No Relief and a Loose Truss Rod Nut

[kaspforeva]

I have just bought two old guitars with this problem, a Yamaha and Yamaki. It is the problem where you have a totally loose truss rod nut and yet no relief at all.
My question is, what causes this problem? From my understanding, a guitar with a single action truss rod gains forward bow simply by the strain of the strings. If there is no forward bow does that mean that it has barely been strung up? Perhaps left in its case unplayed?
And a second question, is there any solution to this problem? I have thought of going up to a heavier gauge string?

[jonfields45]

Differential shrinkage between the neck and the fingerboard. The neck blank might have been relatively green compared to the fingerboard when the guitars were constructed.

[B. Howard]

While a step up in string gauge can help these situations, going to heavy with strings will start to stress the bridge, the top and the bracing inside. Anything heavier than 13-56 is not recommended and will likely finish killing these guitars.

Recently bought these two with the same condition. From the same source? sounds like they may have been stored in less than ideal conditions and the necks have warped..... There are many ways to remedy this ranging from a level and crown, re-fret, neck reset or installing a two way truss rod. It is going to be near impossible to tell which is needed in each case without seeing the instrument in person though....

[jonfields45]

If I recall correctly, this is a common enough problem that Taylor glues up their necks with forward relief and uses the truss rod to straighten it out.

[lonecedar]

I have the same issue with a Harmony Sovereign H1260 that had a neck reset done on it and the luthier who performed it did not tell me that the situation existed. If he had I probably would have paid to have it fixed.

Anyway I am now performing my own guitar work and I am going to remove the fretboard and install a two way truss rod and fix it right.