Cobra Make, Engine: ERA Street Roadster #782 with 459 cu in FE KC engine, toploader, 3.31
Posts: 4,533
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Sounds like "trailer hitching". A healthy cam is usually part of the reason - but light springs on the centrifugal advance can aggravate it with advance swinging in and out with surges in the engine rpm. If it's gotten worse a thorough check of timing and carb settings might be in order.