Hi Jan , don’t mind if I write my opinion. Jo has set the bar very high with his Volvoto and his velvet voice provokes while driving in a way that makes us all envious (joke). 
As far as I can see from your posts , your gasifier is overheating , the only possible cause is too much air in the hot zone , so the charcoal oxidises to CO2 and not CO , it is obvious that at higher load the upward temperature radiation is too weak and not enough pyrolysis gases are formed and suddenly the charcoal starts to burn.
The easiest experiment in my opinion would be , try with finer wood , wood chips , or at least mix them in your fuel , smaller fuel gases faster ,…
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You are so right, when I make really small pieces of wood, the car runs better.
However, is a lot of work.
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I’m a little ashamed to perform in public, but for this glutton I should feed at least two men, and the wood chips as ordered for the gasifier
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Tone, seems you joined JOs club of smileing faces 
I do however need to add an opinion here too regarding fuel size. It does seem Jans gasifier is geting overdrawn, but fueling too small wood can also backfire! The engine, fuel and the gasifier all need to be sized for each other. Stationary gasifiers have it easyer, but a mobile unite needs to cover a wide range, from idle to full power.
Problem is, loading too small wood in a gasifier built for biger wood will prevent air jets to penetrate deeply enaugh in the burn zone at low gas demand. Tar danger.
Jan, can you post a skizz of your current gasifiee with crude measurments?
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@Tone and @KristijanL I appreciate your Slavic way of picking. I can’t help doing that myself sometimes.
@Tone , that video of yours was probably the first step towards becoming a hardcore youtube influencer 
Also, I wish @KristijanL could find the time to produce the amount of videos we’re used to see, but he seems busy increasing Slovenias population lately
I agree with your conclusions. Big, heavy and fast vehicles need lot’s of fuel (surface area).
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JO, dont give my wife any ideas she does apear to forget how childbirth looks like in a couple of months 
Soon… I got a few irons in the fire… Well, the fire isnt lit yet 
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Jo, you always find something juicy in your vocabulary, “YouTube hardcore influencer,” … we’re talking about wood chips and wood chippers, and you have naughty thoughts in your head … There must be laughter.
Today I debated with my brother Primož, who advocates mixing water vapor between fresh air, which forms hydrogen or even methane in the hot zone, and I thought about your gasifier construction (Jan has a similar one) namely the cold walls of the steam condensing funnel, below close the hot zone is the water catchment gutter, so pyrolysis gases with water vapor travel down this wall and enter the hot zone by mixing with fresh air because the nozzles are on top and this is a hit, less CO, more H2 and CH4. How deep in the hot zone does Jan have nozzles?
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Tone, the inner hopper wall has gills down low, right where the funnel starts. That’s where cooled down pyrolysis gases re-enter the chunk pile - hopefully robbed of most of the water vapour. Nozzles are positioned 6-8 inches below that point.
To the sides I doubt much water will be airborne again once it reached the gutter. The gutter area is always cold to the touch.
Don’t you believe there’s enough steam already as is, in front of the nozzles, to do the job you mentioned?
In think Jan will have to answer to meassurements on his gasifier.
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Jan, it seems on the first glance your sizing is spot on. Calculation comfirms this. Your oxidation zone is 4.15l wich is preety damn close to bullseye. Same with the reduction zone.
Can you lower the grate on the existing system? You can try that, but this does smell like a hot leak…
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Here I have drawn my thoughts on the flow of gases. Water vapor does condense in the housing, but only a part, there is still a lot of steam in natural gas, which is good, according to the current understanding of the process
maybe another stupid thought Jan, if the top part is not covered with insulation, less water would be extracted, but the pyrolysis temperature would rise, more gases would be produced, which would improve the reduction process and overheating.
Jan, with the Mazda, at open road speeds, I see 300C coming out of the cyclone as well. I think our firetubes are heating the outgoing gases, making things look like we’re running very hot.
In my Rabbit gasifier the firetube is double walled, with ash inbetween. Pretty much what @Tone is suggesting. It ran slightly cooler, about 250C.
This is one of the reasons I went with a cross-flow on the Volvo, straight from grate to heatex.
Then of course the size and depth of the charbed is what really makes temps drop. I may even have gone slightly overboard with this one.
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Jo and Jan , that’s how I imagine a gasifier rebuild , insulated top , to maintain higher temperature for higher pyrolysis gas production , upward nozzles to mix fresh air , pyrolysis gases and hot water vapour to form H2 , CH4 and CO , CO2 formation has no conditions , and a solid pipe is inserted to redistribute the temperature across the hot zone and raise it , at the same time, the reduction time is longer and the high temperature persistence is increased. However, lowering the restriction opening is unlikely to be easy. Jo, I hope it won’t be too funny, but you’d better say something from your repertoire. 
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Tone, I’ve run out of witty comments for the moment
Don’t you think insulating the hopper would make the charbed overwhelmed with water vapour, pretty much like with the old true Imberts?
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Primož has convinced me to believe that more hot water vapour creates the rich gas that the engine needs, there is no other way. The art is to find and maintain the balance and logically, if energy is lost on the walls of the gasifier and in cooling the gas, this requires additional air, which is ballast and a waste of energy.
Tone, I’m told this is true as long as there is enough heat to do the job.
From what I understand, even completly dry wood contains enough water, chemically bound, to use all the available heat the gasifier can produce in theory. This is why most of try to save as much heat as possible with heatexchangers and so on, and why we try to get rid of excess water already in the hopper. However, it comes to the cost of complexity and has to be considered individually. The most low hanging fruit, in my opinion, is any form of heart insulation.
Running a charcoal gasifier on the other hand, is different. Then we’re dealing with nothing but pure carbon and water can be added.
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Sorry Jo , to be a nuisance , but today is a holiday and I have nothing smarter to do , but you will feel what it is like to be a Christian .
The chemically bound water in the wood should provide all the energy needed for the reduction to proceed , provided of course that it is not lost on the walls of the hopper I imagine a narrow layer of pyrolysis above the charred bed where the resulting gases and heat travel downwards . Of course, some of the gases and water vapour rise to the top, but soon the top of the funnel fills up and the gases cool a little and fall down along the wall into the , some of the water vapour liquefies here and some mixes with the fresh air, forming a rich gas when in contact with the hot charcoal.
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Tone, I always thought enough steam was involved as soon as we catch condensation later on in the cooler - unless the condensation is from pulling through of course.
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Is it not the ash, the charcoal that insulates the fire pipe outwards, is an inner pipe needed to cool the outgoing gas?
I have insulated the outside where the outgoing gas goes, so the heat will probably remain quite good, but if you look at these pictures it looks like the temperature will go down quite immediately, and I have a large cyclone, which must cool quite well , uninsulated.
If I look at the pictures, does it look like I would insulate the lower part of the hopper, to increase the heat during the pyrolysis so more charcoal was made?
Strange too, I drove quite hard today, was out on the main road, and after that the car went a little better, did at least 110kmh. ops forget the picture.
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Well, as you show in th pic the gas leaving the grate is relativly cool. If the outgoing gas flows next to the hot firetube, the gas will pick some heat up.
Ash cones, double walled firetubes, insulation, air voids or the way I’m trying with the Volvo - sending the gas directly to the heat exchanger - are all ways to try resque or keep the the heat in. Every little bit helps.
I’ve noticed the same. I’ve always imagined the charcoal cleans up some - less ash mixed in - vented out.
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