There are a few different ways that your symptoms can occur.
May I suggest that you check a few things on the engine as well before only blaming the head. A lack of compression will make a sparkplug white despite the chamber conditions. The sparkplug displays the combustion conditions in the environment around it, if its white its lean. If the rest of the chamber is black then it means that the fuel condition at ignition time is lean. This occurs because of a lack of fuel vaporization. The gaseous state of the fuel at ignition time is inadequate, by the time the burn has progressed in the rest of the chamber the fuel has all been converted to gas and burnt, thus forming carbon deposits.
The ignition energy may also be to blame for a white plug with a black chamber. If the ignition is weak insufficient energy is available to initiate carbon burn resulting in the immediate zone around the plug being predominately a hydrogen burn. Hydrogen does not create deposits, leaving the plug white. Weak ignition makes the cylinder react like its retarded, getting hot etc.
Another very important point to make sure of is there are no air leaks. A cylinder that is excessively lean will actually make a white plug and still blow black smoke out the exhaust. Its a similar situation to a hydrogen burn, the carbon commences burning too late in the stroke and is incomplete so particles exit the exhaust.
Detonation will also make a plug white with a black chamber in a low compression engine. You cant hear it sometimes but its causes the plug to bet too hot. Typically if the plug is overheated it will be white but with a slight gloss effect over the porcelain. The timing may be too far advanced and it effects the rear cylinder in that manner because of the air cooling stream. The front cylinder will not obtain the same temperature as the rear and may not be detonating.
Just a few things to think about, hope it goes well for you. Regards Bruce.