Strange tach question

flyin '85

New Member
Jan 4, 2004
Hey everyone,
Did a search, couldn't find much on this. Here's the problem. My tach stops working accurately when running the car under full throttle, but only under full throttle.
If you run the car up to 5500 rpm slowly under partial throttle the tach reads fine, never misses a beat.
If you floor it and try to run it through and gear or two, once the thing reaches around 4,000 rpm the tach starts to jump around and seems to get "behind" the engine, I have my rev-limiter set for 6400 and when I hit it, the tach was only reading about 5,000....
Anyone have any ideas what would cause the tach to work ok under parital load when the revs build slow but not work well under full load when the revs are building fast ???

Thanks in advance guys !!!!
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