Mine was doing the same about 4000 rpm, but my crank pulley flange was too big to fit inside the center of my harmonic balancer, so it was off center by more than 1/8". I don't know which was incorrect, but I needed an A/C pulley anyway, so I found a balancer/ pulley combo at the wrecking yard and now I do 5000 no problemo.
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'69 Custom 10 SB
'66 Mustang Sprint
'77 Sportster
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'69 Chev Cust/10 swb, 350/700R4, 5"/6"
'70 Chev C10 swb, 67 front, 454/700r4, 4"/5"
'73 Chev stepper, 350/3 spd
'84 Chev K20 350/4 spd
'91 GMC 1500 Suburban 4x4, 350/700R4
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