synchronise the carb with what? migrating wildebeest? what do you synch a single carb with? I used to do it with banks of 4 carbs
back to the question: are you flooding the engine? if it runs well once started, I'd guess you're flooding it by using the throttle while kicking it. only use the throttle if you need to clear a flooded engine eg after a tumble - pin the throttle wide open for 6-10 kicks then resume normal starting sequence which means finding TDC and then, leaving the throttle alone, kicking it through TDC. use the decompressor to nudge it past TDC if you like, but then give it a good kick. getting the choke setting right helps - mine can be a bit of a bugger. oh, and every now and then random XR650Rs will lose compression. I assume its the decompressor sticking, but I've seen it on several bikes.