I agree with Steve. All wood gasifiers can make tar. When I start up my gasifier in the spring because I do not drive it in the snowy winter months; I will make sure the gasifier is up to temperature at the grate. At least 1000 °f or 537.8 °C , or at the cross over pipe 700 °f or 371 °C . Now this is my gasifier and not yours. I make sure there is no raw wood down by the nozzles on lighting it up. I take it out and only put bigger Charcoal going into the firetube above the nozzles then pyrolysis wood brands above that, then I add dry wood above this.
I might flare my gasifier for 20 minutes to get it up to temperature. If I can not light the flare easily it is not ready to go.
I am sure you know these things Jan.
But persistence will always win in the world of gasification.
Jan, I’m sorry about your valve problems. To me it seems the V4 is extra sensitive to tary fumes. I’ve had several sticky throttle events and intake valves not closing properly over the years (ticking noice that will fade away over time), but never a stuck valve like that.
Maybe your gasifier heart is simply too large for idle purpose mostly - I don’t know. What about a block heater? Is there one on the Iller? That would warm any tar up and lessen the risk of braking stuff.
I have one more trick up my sleve you probably won’t like 
- Feed the engine more soot I have no proof, but I get the feeling an engine that has been breathing a lot of it, is less sensitive to tary fumes - valve seats less shiny and smooth.
I have an engine heater on it, but don’t know if it works, thought I’d try it today, see if I can get the valves loose. But it is difficult to have the power plant in the forest where the ferret is.
Yes, more soot wears out the valve control so it becomes looser, so the tar gets more room, but I don’t think I want it that way.
I got this sorted out last year, by sealing better with soot at the bottom between the cone and the edge of the fire tube, I thought I was careful when I cleaned this summer, but I may have ruined something.
Driven on birch now, only had fir and pine last year. Thinking the tar might creep past the nozzles since they go out so far from the wall, but shouldn’t the choke take care of this?
Hi Jan, tar would maybe pass the nozzles, and restriction when heating up, and on VERY low load, but feels odd, maybe tar/water runs down passing the cone at shut-down, and boils away next start-up?
Do you have the possibility to run it on gasoline, just a minute every shut-down? Or better: 50%gasoline/50%e85.
This clears most tars out, if you suspect some could have slipped by.
Is your cone loose in bottom of firetube? You could help the ash-seal by put ordinary dirt down before putting ash, just clay-ish dirt from a field, blue clay, or red clay.
With a small restriction sometimes the pull can be to hard on a ash-seal, not giving it time to settle enough.
Yes, the cone is loose at the bottom of the fire pipe, I’m thinking about the chimney seal, which is heat resistant, but it usually cracks.
Jan, we haven’t talked for a long time, but maybe it’s time for a few words.
The theory behind the conversion of tar gas and water vapor is to maintain a white glowing layer of charcoal through which these gases travel. I believe that under heavy load you maintain this condition in your hot zone, well, like my tractor, your machine also works with low load a lot of the time, and then the pyrolysis gases cool the hot zone and escape the conversion. There is a solution to forcefully cool the condensation zone, or vent the steam with the help of an exhaust system, as Joni did.
I would do one more measure, I would shorten the long nozzles to 1-2 cm, I would increase the restriction opening to at least 15 cm and lower it, thus increasing the supply of charcoal and most importantly (according to my experience), I would add additional nozzles below , which will keep the charcoal active, will also act as a fuse to burn the tar, should a void occur. I notice that the majority of wood gas users are afraid of hot leakage and the fear of losing strong gas under heavy load, well, neither will happen, the gas will remain strong and “cold” at the outlet.
Haha, that’s funny. I kind of expected you didn’t like the idea
I see your firetube is 340mm dia. That’s 13"+. Sounds a lot to me for a 1.5 liter 65hp motor running 1500rpm for the most part. My 1.8 liter 95hp Rabbit had an 8" gasifier. Maybe you could narrow yours down by dressing it with that white high temp insulation, like @don_mannes and others did and only let the tips of the nozzles protrude?
Hi Tone,
Once again you state another reasoning behind your adding lower pre-heated air.
It makes sense. Another reason that Stephan Abbadessa’s completely different downward pointing six nozzles dual level chipped fueled small series of gassers works for small engines.
A principal; a design to be practical valid has to be useable applied by others. Not just the Master. The Originator. His did. Arvid Olsen’s later, for tractors build-ups; and a few others.
My heart/check-to-you is that others will try and succeed too.
The “Master” I give you for your hollow fabricated lower tubed cage. Not something most of us could dare to do well, and gas tight.
Hey J.O. good idea on reducing the inner diameter. Use alumina silica kawool insulation. But then retain it with a thin drop in SS floating sleeve with holes cut out opposite the nozzles tips. This was a Ben Peterson innovation he shown in his book, to give credit.
I have one of these. They work well.
Regards
Steve Unruh
Excellent tips. A new trick to me, that I may use myself in the future 
Thanks for all the good tips.
I’m surprised why it’s getting tar now again, after working every other day for 1.5 - 2 months last winter.
It must be me who has done something that causes the error, and what I have done is to clean the unit and insert a perforated plate in the hopper for better condensation. ( I measured the distance between the mouths of the nozzles, it was 20cm, not 15cm as I thought.)
JO, it was a new trick to me too. Then I tried it in my double flute nozzle design after using it in my WK Gasifier. Wow talk about holding the heat in that area of the square firetube or lower part of the gasifier. It also protects the welds in the corners of my double flute from to much heat. Good advice from you and Tone, Steve for Jan’s problems. He is in good hands to solve the taring issues problems on his gasifier.
Not a solution to you issues Jan but YT put this in my TV. I knew it was Swedish because of the dots above the vowels. (yes, I know they are Umlauts. I used to badly speak German.) That was before I saw it was Volvo. Of course it reminded me of you.
A nice machine, I’ve had something similar but a Fiat, quite heavy, because of the iron straps, and it had a steering brake.
It was left out a little too long so the engine got stuck, and I didn’t dare drive it on our svamps because of the weight.
I cleaned all the way to the bottom of the fire tube, but I think the ash that was there looked good.
Could it be that the tar goes inside the nozzles and between the restriction plate and the fire tube wall and comes out under the restriction plate?
Or I had in charcol from biltema, (a dealer) can the charocol be poorly made, so it contains wood?
Hi Jan, the way the tar passes the cone i mentioned earlier, therefore i suggested sealing up with clay.
Biltema charcoal should have very little tar, but it takes time before it works at it should, down the reduction, almost all of the barbecue char needs to be replaced by char the gasifier produce itself, the difference in charcoal could be the temperature when it’s produced, cheap bbq charcoal are produced at lower temperature, to make it last longer, this gives a char with less “reactivity” in priciple harder, with a not-so-porous surface. Which could be the reason for tar channeling through the restriction, and reduction zone.
Happened to me.
Yes, I think I know what you meant, at the bottom, but I’m thinking about what I show on the left side, under the edge of the restriction, that surface between the restriction and the fire tube, I must have disturbed that.
Bingo. (that is your answer). Factory made charcoal for grilling, not finished pure gasifier reduction charbed material. 
Edit: This?? good for steaks! 
Yes, exactly these, good for the grill and cheap.
Jan, somehow I don’t like such a small restriction opening (65mm), there is a pressure drop and the gas velocities are high, probably there is always a void below without small coals on the grate, which could easily prevent the passage of tar. My opinion is that the opening should be larger, at least 100 mm, which would allow fine coal to pass through. The long air nozzles also bother me, I would shorten them to 2 cm and cut a groove in the front with an angle cutter, so that the air could also spread laterally.
