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Old 03-21-2005, 07:26 PM
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Timing? or Carb? causing backfire

Vehicle:
1985 Chevy S-10 2.8L 5spd 4x4

Problems:
Hesitant on low end, popping exhaust (Backfire)

I have changed the plugs and plug wires and all are gapped correctly, Checked the timing with a timing light and it is at correct 8 degrees. Checked for vacuum leaks at the base of the carberator and their was none. Replaced vacuum lines to the pvc valve and distributer advance. There are no cracks or carbon lines on the cap but the cap and rotor do look old. The air filter is not totlaly fithy,

So why does it backfire when i deaccelerate and in between gears? And when i mash it will sputter until the mid range and it takes off

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Old 03-21-2005, 07:36 PM
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Did it start all of the sudden? If so, check the timing belt or chain. It may have jumped.
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Old 03-21-2005, 07:41 PM
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Quote:
it backfire when i deaccelerate and in between gears?
Is the vacuum advance hooked up? Have you checked to see that it works?
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Old 03-22-2005, 09:35 AM
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Yes distributer advance is hooked up correctly working right i dont know

I dont know if it did this befor i just bought the truck and replaced the clutch after its been sitting for a year.
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Old 03-22-2005, 04:43 PM
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Poping

Doc here,

Does that engine have an EGR valve on it? If so it may be leaking, Get some carb cleaner and spray around the Base of the EGR and see if the idle speeds up.

If it does either block plate it off (to test it) or just straight up replace it and see if the popping Goes away.

Get a Vacuum gauge, and hook it to any manifold source, watch it at idle, It should read a steady 17 to 20 in Hg at idle. If it flutters rapidly (unless you have a very lumpy cam) you need to look at the valve train, If you eliminated all other Vacuum leak sources.

If you just bought it, and it sat a long time, don't rule out the possibility that the gas may be contaminated with water.

Doc
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Old 03-22-2005, 06:52 PM
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You just changed spark plug wires? Check and recheck your firing order.
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Old 03-22-2005, 07:19 PM
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Might want to rebuild the carb and check your exhaust for leaks.
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Old 03-22-2005, 07:41 PM
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only thing I have to add is that if it pops out of the exhaust it is technically an afterfire. if it pops out of the carb it is a backfire.

-Leo-
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Old 03-22-2005, 11:12 PM
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