Definitely check the cam springs 20+ years on springs is definitely iffy. Another cause of backfires and bogging down is if the needle jet clip is in the wrong spot...as it's position effects the mixture causing it to run rich or lean.
Dropping the needle lower leans it out, raising the needle makes it run richer. If it backfires and pops in mid range lower the clip a notch which raises the needle jet.
So don't forget Raising the clip lowers the needle and lowering the clip raises the needle.
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