i have a 97 580 efi. the thing has always had a bog out of the hole, and kind of doggy till you hit around 6500 where its most responsive. put in an orange spring, and 52 notched weights which makes it engage now at 5000. secondary uses stock helix and spring , just tightened up. have tried the regular cat secondary with some diff helix and spring combo's but found what i have works best. i think the bogs just there to stay.. but you can do what i do, and not left off the throttle...lol
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