I have what seems to be the exact same issue, I installed my Megasquirt today and it still has the same problem.
 
 

Whats yours doing??
I have my mspnp sitting here, but i want it running sweet on the stock ecu first, otherwise you end up going in circles..!!
If you can get it running for a bit, maybe log yours and see what's going on..
.mines def the coil, tested and it's way out of spec... go ahead and replace the coil and igniter and a fresh set of plugs as track mr2 suggested, cause I'm sure if ones gone, the other wont be far behind it.

I realized that the o2 sensor must be ****ed as the ecu was never getting any signal from it at all, I plugged the aftermarket one into the Megasquirt and it helped slightly the AFR would start at 14-15:1 then drop to 10:1 after 10 seconds or so.
 
Took the coil out and had a look and the connector to the dizzy was all corroded but resistance was still in spec. I've cleaned it up and put it back in, then tried to ground a spark plug to the intake manifold to see the spark and now the car will not start at all....I can hear the starter solenoid engage but the starter does not move, fuses all good have to troubleshoot it more tomorrow.
 

Just curious did either of you guys with the rich mixture find the cause?
 
Also Ian thanks again for driving my car and your advice.

No no thanks for letting me drive your car it was brilliant!
 
On my car I think it was the corrosion on the coil, after i fixed that up its now running at 12-13:1 at idle which is still very rich but I think it must just be the base tune on the mega squirt that needs adjusting.
 
It's hesitating when I slam my foot to 100% throttle too the afr will drop to about 9:1 for a moment then jumps back to around 11, running fine otherwise though. I am assuming it just needs a tune...though if it sounds like something else was happening let me know please!

Hmm, not idle though.

As above, I just replaced coil, igniter, O2 sensor and banged a new set of plugs in. Runs as normal. Idles at 14.7 goes 10 when I boot it,

asimovy, im still yet to hook up my mspnp, so I dare say yours is just the tune.