Back fire back fire back fire

[Ernest Montague]

I am putting the race bike back together. Flatslides and gen two cams. 
It ran well last year, and now I can NOT get it running well.  It 
starts and idles fairly well but will NOT take throttle. It backfires 
through the cannister and sometimes the front carb.   I have checked 
and changed plugs, redid the plug wires, checked spark, and checked the 
coil connections. .Fresh gas.  Good battery. Anyone have any 
suggestions?

Ernie

I guess i will take a set of good carbs from a street bike and throw 
them on and see if that changes anything. It will narrow the search 
down anyway.

Yesterday I got it running after some sputtering and backfiring and it 
ran perfectly, took gas well.  Now it  is back to not taking throttle.