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#1
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Backfire after deceleration on freeway
Back info pontiac 400 punched 60 over, raiii cam
Smi quadrajet, Dougs headers with 3" to magnaflow mufflers dumped after mufflers. I get about 10" of vaccum if I recall initial timing 12 total 36 or in around there Car drives around fine some surging at part throttle cruise but will accelerate no problem no matter the speed. On the freeway if I let off the gas it will be random pops and bangs from the exhaust and like a stumbling feel if I hold the pedal in place at same speed, but if I push the pedal down (go faster) it'll all go away then the backfiring will come back after I get off the gas Is this a timing issue? A/f screw issue? Apt screw issue? I've read it could be exhaust leaks but I do not hear any Sent from my SM-G996U using Tapatalk |
#2
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I use to have this in one of my engines sometime ago and I was told it was from unburnt fuel in the exhaust. Try a different carb see if it helps. Never did figure out the cause.
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68 Firebird. IA2 block, 505 cu in, SD Performance E-head, Solid roller 3600 weight. Reid TH400 4:11 gear. 29" slick. Best so far 9.95@134 mph. 1.43 60 ft. 76 Trans am, TKX .81 o/d, 3.73 Moser rearend, 468 with KRE D-ports, Doug headers, 3" Exh. |
#3
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Your condition sounds , especially taking into account the surge is due to a lean condition at just off idle.
I would also double check with the motor out of gear that when you let the motor drop back from let’s say 3500 rpm, that the timing drops right back with the reduction back to idle rpm. The advance section of the Distributor be it shaft or weights may be hanging up especially when the motor is fully warmed up! You need to get this corrected soon because with a surge condition during part throttle the motor is likely lightly pinging also and that’s beating up your Rod bearings little by little even though your not hearing it, Better especially this time of year to be on the rich side, then lean!
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Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! Last edited by steve25; 11-21-2021 at 08:27 AM. |
#4
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Years ago I had a quadrajet that would surge at a steady part throttle...ended up being the throttle shaft had some play around( worn) it allowing air to enter and leaning it out. Ended up having the shaft hole bushed.
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72 lemans,455 e-head, UD 255/263 solid flat,3.73 gears,,,10" 4400 converter,, 6.68 at 101.8 mph,,1.44 60 ft.2007 (cam 271/278 roller)9"CC.4.11gear 6.41 at 106.32 mph 1.42 60 ft.(2009) SOLD,SOLD 1970 GTO 455 4 speed #matching,, 3.31 posi.Stock manifolds. # 64 heads.A factory mint tuquoise ,69' judge stripe car. 8.64 @ 87.3 mph on slippery street tires.Bad 2.25 60ft.Owned since 86' |
#5
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problems like this are why it is nice to have a spare carburetor laying around - you can swap the carb and see if the issue goes away, and that will tell you whether the issue is carb related.
i seem to vaguely recall similar symptoms when i wiped a cam lobe years ago, so maybe pull the valve covers and make sure all the rockers have full movement? |
#6
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Popping exhaust usually indicates retarded timing or malfunctional ignition system.
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#7
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10 inches of vac on ra3 cam seems low.
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#8
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exhaust leak at header can cause a backfire on decel just went through that on my car.
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72 Luxury Lemans nicely optioned |
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