Rebuilt .05 over with new pistons etc.very hard starting and after 5 minutes or so starts farting and popping. Just replaced ignition mod.,new S&S shorty etc.Took out the timing plug on the crankcase.and seemed to start right away but with very exessive blowby.How much is normal and what may cause this?  

Sounds like a combination of things going on here...

Blow-by sounds like the breather-timing on the oil-pump is out of whack. That would probably be one of the key places to look...

But, when you're talking about "Blow-by" and the time plug, there's a lot of splashed oil in that area too. It might not be blow-by...

Your best bet is to do a leak-down and compression test to see if it is actually blow-by..

Farting and popping could be the timing, the ignition module, or the carb.  

Crank position sensor good? Not sure what bike we're talking about here......

Just a random thought, but it's mine.