Cam timming?

[Peter Lucas <pllj@earthlink.net>]

I have a question for you. I started up my new engine for the first time
last week. It started right up and idled ok for the first few seconds.
After that it started to back fire out the carbs- but only a little bit.
Even after timing with a strobe and re-adjusting my points the problem
continued. Opening the throttle  only seems to enhance the problem. Does
this sound like a cam timing problem or have I overlooked somthing
simple? 

Thanks in advance
PL