had similar problem with a 350 i had once turned out it was the breather pipe on the bing carg was to long i cut it back and bingo sorted the problem. also i was told to lay the bike on it's side this worked before i cut the breather pip back i use to lay it on it's side every i started it. as boghopper says be good start to put the new crank seals in if it's been stood 20 plus years they will be rock hard.
