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Old 04-01-2010, 07:37 PM   #1
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Join Date: Apr 2007
Location: Niagara Falls
Posts: 171
Car: 1988 Firebird Formula WS6
Engine: 305 tpi lb9
Transmission: 5 speed
Axle/Gears: 3.45

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backfire under acceleration?

I just took my rebuilt engine for its first run (305 tpi). it has been bored 30 over, heads machined, new lifters, 1.6 roller tip rockers, new accel distributer, new wires, new accel plugs at .035 gap and 1 year old accel coil, hooker headers new converter. never backfired before rebuild.

I set the timing to 6 deg. before tdc with engine warm and black and tan wire unplugged near firewall when it is running with the wire plugged in it is around 20 deg or 25 deg tiiming (off the scale).

What is the first logical step to take when diagnosing a backfire or pop from exhaust when under a load or accelerating, car sound good at idle but kind of shaky? is my timing ok? I replaced the knock sensor also could this maybe be the wrong one? If I over adjusted a rocker would it backfire all the time?

Also once when i got to about 3k rpm the car died and wouldnt restart for a minute. it was in third and i felt it die out, had to pull over to get it to start. I bought the distributor from someone on this site and am not sure if the module under the cap may be the cause.
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Old 04-01-2010, 09:42 PM   #2
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Join Date: Jul 2009
Location: morrow, ga
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Car: 82 S10, 83 280ZX, 84 Z28
Engine: 355 smallblocks..na, 2.8 turbo
Transmission: 85:th350, 84:700R4
Axle/Gears: 85:ford9 4.85, 84:stock 3.24

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Re: backfire under acceleration?

may be a lean skip/backfire. sounds like you may have increased your engine's need for fuel with your mods.
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Old 04-01-2010, 10:55 PM   #3
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Join Date: Apr 2007
Location: Niagara Falls
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Car: 1988 Firebird Formula WS6
Engine: 305 tpi lb9
Transmission: 5 speed
Axle/Gears: 3.45

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Re: backfire under acceleration?

wouldnt mass air flow compensate for this? I didnt think 1.6 rockers and an upgrades ignition and exhaust was that big of an upgrade but i may be wrong. That does sound like a good place to start. should of boosted my fuel pressure a bit with a fpr i guess. I am gunna double check my timing and make sure all the plug wires clicked on. It sounds really good when I rev it in neutral only under a load it does this. Should there be any play in when i move my rotor back and forth (gear play)?
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Old 04-02-2010, 12:14 AM   #4
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Join Date: Jul 2009
Location: morrow, ga
Posts: 1,350
Car: 82 S10, 83 280ZX, 84 Z28
Engine: 355 smallblocks..na, 2.8 turbo
Transmission: 85:th350, 84:700R4
Axle/Gears: 85:ford9 4.85, 84:stock 3.24

Classifieds Rating: (2)
Re: backfire under acceleration?

no, unmetered air causes a lean condition. higher ratio rocker increase lift and duration causing a drop in map. check the fuel pressure. try to get it around 50psi. check for vac leaks with some wd40 with the engine running and yes check your plug wires.
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Old 04-02-2010, 12:32 AM   #5
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Location: San Diego, California
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Car: Trailblazer EXT 4.2 (Firebird Form)
Engine: 5.1 4bbl
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Re: backfire under acceleration?

when you disconnected the wire to set the base timing, did your RPM drop like an anvil when you unplugged it? if it didn't you may have to disconnect one of the ignition module plugs, don't remember which one it is, but it should sound lopy ... RPM should be about 500 rpm, check the timing... another thing, try running higher Octane fuel...
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Old 04-02-2010, 08:55 AM   #6
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Join Date: Apr 2007
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Car: 1988 Firebird Formula WS6
Engine: 305 tpi lb9
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Re: backfire under acceleration?

the rpm did drop when i disconnected it but didnt change back when reconnected it untill i shut it off and restarted it. rpm drops when i disconnect it so im pretty sure the base timing is correct. Im going to get some new fuel today since this tank is from being stored over winter and is 87 octane. I dont think this is the problem but its worth a shot. ill post back later if it fixes it.
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Old 04-02-2010, 10:25 AM   #7
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Car: 1988 Firebird Formula WS6
Engine: 305 tpi lb9
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Re: backfire under acceleration?

I really dont want to post this but i will. I double checked the wires and found 6 and 5 were switched. It still sounded good at idle and reving and i thought the xtra shake was from the 1.6 rockers. I still havent test drove it but i am certain this was the problem. at least it was a simiple fix.
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Old 04-02-2010, 11:29 AM   #8
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Join Date: Apr 2007
Location: Niagara Falls
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Car: 1988 Firebird Formula WS6
Engine: 305 tpi lb9
Transmission: 5 speed
Axle/Gears: 3.45

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Re: backfire under acceleration?

Just drove it 20 miles without a hicup. got some fresh 93 gas and it runs and performs like new now. Still cant believe I made that rookie mistake after rebuilding the engine and all, doh!
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Old 04-02-2010, 11:29 AM
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