My engine has just been rebuilt. Last night it would'nt start. Today the engine runs as usual but it stalls when I decelerate, then I start after one pull. I also changed the throttle cable.

probably your throttle saftey switch. You might have to re-adjust your throttle slides.

also look at the connection to your cdi/coil...I use dielectric grease on mine As well these sleds seem to have many cdi issues

I have a 1998xc 7 that would run like a raped ape....then stall. start up, run like a raped ape......stall
I changed the cdi........1000 miles later, still makin tracks

Throttle safety switch engaging. Adjust the cable so you have .030" freeplay gap at the lever to throttle block. Adjust both cables at the top of the carbs out equal amounts, to keep the carb sync proper.