The T-20 engine is an improved version of the F-20. The T-20 has a different design cylinder head with larger valves. The T-20 also uses the same larger air cleaner as the F-30. The F-20 carburetor has the number 17 venturi and 21 main jet. The T-20 has a number 20 venturi and 25 main jet. Chris Klossner http://home.att.net/~klossner/