Just got a 1992 Tdm 850,

It's missing between 2500 to 3000 rpm.s any idea.s folks to what might be wrong,

It's runs better on reserve than it does on main tank too can't seem to work that problem out,

if it runs better on reserve than on main then you may have fuel tap problems ie dirt or something blocking the tap

solution strip the fuel tap and clean

this will probably sort the missing problem

also check your float levels in the carbs the carbs could be needing cleaned/balanced

hope this helps

Definitely sounds like a fuel petcock issue