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muffled backfiring out tailpipe when de-accelerating

479 Views 9 Replies 5 Participants Last post by  briann510
i just tuned the carbs up and synced them and it idles great now and pulls hard through the whole rpm range. i adjusted the idle mixture screws evenly by the book and per instructions how to set them and now i have some slight muffled backfiring out the tailpipe when downshifting. it isnt bad, but it didnt do that before so im thinking im a little too lean maybe? or would it be a little too rich?

the idle mixture screws are out 2 turns right now for 80 degrees temperature outside for the SU carbs.

these are 6 month old z therapy 4 screw carbs.


Post Edited (Oct 7, 6:50pm)
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i had them both at 1-3/4 turn out and thought i might be a bit lean as most of the books say they should never be less that about 1-3/4 turns out but ill turn em to 1-1/2 out and see what that does. i know when the choke is on in the mornings its way too rich and i cant even use it once the engine fires. whats weird is i turned them in from their original settings 1/2 turn when i tuned the carbs.

i also heard tho z therapy carbs are pretty much set for 280 motors wanting more fuel and i will be really rich from the get go jetwise. i would just hate to go too lean and hurt the engine.


Post Edited (Oct 7, 7:09pm)
When I had my AFM set a bit too rich, I had the same gurgle and muted pops from the exhaust when letting off the gas after a peppy burst.
i also unhooked my throttle dampner vacuum thingamajiggy thats on the balance tube that slows the throttle down when letting off the gas (emissions). that probably makes it a little worse having the throttle slam shut now when letting off the gas.

That is your problem right there leave your settings on the carbs alone and rehook up the decelleration vavle. This is what is richining your mixture when decellerating. Just like pulling the choke.

you guys don't like the sound of the gurgle and minor backfiring??

Makes me feel like I have a race car... spitting baby flames... or even a Rally car.... pop! pow!

hehe.. i wish

but with my twice pipes exhuast my little backfires and gurgles.. are really LOUD
throttle dampner vacuum thingamajiggy thats on the balance tube that slows the throttle down when letting off the gas
FYI its called a dashpot.
i unhooked the "dashpot" thingy because it had a vacuum leak.

the earlier Z's didnt have those, so what keep those from "gurgling"?

with that thing unhooked when you let off the gas it really slows down the car in gear, before it just kept on going for a bit as the throttle closed more slowly

Post Edited (Oct 7, 11:58pm)
hooked the dashpot back up and it sounds back to normal now.

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