I changed the fuel filter out (it was clean) so supply does not appear to be the issue. Pulled the plugs, light soot coating which would point to too rich, but I think it's a temporary conditon from the injector cleaner. so I'll run it for another week and pull them again. In answer to the question, no clatter when the timing is set at 12 degrees, but it hesitates
all through the pedal range. If I set it at 14 degrees, hesitation is gone in the low range of acceleration and diminishes in the upper range although there will be a slight valve clatter initially. There has been no change in the "13" code flashing up while driving and it still disappears when I let off the gas or tromp on it. Another thing I've noticed in road tests is if I feather the pedal, back off slightly when I fell the hesitation, then press the pedal again it's as if something has reset because the hesitation is momentarily gone. How do you check the MAF?