Here is a super abstract drawing of a charcoal updraft gasifier. You want two containers with lockable lids and heat resistant gaskets. I used for my truck a 55 gallon drum with a fireplace rope gasket ultra red RTV'd into place to the lid, and I used a tall ammo can and covered the neoprene gasket with the same RTV. Permatex Ultra Red is good up to 600F.
For nozzle size, we've figured it takes a half inch sized nozzle for every 500cc, you can oversize for your engine it won't hurt anything. Might be a benefit and make richer gas. For my 2 liter Mazda engine I used a series of holes making the pipe look like a flute.
If you make a simple nozzle just coming into the side and not pointing upwards it will burn away, but if you point it back upwards with a 90 degree elbow it puts the nozzle below the extreme heat. The reaction is endothermic because the fuel is also it's own insulator from heat. You have a small lobe of anywhere from 2000F to 1300F depending on if you use water drip or exhaust gas return to cool down the reaction. Water drip is beneficial because it turns to steam when it hits the hot nozzle, then as the steam passes the lobe it cracks into hydrogen, a small amount but it still helps, as well as brings the reaction temps down.
You want the upwards facing nozzle as close to the center as you can get it, but it doesn't have to be perfect. This is just to keep the heat as insulated away from the barrel and keeping it from getting hot.
For my gas exit of the main barrel I welded in 2" thin wall conduit, with about 4 inches protruding either side. I wrapped up a cylinder of hardware cloth and hose clamped it down, this keeps the little charcoal bits from flying into the filter and being a general annoyance.
For a small engine you could use 1" plumbing pipe or conduit. I will be using 1" conduit for most of the gas routing for this bike.
The gas exiting an updraft usually isn't super hot but you should still use a metal line to connect to the filter where it will be ambient temps. I used 2.25" flex exhaust hose, RTV then exhaust clamped into place on the conduit pipe studs. I used the same conduit for the bottom and top pipes on my ammo box filter. With the gas coming in from the bottom it lets the heaviest soot settle to the bottom, leaving only the finer stuff to get caught by the sack/bag filter. In a mobile unit the soot being collected on the outside of the bag allows it to be shaken off by bumps in the road. After the filter you can use PVC pipe because the gas is at ambient or below temps.
The added benefit of a sack filter is if you have poor quality charcoal and it produces tar, it will clog up the filter instead of your engine. To clean a sooty filter I just take it out and shake it. You would probably need to machine wash one that had tar on it.
For an air mixer, you can use a simple ball valve. I find the brass ball valves move easier than stiff PVC ones. Your fresh air filter should go where the ball valve is. If you have a Honda 196cc with the stock carburetor, you can use those pod filter adapters and attach the charcoal gas with rubber boot connectors. Some guys have used shop vac hose but I've never had luck with them, they're leaky and bring in too much air making it run lean. Most hardware stores sell rubber connectors that are made to fit both PVC and metal plumbing pipe, I use these combined with PVC pipe to route to the engine. You should check for air leaks by blowing air into the unit and spraying soapy water, and patch any leaks with red RTV. The system is under vacuum when running so it isn't dangerous to have air leaks, it's just annoying and can make the gas not strong enough to run the engine or give poor performance.
To start this unit up, you can use a shop vac or a mattress blower using the suction side. A handy place would be at the air mixer with the valve set wide open. Turn on the source of vacuum and use either a propane torch or some burning paper, one guy uses fiberglass rope soaked in just a little kerosene held by a long wire to light his. You should see a glow pretty quickly, dry charcoal lights rapidly. Once the unit is warmed up you can introduce water drip, I use a small petcock valve on a rubber hose with a stainless steel straw at the end and place that straw inside the nozzle pipe. You want about 1 drip per second for a small engine.
To see if you're ready to start the engine, take a match or lighter to where your vacuum is exiting. You should see a mostly blue flame. Don't worry if you see purple or a little red. Typically you're ready to run the engine after 5 minutes of warming up.
This is where having electric start would be handy. Close your air mixing valve almost all the way, just barely cracked open. You could fill your float bowl with gasoline and then shut off the liquid fuel supply. The engine running provides vacuum demand on the gasifier, so it's only making gas as long as something is sucking on it. With my truck I usually would start on gasoline to both light and warm up the gasifier and once I noticed it was running really rich I would shut off gasoline supply and let the float bowl empty.
Hopefully once gasoline is used up in the carburetor it will transition over to pure charcoal gas. If you have a lot of air leaks you might have to close the mixing valve even more or close all the way. If that's the case then you should find the source of the leak.
Woodgas and charcoal gas really like high compression engines, it can withstand up to 18:1 compression before knock occurs. They also like really advanced spark timing. Carbon Monoxide is a very high octane gas and resists knock like crazy. I adjust my distributor on my truck all the way forward and all it does is smooth up.
If you have any questions let me know, I just wanted to share the process of how it works.
