My car starts easily when cold, but hot it has been a struggle. I put a variable resistor on the CHT as an experiment without success. Engine's not even catching.

Plugs look good, 123 distributor, blue coil, valves adjusted, runs fantastic once started.

I am using a 2 year old motorcycle AGM battery(which has been working prior to 123 distributor install). Open voltage measures at 12.6V.

I stuck a lead on the coil while cranking and voltage drops to around 10V and will bounce down to 0 occasionally if I crank for more than a few seconds. With ignition only the coil is at about 12.3V.

The blue coil is an NOS coil that I had. It's probably over 30 years old but resistance is good when cold. I did not think to measure the resistance hot, but would it make a difference? Does the insulation breakdown over time even if resistance is good?

Thinking my little battery doesn't have enough power to crank and fire the coil. Any opinions? Cranking is strong.

If it is cranking strong then I would look elsewhere. Is the spark strong? Pull a plug and watch?

When I installed a 123 disty on my 2.0, their tech rep advised that the coil needed to have > 3 ohms resistance across the terminals. 30 years is pretty old for a coil, you might consider replacing the coil. Also, to rule out the battery as a potential cause, you could connect jumper cables to another standard battery and try starting it.

I'am running an AGM which is commonly used in my country as a speedway car battery ,only issue I had with it is when they get too flat they need to be woken up by a low amperage charge,this isn't often as it has a very low discharge rate even when not used.
One other issue I had was a non-matching regulator which appeared to confuse my reconed altenator so the charging was not consistant to load ,changed to NOS Bosch regulator and the issue went away .
I had a original coil in one of my VW buses that would break down after about 15mins or less if a hot day so maybe you should start there ?.
If you have good cranking speed I don't think you would have a voltage issue @ the coil unless you do have a suspect one.
I just checked my AGM and it is 12.97 volts sitting out of the car ,I topped up the charge around 2 months ago so maybe a full charge on yours may help.
Good luck.

I know I'm getting fuel, so I've been focusing on the ignition. Tried swapping to a "cool" coil after a run with the same results. Also cleaned up the ignition wire contacts hoping that would make a difference (at which point I planned to buy a new set of wires).

Should have done this first, but I unplugged the FI trigger points and cranked the motor to look for spark on an exposed spark plug (it did spark). The motor tried to start with no fuel. I tried starting the car with open throttle (set in place prior to running pump) and was able to get the car started. On a cold start I just turn the key and the car starts every time with closed throttle.

So I'm way too rich on hot start. I don't understand what controls pulse length during starting. Aside from the CHT (I did not add a resistor), are there any other controls? Does the ECU see the MPS at full throttle during starting? As long as the throttle is in a fixed position prior to the pump starting, does the TPS play a factor? Does ECU idle knob effect starting pulse length?

My cold start injector is not hooked up and injectors were rebuilt by Mr. Injector so I'd be surprised if all 4 were leaking, but I guess it's possible. The plugs look good with that brownish white electrode color so I think my mixture is okay while running.

So what knobs can I turn to lean out hot start?

Does your ECU have the idle adj knob? If so, lean it out there.

It does. If I lean it out at the ECU, does the throttle plate have to be in the closed position or does the ECU use the knob position during the start cycle regardless of throttle position?

is this D-Jet? If so is this a classic hot start problem?

Really, CHIT is the biggest variable. I'm not aware of any "start mixture" control in terms of adjusting pulse width. It's going to be controlled by the usual suspects: CHT, MPS, fuel pressure, and TPS (if you have throttle down the idle mixture knob should be irrelevant).

Brad Anders demonstrated a few years ago that the classic hot-start problem is not going too rich but going too lean. Worth reading: http://www.914world.com/bbs2/index.php?showtopic=298261

That idle mixture circuit is bypassed when the throttle is open (assuming your TPS is adjusted properly). So that knob has no effect once you open the throttle.

If your cold start is bypassed I wonder if something else is mucked with or broken to get you a good cold start which then bites you in the rear on a hot start.

Are you sure everything is kosher on your fuel injection system? Above you mention a 123dizzy. Is that still the case? Is this a 2.0 or 1.7? What year is it?

The car is a stock '73 D-Jet 2056 cc. FI is all stock. The cold start sensor is hooked up electrically, I just don't have a fuel supply going to the injector. Based on the activation temperatures, I don't think it would make a difference in SoCal, but electrically everything is connected.

I have read Brad A's analysis and that's the first thing a tried with a variable 2K resistor. I'm just wondering during starting how the ECU controls fuel output. Is it just CHT and fuel pressure? Is it taking MPS input during cranking and would that be the equivalent of nearly full throttle?

It gets complicated. I spent a few minutes reading (or rather scanning) this: https://members.rennlist.com/pbanders/ecu.htm#ES

I got lost on some of the abbreviations and had a hard time following in some cases. Reading this reminds me how impressive D-Jet is from an engineering standpoint given technology at the time.

@PBAnders mentions something about a "start signal" in the context of Idle Cold Mixture Compensation and in the context of fuel pump relay activation, but it doesn't appear (best I can tell) that the ECU has a specific starting mixture circuit or input to pulse width.

If that's the case, I would assume pulse width would be driven by inputs the same as normal operation, with the two most important being manifold pressure and head temp. If the idle circuit is engaged (because the throttle is closed), then I assume IM has a large input to starting/cranking enrichment. Maybe you can glean more from reading that page than I was able to.

FWIW, my hot start problems on D-Jet were significantly mitigated when I installed my hi-torque starter. Have you tried the starting procedure Brad describes on the last page of that previous post?

Cold start has a few main bits. In socal temps the cold start valve gives a quick burst, and the AAR is open increasing idle speed. During this time the throttle switch is telling the computer the car is on the idle circuit since the throttle should be closed and it will be taking mixture info from the knob on the ecu and the cht, which on a stock 2.0 should have the resistor to make up for the 1.7 ecu used in 73. Fuel pressure is static on djet. The dizzy tells it when to fire, and the ecu, using the mps and the cht tell it how long to fire.

Now, in your case I would check to see what year ecu you have. I would also verify your injectors are stock or not. Since you have a bigger displacement you may have something else.