Rocket Mass Heater tweaking

Heya all,

We've built ourselves a small rocket mass heater in our trailer.
You can see some pictures here:

bench (still needs toplayer of stone)
rocketmass heater above
rocketstove front
exhaust start and end
fire with helper plate
fire without helper plate == smoke
rocketstove behind
rocketstove side
close-up exhaust
full view with exhaust
inside barrel from above (note that heatriser insulation is vermiculite covered with ash)
inside barrel the connection to outside t-piece
ash outlet in t-piece directly after barrel

Now the problem is that even though the system works fairly well - and we do get enough warmth - it is not functioning really rockety.
It still gives smokeback often, which we mostly cancel out with a bar of pressed vermiculite which I use to control the air flow, which helps suck the smoke in.
It also still gives too much real smoke outside from the exhaust.
Therefore I think it is not burning hot enough.
Without the barrel it is actually burning much stronger.

The whole setup is small and the average pipe width throughout the system is 130mm (5.1 inch) The heat riser and wood feed are somewhat larger.
The barrels are 38cm diameter.
The bench is 2 meter long and the exhaust pipe goes through it on two sides (going 180 degrees at the end to come back)

It also seemed to be working slightly better when we hadn't finished the bench. The end of the exhaust pipe used to become 'warm' (still touchable), while now it's almost cold.
Also the bench itself is staying pretty cold - whilst before it did get warm.

I'd be happy with any ideas
Rein

what is the heat riser height? it should be around 40 inches for a 6 inch system.
burn tunnel length should be as short as you can make it and still be able to remove the first bridge brick with ease.
how well is the heat riser insulated? in very small systems you need very good insulation around the burn tunnel and heat riser. if you think of the scale a smaller RMH tends to loose heat faster than a large one (more surface area exposed for the size)
so you need better insulation than are normally used.

you will also need to make the CSA the same throughout the system, larger CSA in the heat riser and burn tunnel menas it will try to draw in air but will not be able to because the volume is then restricted by the small CSA in the ducting. Essentially you have made yourself a problem; not unfix-able but definitely one to watch out for in the future.