Generally 2-strokes don't like cold. They were designed to be running at a fairly high temperature - colder block temperatures leach heat from the compression chambers and make it harder for the diesel fuel to auto-ignite under the high-pressure. The quick compress-and-soak meathod works for me, but it can take a few trys to get the mill running (something that may not be possible with less-than-perfect batteries/electrical.
I don't think this was covered - but:
You recently replaced your batteries (8D's you said - good choice, lots of cranking power available). What is the condition of the cables from the batteries to the starter and mill? If there are any devices (fuses, shunts, switches) between your positive terminal on the batteries and the positive terrminal on the starter (assuming here a negative ground system) - remove them (fuses should not be in the start circuit - although, there should be a fuse within 8" of your alternator especially if it is wired to the positive terminal of the starter). A large gauge cable (0 - 0000) should be used for your starter connection to the batteries as the starter draws tremendous currents to get that high compression ratio squeezed. A thin wire (I consider anything "thin" if it's 2-gauge and up), or devices in line, or even corrosion on the positive/ground paths will add series resistance to the circuit thereby acting as a current limiting device for that circuit (and the last thing you want to do is limit the ammount of current going to the starter!!).
On some rigs, the mill is not directly grounded to the chassis with a cable (which it should be - in fact as an upgrade, the same gauge or larger than the positive should be used to connect the starter back to the battery
ground) - in this case there may be rubberized vibration isolators keeping the return path for the voltage in a high-resistance state (i.e. the voltage return path and current souce will need to find a way through the path of least resistance - this can even be your transmission-gears/driven-axle-bearings in the worst case). If you don't have a ground strap and the resistance from the mill to chassis ground is more than 1-Ohm, add one.
With a DMM, check your start-bank voltage before you crank the engine and watch what happens to the voltage when you crank. On a 12-volt system with good batteries this shouldn't get below 9-volts on the low side in the cold. On a 24-volt system with good batteries this shouldn't get any lower than 17-volts. Both of these values are "at the battery".
At the starter you shouldn't see any more than .5-Volt difference from the battery voltage during the start. If you see something like 1 or more volts lower than the batteries, a lot of your cranking current is being wasted to the wiring/ground.
If you have a ground strap from the mill to the frame, double check with a DMM that the resistance from the mill to the frame is less than .75-Ohms. If it's higher than that - pull the strap, and check the ground points for corrosion (especially important if your rig is on the east coast where they use salt!!!). Make sure the points are clean and after you've put the strap back in, spray the points with anti-corrosion spray for battery terminals. If you have to pull the strap, check to make sure it isn't the source of the resistance (measure at each end to see the strap's resistance). If the strap shows significant resistance, replace it (and while you're at it, make it bigger
As a build-on to Hi yo silver
's idea about having a breif load to warm the batteries, consider a chassis-voltage air heater between the blower and the intake plenum (I believe Kim-Hotstart makes these). This should warm the air in the intake plenum (after the roots blower) enough to help ignition during compression (and will let a bit of heat into the side of the cylindar walls from the plenum cavity), and would have the side effect of raising the temperature of the batteries due to the load (so they may be able to put out a bit more current during cranking). If you do this, I recommend that you not have a simple toggle-switch to turn it on (as you may forget about it, and suck down all of that starting power), so use a push-button (good), or a dial-timer switch (even better)
. You'll also want to make sure you can't use it while the engine is running (once at temp engine's like cool air) - so a relay that "breaks" this circuit once the Run-Stop switch is set to "On/Run" would be a good idea (you can even go further, so that the heater circuit will stay active until you press the start button - and use the start pulse to initially energize the air-heater lock-out relay through a diode, and with a double-pole-double-throw relay, have it self maintain as long as the run switch is on). I whipped up a quick wiring diagram to explain what I mean... (attached below).
When I picked up my bus from LTD in Oregon, the mechanic had to do the "crank, wait 30 seconds, crank
" to start the engine (Williams air-throttle) and hold in the throttle on from the back as it had been sitting in high-altitude/<40DegF-weather-for-over-a-week (with poorly charged batteries).