As mentioned, check compressions are all good then go through the carbs & fuel pump. Check timing is correct. If all this is good then there are some small plastic restrictors located in the transfer ports on cylinders 1-3. By all accounts these can work free & get ingested, after that the thing will never idle any sense.
I believe I may have the restrictor issue on my '84 115 as it won't idle any sense & everything else is done.