Hi. Having problems with Bedford engines in the past, having purchased an RL Recovery that had been off the road for many years, it too back fired and would not run correctly. The remedy to this was setting the points correctly, which took some time to perfect. Once set, it has run perfectly since. The other problem you may find, as it is running fine at idle, is the accelerator pump may be stuck in the carburettor body, which will give you a backfire when you try to accelerate the engine. This can be freed up by removing the carburettor and gently warming with hot water or hot air until it becomes free. Hope this helps. Cheers.