K11 hot rod performance issue

Hi All,
My sons k11 suddenly started almost miss-firing or hesitent to rev. Its the dizzy type engine and ran perfectly for many races but all of a sudden it developed this issue.
We have a spare car so we swapped almost everything with no improvment, trottle body, fuel rail, injectors, FPR, distributor, plugs and leads, ect
I need to rig up somethingto check fuel pressure but the flow looks very strong.
The spark plugs are fouling up with black dry soothe
I cant be sure of the quality of the second hand parts so im aware it could still be a fualty distributor or maybe any of the other parts.
Since the rev limitor is by passed and the engine is at high revs most of the time is it possible that the timing jumped a tooth? something i wont get the chance to check for a day or two..
Any assistance would be great,, thanks
 
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