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Backfiring trough the carb?

Blue71

Contributor
Bronco Guru
Joined
Aug 27, 2001
Messages
5,144
'70 Bronco with an aftermarket carburated 1987 302 motor. Someone has put an Edelbrock intake and an Edelbrock 4bbl on it. It ran and idled good a few days ago after purchasing it, but now it will barely idle. Id did set for a while bu it has new ethanol free gas in the tank and a new fuel filter. When you try and rev it up...it backfires through the the carb and runs rough then dies. Sometimes shoots a small flame or two out of the carb too. Wahoo!

Guide me.

Thanks,

Blue71
 

SDlivin

Sr. Member
Joined
Feb 5, 2010
Messages
781
Loc.
San Diego
X2 on timing.
when I changed my cam / intake, it took me quite a bit to get the correct timing down. when it was incorrect, I would get the backfire through the carb etc....
Not sure on the gas though...

Check timing and Vacuum.
Should have around 8-12 BTDC for timing at idle, and 15-20 vacuum pressure @ idle.
Results, may vary accordingly....
 

77bronco_ed

Bronco Guru
Joined
Jul 3, 2004
Messages
1,784
If after checking and setting timing if this happens again it could be timing belt?
 

oldiron

Bronco Guru
Joined
Jul 21, 2005
Messages
1,032
No timing belts on 302's.
Other options for backfiring through carb:
Waaayy to lean, check for fuel enrichment (power valve on a holley)
Vacuum leak (could also explain the rough idle)
Burnt valve (could also explain the rough idle) easy to check with a vacuum guage.
Clogged fuel filter (again, causing poor idle quality and overly lean condition)

Need more info to be more accurate, such as how long did it set, driven much. etc,etc.
Greg
 

El Kabong

Contributor
Driving stuff Henry built
Joined
Oct 8, 2009
Messages
1,486
Also double check the firing order & condition of the wires. Reversed wires or crossfiring can cause backfiring by lighting the mixture while the intake valve is open.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,346
And speaking of ignition, are you sure you're getting enough current to the ignition? I ask because you're running into all those other wiring issues right now.

Paul
 
OP
OP
Blue71

Blue71

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Aug 27, 2001
Messages
5,144
And speaking of ignition, are you sure you're getting enough current to the ignition? I ask because you're running into all those other wiring issues right now.

Paul

Hey Paul..Me? Wiring problems? :) .I should be fine on the current. This is on another Bronco that I picked up a few days ago. Its a '70. That '75 is the booger with the starting issue we talked about before. Will double check though.

Thanks,

Andy
 

bax

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Old Member
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Aug 22, 2005
Messages
14,491
Do I need to come pick it up from you?
 
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Blue71

Blue71

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Messages
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This Bronco sat for a few months. But ran fine for the past several days. Really strong 302 and idled well. Just the last day or so its giving these problems.

El Kabong- Firing order should be fine since it ran great a few days ago and nothing has been changed. Will check condition of wares again, but looked in good shape, even on the ends.

Oldiron- Might possibly be a valve. It did set a few months so will go back and double check fuel filter. If I can get her to idle..will check vacuum too.


Would there be any reason that the timing would be off since the other day? Haven't messed with it since it ran good. Might have jumped time possibly??? Really seems more like a carb issue. It is like it almost loads up or something.
 

gumbydood

Jr. Member
Joined
Jul 5, 2009
Messages
81
A broken valve spring might also cause this problem... i had some serious backfiring/misfiring issues with a broken intake valve spring, and it could be a "fine" one day and "bad" the next. Just a thought...
 
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Blue71

Blue71

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Aug 27, 2001
Messages
5,144
Ok...I did get it runng bnetter today. Found a vacuum hose off behind the Carb on a vacuum tree fitting on the manifold. There was also another open port on a back there as well on that same tree. Doesn't appear to go to anything. Vacuum advance vacuum hose is in good shape and it comes off the front of the carb. Can't see any more vacuum leaks. Now she will idle fine. Revs up pretty nice and strong too. I took her for a spin, and first run was pretty good with a small bit of back firing from the carb, but not to bad and she ran strong. The next run down the street...it started out strong in the throttle, but would fall on her face in the mid range. Also seemed to fall on her face while pulling up hills too. Tried this several times and same thing happens. Timing seems to be pretty close too according to throttle response and starting. Brought her back in the garage and reved her up hard and no bumbles now. Just seems to do it mainly when driving.


Thanks,

Blue71
 
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