Related to Dan's suggestions...

What carb and how many carbs? 
Did you set the carb(s) back to the generic, first start position when installing the new engine?
Have you tried disconnecting the choke before starting the engine?

Start with the carb. What kind do you have?
1. Make sure the throttle plate is actually closed - if it has been dismantled and incorrectly reassembled, the throttle may be full open. 2. The spring or springs, depending on the type of carb, should hold the throttle closed. Are they in place and pulling in the right direction?
3. Check the fast idle screw is retracted enough to let the throttle almost close.
4. Make sure the throttle cable is correctly installed, with a little bit of slack.
5. If an SU type carb, make sure the dashpot has damper oil in it. If you have removed and installed the air cleaner elbow or gasket, make sure the air ports are open and clear. If the gasket is on wrong it will block the ports.

( The good news is that the engine started and runs!)

Hi Everyone, Just completed install of newer 998 motor, on start up the rpm's race straight up with out any signs of leveling off. Thoughts on reasons why? Thanks