Plans for the Keith gasifier are available from Drive On Wood. Save yourself a lot of time and effort with Wayne Keith's proven gasification system. Get special access to Premium content and see the details for yourself! Visit the store.
Here are some free plans to get you started. Also, don't miss these other free PDF downloads.
These are the critical dimensions from the most successfully deployed gasifier in history.
Free plans put out by FEMA. Great for a weekend "lets see if this actually works" project.
NOTE: this is an emergency-use design - stratified gasifiers are known tar makers, placing your engine at risk. Best used for flares and heating applications.
A very easy to build charcoal gasifier designed by Gary Gilmore. Works well for backup generators and most small engines.
The team at Mother powered a truck back in 1981, read the articles here and here. You can contact them for the plans. OK, it's not free - but it's pretty cheap. A well documented build is here, videos here.
Jim Mason's design. He makes the plans freely available, so you can build one straight from the CAD files.
General design schematics for Larry Dobson's design.