Have fitted 1800 Kombi motor to my Trike, have trouble achieving top speed of 120 kph, compression 125 accorss pots, dual carbi's (balanced). Timimg set by book. Have been adviced primary to hot dogs to short, less than 400MM. Recommend go back to dual 1"3/8 from heads to muffler atleast 1.2 metres then Hotdogs to single pipe. anyone have advice or experience. HELP!!!!

Are they the stock carbs with the stock jets? If not, what is the jetting?

what else has been done to motor?? Aftermarket dizzy etc etc.

No being capable of 120k's is not a design problem,
it must be a fault.

If your exhaust is restrictive that may be trouble(check for bumble-fly nests too!)

Make sure the ignition timing IS advancing from tha static setting,
and use a probe, or at least check the plugs to make sure it is not lean due to
inadequate fuel supply to the carbs(or jetting if not stock).

A dodgy fuel pump, or restricted filter may be one of the causes of "inadequate fuel supply".

Your compression is more than enough,
if your spark is at the right time,
all it needs is

fuel (supply and jetting)

and

air (restricted air cleaners?).