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302 Hesitation and Backfire at 4k

ECStein

Newbie
Joined
Oct 16, 2016
Messages
23
I have a 72 with a 302 and Edelbrock 1404 carb. Vehicle idles and drives fine. If I dig in at WOT in 1st or go to WOT in 2nd the engines sputters, feels like its losing power, and backfires out of the exhaust.

I've checked timing (10 BTC), vacuum (18 @ idle) and fuel pressure (4 at idle). The problem persists - it always happens at 3.8k to 4k and causes the needle on the tach to bounce.

Any thoughts?
 

B RON CO

Contributor
Bronco Guru
Joined
Jun 29, 2016
Messages
2,412
Loc.
Statesville, NC
Hi, I would check the ignition system. If the tune up parts are old I would probably replace them. If the wires are arcing ( look under the hood at night), or the points are corroded, or the cap has cracks or carbon tracking that could cause your problem.
You could unhook the tach to see if it is causing the problem.
Good luck
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Bouncing tach is a big clue. The tach reads ignition speed. Doesn't care about fuel. You could be out of gas coasting down a hill and the ignition will still fire away and the tach will still read correctly.

Since the tach goes funky at the same time the engine runs bad, that tell me that you have an ignition issue. What parts are in your ignition system?
 

bax

Contributor
Old Member
Joined
Aug 22, 2005
Messages
14,491
You are lean. The backfire at high RPM is a lean backfire. Not ignition in my opinion. Check your float levels, your pump PSI and your fuel filters.
 

ENDLIFE

Sr. Member
Joined
Nov 17, 2011
Messages
453
If you check your fuel mixture and you're not running lean, check the total timing of your distributor, as you might not getting enough timing and need to adjust your distributor. 10˚ is a good base timing to get your engine going, but in my experience, it rarely stays at 10˚.
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,833
Does the tach bounce match what the engine is actually doing? IF it is dropping 1000 RPM and back in under a second type of thing that the engine can't actually be doing. That shows electrical issues. Think of the symptoms if you were driving and flicked the key off and back on. The tach will bounce around (even though engine speed doesn't really change) and you get backfires through the exhaust. Exactly what you describe.
 
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