Took a long time to find this skech and its without measurments but from the top of my head l remember my measurments being about the same, if not bigger thain yours. For a 1.6l engine!
I strongly belive your gasifier is too small. It is too restrictive but most importantly it can not produce enaugh small enaugh charcoal in time at right places for the gasifier to work good.
I am afraid to say there might not be much choice but make a new gasifier, if you want this to work good.
But for the mean time, make 120% sure you have NO hot leaks ANYWHERE! Ash doors, fittings…
Can you take the restriction out?
If you can, try puting in a temporairly bigger one. 15cm without a problem. Then, take your small and dry wood chunks and mix with some walnut and hazelnut size charcoal, about third or a half by volume. Try runing a hopper or two of that. Do this allso if you cant change out the restriction.
This fuel is what Bob calls “rocket fuel”. It is a abuse resistant fuel. Resistant to overdrawing, moist wood, makes good quality gas over a small reaction area and a lot of it! It will make tar free gas even without much of a restriction.
What this will show you is what your engine is capable of on woodgas. You then have a comparrison point when you experiment with pure wood again.
BUT! your gasifier is not built for this fuel and it will melt down fast if you use it long term. Imagine feeding Nitros to engine. Not gonna hurt it at a few bursts but if you feed it non stop…
A hopper or two will not hurt anything.