Hi Joe
Do you mean "backfiring" (as is loud pops or fartz from the exhaust) when accelerating, I ask because that would be unusual. Backfiring normally happens when slowing down. Reason = it is un-burnt fuel getting into the exhaust, often mixing with air being sucked in from a leak in the exhaust, then exploding. So, if it happens when accelerating then a lot of un-burnt fuel must be getting into the exhaust + air, and this is unusual when accelerating (because the pressure of the exhaust gasses forces stuff out of any leaks rather than sucking air in). Is the choke stuck on or other reasons why it might be running very rich perhaps ? - Have a look at the colour of the plugs, they should be the colour of a weak cup of tea.
Or perhaps it is "miss-firing" under acceleration ( i.e. holding back and generally spluttering) in which case most bases have been covered by those folks that posted above
Do the tappets sound OK ?
After being stood, these engines tappets can take a good drive to start working properly again, and whilst they are not working properly, they can cause a slight rough running and lack of power.
How long was it stood for ?
(unleaded petrol can go quite badly "off" in 6 months)
E D I T = Just thought, if it is really backfiring a lot when accelerating, you did put the HT leads on in the right order, didn't you ? - 2 the wrong way round would certainly cause a lot of backfiring
