This mid RPM range hesitation can be traced to the secondary fuel injectors not firing properly.  This can be caused by several culprits.  If the engine is not run very hard, the secondary fuel injectors will almost never switch on.  Having the secondary fuel injectors dormant will cause deposits to form over time.  Also, due to the age of the electrical systems, the sudden increased current draw of the secondary fuel injectors turning on, the electrical circuits must be in good shape.  Bad grounds and dirty electrical connections will induce voltage drops that can cause the hesitations.

The 3,800RPM hesitation problem is covered in detail in our FAQ section.  Please check out the page here:
FC3S Pro - FAQ, 3,800RPM Hesitation

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