I have done a complete tune-up, set the timing correctly, curved the distrbutor and still get a rattle at cruise rpm and light thottle using 93 octane. I believe that the previous owner drove around with the vacuum advanve disconected which is ok if your drag racing but will carbon up the top half on the street. I think my rattle is the carbon deposits that have collected in the cylinder causing a mild knock at cruise. The engine does not use any oil but the exhaust pipes are carbon black. I disconnect the vacuum advance the rattle goes away, but it loads up the plugs if driving with light thottle after a few hundred miles. I think I'm on the right track with the carbon deposits, just not comfortabe dumping somthing down the carb that I'm not familiar with. I'm looking for someone who may have used Sea Foam and can commet on the product.