Hi Touie, thanks for posting here. Nice work on your LJ78!
Generally these little motors have a lot of torque in the low rpm range (out of boost), especially when the throttle plates have been removed and the motor can get all the air it wants. Low rpm torque is good because they have a very conservative cam grind that flows very well at low rpms (perhaps Toyota did this to make up for the laggy turbo). I never run out of power in the low rpms. Dougal is right that the turbo does take a long time to spool on these motors (not only is the turbo an old design, but it is actually a bit big for the displacement of this motor), but I don't think that is the problem you are describing. Especially if it is a new problem that did not previously exist. I've never been able to stall my engine.
A few things come to mind:
The first item I would be wary of is the wiring work that was done to convert to LHD. Does your check engine diagnostic light work? You can jumper the T1 to E1 pins in the diagnostic connector in the engine bay and turn your key to the 'on' position. Look at the flashing pattern of the check engine light in the instrument cluster. Record the patterns and see if there are any sensors out of range. The hilux surf au website has a list of the patterns and what sensor they correspond to in their tech section.
Check all the wiring visually and repair any poor splicing or work that was done.
Is your transmission shifting right? Even with the shifter in 'D' position, the transmission should start off in 1st gear. If it does not, there is something wrong with the electronic control (done by the Engine computer). Again this could be a wiring problem.
Is the injection system limiting fuel off boost? When you are having this problem and you have the accelerator pedal all they way down, do you see any smoke? I know if I stomp on my accelerator in the low rpms with no boost, I will get some black smoke. Does this happen to you? Did you change the injection pump calibration resistors? This could reduce fueling and cause problems. Also, when you turned the spill control valve screw, did you turn it in, or out? Maybe you reduced the fueling? By the way, I would not recommend turning up the fueling until you have intercooled the motor (or at least have a pyrometer gauge)....you are pushing your luck. I turned down the fuel until I had an intercooler.
When you removed the throttle plates did you also remove the throttle position sensor? This needs to be adjusted very carefully with feeler gauges to get the right setting. If this is out, it can cause the problem you are describing, although it would be very consistent (not random).
Some of the other Denso pumps (mechanical) can have issues under certain circumstances when the vehicle is on an incline. So there is a remote chance that for some reason your fueling is being limited. Have you checked for fuel leaks or air getting into the system? When you pump the hand primer does it get stiff quickly? The top cover does start leaking on these injection pumps after time. I had to reseal my whole pump.
Resealed my rotary injection pump
OK, a couple comments on the water/meth injection. I've been using it for a few years now with no problems. I've had my cylinder head off (carefully inspected top end), and also have replaced my big-end con rod bearings (carefully inspected bottom end) after running the meth injection for years. There were no indications what-so-ever that it was causing any damage to my engine. Big end bearings looked brand new (204,000kms) and no indication of being hammered by the meth (
Did my BEB...). So if properly used, I don't think you should have any concern. Improperly used, Dougal's concerns of engine damage are valid, although I personally feel the piston ring lands will go long before the head or head gasket.
There are a few things that can go wrong with the meth/water:
The first problem which is most likely is cracking or shattering the piston ring lands. This will occur from pre-ignition of the meth/water mixture before the piston has reached TDC. This is a VERY REAL RISK and can be controlled by using the appropriate mix of water/methanol. The idea is to have the methanol burn only when the injected diesel ignites, and not auto ignite before this time. Methanol auto-ignites at 470C. These temperatures can occur in the pre-combustion chamber. I have found that in the winter season where I live, I can run a 50/50 meth/water mix when ambient temperatures are less than 15 degrees Celsius.. In our summer season, I can only run 30/70 meth/water mix maximum. 25/75 is even safer. If I run 50/50 mix in our summer, I definitely get pre-ignition of the methanol. It is very audible and sounds like a pinging/knocking sound. The sound gets worse the hotter the engine is (sounds exactly like pre-ignition in a gasoline engine). This is very dangerous for your engine and should be avoided at all costs! Piston failure WILL happen under these circumstances.
Second: if you run far too high a flowing nozzle (or a poorly placed nozzle), with it turning on early in the rpm range, you could cause hydro-locking type problems which might impact the head-gasket (or bend a con-rod, wreck big end bearings, etc). This would be a pretty far fetched scenario, and you would probably quench the ignition long before this happened. I'm running a D04 252ml/min nozzle. It's a perfect and safe size for this motor. If your nozzle is around that size I think you will be fine.
The third problem that could occur is if you inject before the turbo, you could erode the compressor wheel over time so is not recommended. Also, make sure you inject AFTER the intercooler (when you install one).
In our winter when I run 50/50 meth/water mix, I gain probably in the region of 20hp at the flywheel. A massive difference in power and a lot of fun around town; feels like a big V6 or small V8 (with the addition of my other modifications). When I run the summer 25/75 meth/water mix, there is far less power (maybe 5hp), but I use it mainly when towing to cool the combustion chambers. When the water turns to steam during piston compression stroke it absorbs heat energy helping to control EGTs.
Basically, the meth is extra fuel and provides more power, and the water is for cooling the combustion temperatures. There is no point in adding the extra fuel if your air fuel ratios are already at their limits (rich). The 2LTE injection pump can inject a fair amount of fuel and is totally adequate until you've intercooled the motor. Once I intercooled I simply could not get enough fuel into the motor. EGT's were very low. The injected methanol was able to take advantage of all the extra air to make some extra power while still having low EGTs.
I am still working out some of the bugs in my 2l custom turbo setup. I am getting a some black smoke at low rpms and when I shift at higher rpms. I might have to fuel cranked up a little too high. 
