John, generally speaking, the less HP you have, the less stiffness you need.  So a 40 mm axle will generally be less stiff then a 50.  A 28mm frame less stiff then a 32.

The Euro folks are now making 4 cycle, low HP karts.  The tend to be 28 mm frames with 40mm axles.  I believe Margay makes one as well.

We put a 206 on an old Intrepid 32 mm X 50 mm axled shifter chassis!  There was NOooooo way we could free it up enough!!  We just put an older TopKart with a 28 mm frame and 40 mm axle together- haven’t run it yet.  But I suspect it will be MUCH freer!!!  I’ll let you know next year!

By the same token, it’s very unlikely a 28 mm frame would work well with a Stock Honda shifter either!

Hope that helps.  But I’m sure someone with a lot more knowledge then me could better answer your question.

Clark Gaynor Sr.