.

[Schurkey]

Originally Posted by many birds

I checked the dwell the using an old SunPro tach/dwell meter from the early eighties. The dwell is not varying with engine speed, it is fixed at 30 some degrees from idle on up. Could this module be the problem? Is there any reliable way to test it or just replace it with another module?

Is the dwell meter connected properly? Does the dwellmeter show varying dwell on a different vehicle with HEI? Any HEI module that doesn't vary the dwell is either designed improperly or is defective. Shiitcan the module, install a QUALITY replacement (Delco D1906, for example) and see what happens. There is the outside chance that a faulty ignition coil could also cause dwell problems, but I'd start with the module since what you have now is a boutique aftermarket item. (i.e., a generic aftermarket unit with a fancy sticker on it, no verification that it ever worked right, stuffed into a custom-printed box, and the decimal point on the price moved over at least one position.)

There have been rumors and mutterings that even the Delco module isn't what it used to be. I'm scouring the Treasure Yards buying HEI cores.

Originally Posted by many birds

Just checked. 12.35V at battery terminals and 12.04V at the distributor.

Battery voltage if fully charged should be 12.6--12.7. Your battery is way under-charged.

[aceaceca]

I have had 2 mallory distributors for Chevy's that were bad out of the box .As soon as the car warmed up the motor would just die. One was a Unilite and the other an HEI. For the GTO I got the cheap China HEI.
It was a joke. So poorly made. I bought 2 used Delco HEI 's for our two GTO's and no problems. I would definitely follow Schurkey's lead.

[many birds]

I did some more testing regarding the dwell. Initially, on cold startup the dwell does vary with RPM. It goes between about 15 degrees at 900 to 30+ degrees at 2000 ish RPM. However, as things warm up, not totally hot yet, the dwell pins itself at about a little less than 40 degrees—RPM changes make no difference.

[Scott65]

I think you're working with a mathematical impossibility there at some point. I'm surprised that it showed up at low rpms first. 40 degrees of dwell becomes impossible at a certain speed. I'm to tired to do the math...

[many birds]

Nope. Mathematical limit is less than 45 degrees.

Here's how I arrived at that:

360 degrees divided by 8 cylinders = 45. Therefore, dwell cannot exceed 45 degrees.


And yes, I have questioned my dwell meter and plan on checking it against my '75 TA with the stock, oem module sometime later this week.

[many birds]

I opened the dwell meter to see if something was obviously amiss. All the resistors and caps tested ok on my multimeter. That's all I can test with my multimeter.

I ran the engine for about 7 to 8 min while checking the dwell--didn't want neighbor complaints

I'm guessing it would have eventually settled at 30+ degrees like it used to before once it got fully heated up--just an assumption.

[Scott65]

Yes, but you have to factor time I believe. How fast can the the coil charge/discharge at the proper volts/amps in seconds/milliseconds etc.

[70GS455]

In addition to rpm, a GM HEI will also vary dwell based on supply voltage (more dwell at lower battery voltage) and coil current (tries to saturate the coil until current limit is reached)

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[389]

Put the stock single points system back in. There is nothing wrong with it up to 6000 rpm except the nonsense that people who don't understand ignitions say..

[many birds]

Originally Posted by 389

Put the stock single points system back in. There is nothing wrong with it up to 6000 rpm except the nonsense that people who don't understand ignitions say..

Agree, it’ll fit better in the limited space also. Only thing is that I’m missing parts. Previous owner disassembled it and ground the gear down and I I don’t have the coil or ballast resistor or breaker points. Or the knowledge regarding rebuilding it. Bottom line, to get it working is going to cost about the same or more than changing the module. I’ll probably look into it during the winter months when the car is resting. It’ll be a good learning experience.

[Scott65]

Msd ready to run is the size of a points unit, and will run with or without an msd box.

[many birds]

Originally Posted by Scott65

Msd ready to run is the size of a points unit, and will run with or without an msd box.

I’ve actually been thinking about your awesome write up on the SFI with mega squirt. I love the idea, but cost is a major question.

Since I’m cheap, I was thinking of getting a stock 301 computer controlled distributor for an ‘80 or ‘81 TA and using megasquirt 2 or microsquirt for timing control and adding an old Holley Projection TBI (that I have sitting around) for fuel. The Holley kind of sucked with the crappy controller it came with but should do pretty well with the MS— I think?