Quick Tech With Mar­lan Davis

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Main­tain­ing idle sta­bil­ity as cam du­ra­tion in­creases man­dates pro­gres­sively more car­bu­re­tor throt­tle-blade open­ing (via the idle mix­ture screws) to com­pen­sate for the re­duc­tion in cylin­der pres­sure in­duced by the larger cam’s later-clos­ing in­take valve. Even­tu­ally, the at-idle throt­tle-blade an­gle in­creases to the point that it un­cov­ers the tran­si­tion slot, thereby ac­ti­vat­ing the main cir­cuit and ren­der­ing the mix­ture con­trol in­ef­fec­tive. One so­lu­tion is to raise cylin­der pres­sure by run­ning more base (ini­tial) tim­ing. With cylin­der pres­sure re­stored, less car­bu­re­tor throt­tle-blade open­ing is needed, so the idle mix­ture screws should again be­come ef­fec­tive. At this point, you would want to shorten the cen­trifu­gal curve so over­all to­tal tim­ing re­mains op­ti­mal for the par­tic­u­lar en­gine com­bi­na­tion.

[ If af­ter in­stalling a longer-du­ra­tion cam you have idle sta­bil­ity prob­lems, try in­creas­ing the ini­tial tim­ing and short­en­ing the cen­tifu­gal ad­vance curve.

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