what causes backfires - Engine Troubleshooting


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what causes backfires

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1aeco1
New User

Dec 12, 2009, 11:19 AM

Post #1 of 4 (2378 views)

what causes backfires	Sign In

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.

steve01832
Veteran
steve01832 profile image

Dec 12, 2009, 6:37 PM

Post #2 of 4 (2366 views)

Re: what causes backfires	Sign In

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

Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Dec 13, 2009, 12:13 AM

Post #3 of 4 (2363 views)

Re: what causes backfires	Sign In

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

Loren Champlain Sr
Veteran / Moderator
Loren Champlain Sr profile image

Dec 14, 2009, 7:06 PM

Post #4 of 4 (2349 views)

Re: what causes backfires	Sign In

To add; Flat cam lobes on #7 or #8?
Loren
SW Washington

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