Actually if instead of sending raw data you send manchester encoded data. Another trick is to send a 10101010 preamble then data then pause for 10ms before sending again. I have a few chinese made ON/OFF keying RX/TX modules and they work if you talk to them correctly, what that usually means is not necessarily how Serin/serout sends data.

If this is a one off project, invest in FSK TX/RX like Linx technologies. If mass production, then it is worth the time to find out what mods in the data format will give you what you want.

Nick