Timing too Retarded?

[MIPS]

Slightly related to previous issues with my rebuilt engine running too cool, on cold days if I try and accelerate hard the engine has no beans and stalls. Sometimes it will even backfire. Once the engine has completely warmed up however it isn't a problem. I've been running tests since last week and I can conclude it's the computer keeping it too retarded, but it's not the computer at fault. Here's my theory:

The CeC will not enter closed-loop until the TAC switch opens (and the coolant switch closes) and signals the intake air temperature (and the engine) is sufficiently warm for the carburetor. I've verified the switch is good and the warm air duct and the vacuum switch for it are working, just with a colder than normal engine the manifold's not getting hot enough fast enough for what I'm used to and it's taking longer to satisfy the TAC switch. While the computer is being told the air cleaner is too cold still it retards the spark as much as it can get away with to generate waste heat and warm up the block and the exhaust. As soon as the TAC opens it adds spark timing again and you will hear the RPM drop. You can see this on a hot engine by unplugging the TAC and jumping the pins. Without activating the sol-vac or idle control solenoid the RPM will increase and it will exit closed-loop until you pull the jumper.
When idling and under light loads this isn't a problem but if you try to accelerate hard (EG: you turn a corner and give it the beans to enter onto the highway) you still have a very retarded spark when it doesn't need it. Because the Eagle doesn't have a Throttle Position Sensor it relies on the two vacuum switches to detect a heavy load condition, at which point it takes a second but it adds as much timing electronically as the engine wants (on top of whatever the vacuum advance can deliver), BUT if the base timing is too retarded you'll feel that brief lag before it can advance the spark and that's the issue I'm getting.
I've been getting around it the last few weeks by creeping into the pedal before it completely stalls and eventually the computer bumps the timing and you're good to accelerate normally.

So when the engine was rebuilt I had the shop verify my Dorman harmonic balancer had the timing mark in the correct location because I've seen a lot of people say it isn't. They found it was two degrees off and marked the actual TDC with silver pen. Well I'm dumb and didn't make a more permanent mark and the marker has faded away and I also can't remember if the drifted mark was two degrees too advanced or too retarded.

[MIPS]

And I've also now confirmed it's not a bad ignition module, a misbehaving EGR or a vacuum advance problem. (or a misbehaving CTO, TVS, delay valve or vacuum switch)

[MIPS]

Plot twist: It WAS the Duraspark!

In the middle of November I ran the tester on it again and randomly got a "DO UIM SYS TEST" which means it failed the module for some reason I couldn't see or hear.



Ran it again without touching anything under the hood and it passed. Did the UIM test as per the manual and it passed. Pulled out the Sun Engine Analyzer and saw nothing abnormal.
If at this point I had the magnetic timing pickup for the analyzer I probably would of caught the fault (If the probe and the light are used at the same time the light and the analyzer should show the exact same timing, otherwise your balancer has drifted or the module is unable to maintain spark timing...or both) but because the only way the tester, the analyzer and a timing light can otherwise check timing is by comparing to the secondary voltage on the #1 plug which is all downstream of the Duraspark, if the ignition module is at fault but not actually failing it looks like an unrelated timing issue.

Changed the module and the problem went away completely the next morning on a cold start. It's been excellent for a month and a half. Was curious if it was because it has not gotten as cold as previous years but the last few days it dropped below 0 and it's still running like a champ. Sure, problem solved but I wanted to catch the module failing even though Durasparks never had a great reputation to begin with. I'm used to them just overheating and dying and this one survived the 2021 heat dome, so it certainly wasn't dying the usual way.

Put the old module back in, let the engine sit overnight and test drove it this morning. Boom. Pulled over, restarted the engine and tried accelerating as I entered onto the main street. Boom. If the TAC says it's still too cold the spark retard control signal going to the module arrives on-time but is applied to the primary side of the coil late, resulting in either a horrible timing miss or it completely stumbles and the engine stalls. As soon as the engine is hot or the outside air temperature is warm enough that we don't need to deal with weird air density issues (Summer) you never see the problem. So the module is actually bad but in a way that it only fails under specific conditions. Case closed.

[vangremlin]

Glad you were able to get to the root of the problem!  Thanks for the thorough explanation.