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05-04-2005, 11:29 PM | #1 |
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What is the deal with backfire?
I'm trying to trouble shoot my back fire issues, and I can't fix it.
Ping Bing pop happens at idle and some levels of RPM's But at a steady high RPM like 2500 it runs fine? I always thought that backfires were from to much fuel, but I was just reading my hanyes manual and it says "just taking the air cleaner off sometimes will lean a carb enough to create a backfire". So I not sure if I'm to rich or lean. The plugs are ok from all the "pops" but I HATE hearing that noise. Next thing you know your trucks on fire and your running down the road with grenades. The book reads for trouble shooting a back fire: 1 Emissions system not functioning ( emissions?) 2 Ign. Timing Could be! 3 Sec. ign system. Plugs wire cap roter all new. 4 Carb neds adj or worn. Its a Qjet from a 78 C20 might need adj. 5 Vaccum leak Maybe the carb gaskets are still to loose 6 Valve issues Can't see it 7 crossed plug wires ? runs to good at 2500rpm What do you guys think?? Thanks a TONN |
05-04-2005, 11:36 PM | #2 |
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A backfire is one indication of too lean of fuel mixture.
Dan
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05-04-2005, 11:42 PM | #3 |
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So I should try turning out my mixture screws?
As they are only 1 turn out. |
05-05-2005, 12:01 AM | #4 |
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I normally start with the Idle mixture screws out 2 turns and go from there.
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05-05-2005, 07:52 AM | #5 |
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is it a points dizzy? check the condition and the dwell then your timming. may also need a carb rebuild if you getting a lean condition
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05-05-2005, 08:35 AM | #6 |
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Its a HEI dizzy. The carb ran good on the other truck?
Tonight I'll try a few carb ajd.'s. Would the timing change when changing carbs? |
05-05-2005, 10:20 AM | #7 |
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Timing shouldn't change. Check for intake leaks at the carb base. I would also double check the carb base gasket to make sure it is the right one. I have seen the gasket look right for the carb, but wrong for the intake, or vise-versa.
Dan
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05-05-2005, 11:48 AM | #8 |
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I have a square intake with the adapter for a Qjet. It may be the gaskets aren't sealed
The Qjet one has some wear on the two humps near the front. I had a Holley on there two days ago with no adapter and it was backfiring too!???? |
05-05-2005, 01:20 PM | #9 |
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Line the gaskets up on the adapter to verify they cover what they should. Possibly carbon tracking in the dist cap causing a misfire? Maybe a crossed plug wire. If the firing order is correct try re-routing the plug wires so they don't run right next to eachother. Still sounds kinda like a vacume leak to me, though.
Dan
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05-05-2005, 03:14 PM | #10 |
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Might want to check your fuel pressure.
I recently went NUTS putting a known good carb on an engine that ran reasonably well with a clapped out carb and it was waaaay lean. Finally ended up pulling the pickup 'sock' replacing some NASTY old rubber line and fuel pump and all is well. If this truck has backfired with 2 different carbs, one known good I'd be asking myself what non carb problems could be causing this. Has this truck ever ran well w/o backfiring for you ? To look for vaccum leaks you can spray around the base of the carb with some carb cleaner and see if the idle speed goes up.
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05-05-2005, 03:36 PM | #11 |
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I bought a Holley 4150 carb 5 years ago to go on a 3000km trip which worked out great.
Then from sitting the gasket on the seat went causing the truck to run terrible. So I rebuilt another 4150 that came with the truck. It runs great but back fires. So I took a 78 Qjet got the adapter cleaned it up and it will run great too but back fires? I'll check for leaks tonight with some spray and see what happens. I don't want to buy a new carb but it's getting tempting. |
05-05-2005, 07:32 PM | #12 |
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the emissions they reference are the air injected systems, if there is a problem in the pump or diverter valve it could cause a backfire........
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05-06-2005, 08:32 AM | #13 |
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Holy fn shoot bags! Turned the mixture screws out 2.5 turns and BAMMMMMMMMKMMM
Were GONNNNNNNNNNNNNNEEEEEEEE~! Thanks Guys YEEEEEEHAAAAAA To Love Thy Chevy |
05-06-2005, 12:44 PM | #14 |
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Oh ya I also tightened the carb bolts a whole turn.
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05-06-2005, 07:37 PM | #15 |
got boost?
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to get the idle mixture correct you need to put a vacuum guage on it.
adjust one at a time correcting the idle speed if it drops or increases. shoot for the highest stable vacuum reading with it in p or n.
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05-12-2005, 03:22 PM | #16 |
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chevibubba, Where does the vacuum gauge hook up to???
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05-12-2005, 04:01 PM | #17 |
got boost?
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any manifold vacuum source such as the tranny vacuum modulator port or the port below the throttle plates.
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'70 chevy lwb w/'67 hood. 357 s.b.c. built, 4l60 w/vigilante lockup high stall converter, edelbrock pro-flo,brodix heads,comp cams rollerized w/ belt drive, one piece d-shaft, yada, yada, yada..... '82 honda v45 sabre"the black" |
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