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1aeco1
New User
Dec 12, 2009, 11:19 AM
Post #1 of 4
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what causes backfires
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i have a 1972 chevy k5 blazer with a 350 small block which will run very well when idling. but when i place the vehicle in drive it bogs down and starts back firing and will die if not given copious amounts of fuel.
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steve01832
Veteran
Dec 12, 2009, 6:37 PM
Post #2 of 4
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Re: what causes backfires
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There can be many reasons why this is happening. Could be ignition timing and centrifugal, vacuum advances. Carburetion, low fuel pressure, valve timing, excessive slop in timing chain, etc... You may want to start with doing compression tests, fuel pressure tests, vacuum guage readings at idle and snap throttle conditions. Steve
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Dec 13, 2009, 12:13 AM
Post #3 of 4
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Re: what causes backfires
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1972 - If some miles on it and timing chain hasn't been done it might even need another. Check how much slack there is. Trick to tell is set to TDC, take dist cap off and turn engine by crank bolt, get it tight and see how many degrees you turn engine by hand it goes before rotor moves. More than 5 or 6 degrees is enough reason to replace the chain. Getting old now - I used to do many back when especially if you were replacing a water pump you are most of the way there on many. If that's it - it's usually intermittent at first - then the popping and may snap out of it for a while are the common symptoms for that with real chains. What carb is on this? Quadrajets of the whole era are tricky to adjust and choke pull offs would go bad. Laugh: They used be nicknamed Quad Bogs! I loved them as when everything is right they are an excellent carb but if it's the issue get some good specs and know how to adjust it. Owned a zillion of them and every one brand new (family + others) had a different character! The other 2v should be a DualJet - a bit simpler, T
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Loren Champlain Sr
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/ Moderator
Dec 14, 2009, 7:06 PM
Post #4 of 4
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Re: what causes backfires
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To add; Flat cam lobes on #7 or #8? Loren SW Washington
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