If you can narrow it down a bit, it will be easier for you to sort out. If it's from the pipes, it could be a very rich idle mixture or weak spark allowing raw fuel to build up in the exhaust system and being light off when you open the throttle.
If it's from a carburettor, the mixture is a bit lean. This could be the idle mixture adjustment, a clogged transition jet (the small holes in the bottom of the carb throat just above the throttle plate), a slide stuck partly open, an air leak or too small pilot jets.
