Hey, good work on getting the generator up and running! Koen made a spreadsheet that one can use to calculate nozzle airspeed based on displacement, which you can find here
The displacement and RPM are the most critical numbers in the top section, dont worry too much about the other ones for now (and treat your results as a decent estimate).
I could not find the original chart that he is referencing, but here is a version of it that I have marked up a little. The spreadsheet gives you the air rate in Liters/sec, so to make heads or tails of this chart, you need to convert that number to Cubic Meters/hr (I.E. Multiply by 3.6)
It took me a long time to figure out what this chart was saying, but it basically shows the percentage of the CO2 that is being converted into CO up the Y axis. Each line is labeled with the nozzle size that generated that resulting curve. (if anyone can find where this was originally posted, I will link back to it, as I recall it had some explanatory text that went along with it) What I took this to mean was that with my setup (using about 5 m^3/hr) a nozzle of 3.1mm would give me 92% conversion, and a nozzle of 12.7mm would still be like 88%
So, is nozzle size then actually that critical? After looking at this chart I am inclined to believe it is not. Anecdotal evidence seems to back this up, as many sizes have been tried with good success. As the nozzle size gets smaller, you get higher conversion efficiency, but you also create more restriction, which forces the engine to work harder to fill the cylinder with woodgas. (this is what one of those other values in the spreadsheet is trying to account for, I believe) I suppose there is a sweet spot somewhere in there, but without the means to measure the quality of the gas being produced, its pretty hard to find the true peak. Generators have the nice feature that you can see how much load they will put out, and that lets you at least approximate the quality of your gas.
Now, all that being said - if you want more power from any given setup, you are going to have to start thinking about water drip. I think there is a pretty recent thread on that topic that might shed some light. Keep up the good work!