Hi Russell45,
In order for the 26 MM to work the cylinder & head must be re-worked.
Your carburetor is a 16 MM.
I have a few of the 26 MM carburetor in stock, however I am not sure I want to sell any of them, and I doubt many would want to pay the HIGH price. They normally sell for $130.00 [what I paid for them]. Be careful, not to buy any listed on EBay as they don't work correctly on the Whizzer motor. They are not setup correctly, and require a new slide, pilot jet, needle jet, and main jet to work close to correct.
You will need to port the cylinder, mill the head, re-work the mushroom lifters, and match the intake spacer, to get the 26 MM carburetor to work. When completely modified the motors have been known to exceed 70 MPH, however most run in the low to mid 60 MPH range.
The last service manual was printed on the 1999 model and later manaul were never supplied. Even the 1999, when available were copies of the original 1999 version.
If you want to send the Tachometer I can test it on my Ambassador to see where the problem is [wiring, Tachometer, Analog vs Digitial, etc].
I will check it for free, you just need to cover shipping cost both ways.
Have fun,