Tarkus,
That is probably it. Thanks
By some more experimenting I found out that I can increase the gas output using different cell configurations. For example, if I connect the negative wire in the center plate and two positive wires on each side, I get two 15-cell compartments in parallel, this way the cell eats those 20A at 30.6V. This gives me 3.3 LPM gas output, but the efficiency is still around 70%. I can also get the same LPM's using only one 15-cell block, but this makes alot more foam and much of the electrolyte is escaping to the outlet by the form of foam till the electrolyte level in those cells is so low, that not much amps can flow. If I use this on two 15-cell blocks in parallel, the foam problem is much less, because the amps now are divided through two cell blocks. This is great for hho torch, still, the efficiency is bad. Also if I use the cell in the two 15-cell mode, a good electrolyte level (where the foam does not overflow through the cells) is about 4-5cm below the vent holes, that is a lot of plate area wasted.

Will try the 4mm (or better 5mm?) vinyl spacers, if that wont work well, we will need to think about other designs

Also I have some other questions. Sometimes I see some white smoke coming out of the output tube, even in the gas goes through a bubbler. What is that? Water vapor? Under what circumstances does this effect appear? And the other question is is there any difference in improving gas mileage using hho booster in a vehicle that is already converted to run on propane/butane?