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I have had a flutter occasionally at idle and somtimes cruising, and could not find it until I checked the timing and saw that the timing was jumping around like crazy below 2500rpm, checked the ProComp HEI distributor mechanical advance springs and found that the attachment posts had broken, so basically the weights were just flailling around. Needless to say, it will be scrapped and a MSD or something USA made substituted.
I had wondered why it had such a useless idle lately.
Used the Summit brand distributors and never a problem also.
Lots of times they are MSD or Mallory with their name on it. Summit usually wont sell junk with their name on it.