If you want to adapt the popular FCR 39 MX (also the FCR 40 and 41 MX) carburetor to your DRZ400S or DRZ400SM you have a few options and they typically involve finding and earlier FCR MX  with a removable intake bell. These are expensive and/or harder to come by. I have fabricated an aluminum adapter (more durable and better finish, meaning smoother flow, than plastic printed parts) that can be used on the later model non-removable bell style FCR MX carburetors. These carbs are commonly found on the later model carbureted YZ450F, for example. These can usually be picked up a bit cheaper and are newer models (I used an 08, but this will fit the other years as well). The adapter simply needs to be installed on the carburetor with a little rtv sealant and then aligned and locked down with the set screws. I believe you can use the pull cable on your S or SM model or just buy a complete throttle assembly that matches the donor bike. You will need an "E" model carb to cylinder intake boot (about $25) and proper jetting. I can provide that info as well. This should fit other size FCR MX carbs. I have confirmed and sold units that have been used on the FCR40MX and FCR41MX. You will need to grind the lip off the bell just a little. Here are my dimensions so you can confirm for other carbs. Diameter that slips over your intake bell on the FCR - 56mm, diameter that slides into your airbox boot - 57mm, air flow through-bore diameter - 44mm. All the info I have gotten is from the DRZ group on Thumpertalk and my own measurements. I have no problem guiding you on the conversion if you need any additional info.I'll include instructions and jetting info with the adapter.