back fire at 3/4 throttle
Thread Starter
Burning Brakes
Joined: Dec 2004
Posts: 825
Likes: 0
From: Hanging out back with the dark horses. Wa.
back fire at 3/4 throttle
81 350/350 basic bolt ons. edelbrock performer and carb, replaced computer distr with HEI.
Car seams to run good until I hit 3/4 throttle and then she starts popping and backfires. keep the rpms up she runs ok until 3/4.played with the nidles and springs in carb and got it that far. unpluged all vacume ports looking for a leak and found nothing still drives like
took carb apart and blew out jets.
with out throwing parts at it..... anybody
Car seams to run good until I hit 3/4 throttle and then she starts popping and backfires. keep the rpms up she runs ok until 3/4.played with the nidles and springs in carb and got it that far. unpluged all vacume ports looking for a leak and found nothing still drives like
took carb apart and blew out jets.with out throwing parts at it..... anybody
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
You can also check your spark plugs : If you find one which is fouled, with an abnormal gap, or just not the same color as the others, bingo ! This is probably a faulty cylinder.
Also check the continuity of your spark plug wires with an ohmeter, and check if they are insulated correctly.
( Not burned by the exhaust pipes, which is a frequent cause of spark shunt ).
I have had this kind of problem with mine and it was because cylinder #7 was not receiving any spark, because the tiny spiral wire was cut.
If all your spark plugs look great, then check if you're not too lean with your carb setup.
Also check the continuity of your spark plug wires with an ohmeter, and check if they are insulated correctly.
( Not burned by the exhaust pipes, which is a frequent cause of spark shunt ).
I have had this kind of problem with mine and it was because cylinder #7 was not receiving any spark, because the tiny spiral wire was cut.
If all your spark plugs look great, then check if you're not too lean with your carb setup.
Last edited by 73StreetRace; Apr 6, 2009 at 05:51 AM.
Melting Slicks
Joined: Jul 2005
Posts: 2,740
Likes: 3
From: Aurora Ontario
81 350/350 basic bolt ons. edelbrock performer and carb, replaced computer distr with HEI.
Car seams to run good until I hit 3/4 throttle and then she starts popping and backfires. keep the rpms up she runs ok until 3/4.played with the nidles and springs in carb and got it that far. unpluged all vacume ports looking for a leak and found nothing still drives like took carb apart and blew out jets.
with out throwing parts at it..... anybody
Car seams to run good until I hit 3/4 throttle and then she starts popping and backfires. keep the rpms up she runs ok until 3/4.played with the nidles and springs in carb and got it that far. unpluged all vacume ports looking for a leak and found nothing still drives like
took carb apart and blew out jets.with out throwing parts at it..... anybody
Jim
Thread Starter
Burning Brakes
Joined: Dec 2004
Posts: 825
Likes: 0
From: Hanging out back with the dark horses. Wa.
I yarded all the computer stuff off and put in a summit HEI dist. all parts are new out the box. triple checked firing order, and will pull the plugs today. I'm having to use short plugs because I used blockhugger headers so I could run sidepipes.
I'm building another 81 with a 4spd now and the will be using long tube and performer RPM set up on that one. It's just one of those things that is driving me
I'm building another 81 with a 4spd now and the will be using long tube and performer RPM set up on that one. It's just one of those things that is driving me
Thread Starter
Burning Brakes
Joined: Dec 2004
Posts: 825
Likes: 0
From: Hanging out back with the dark horses. Wa.
Thus far...... number 6 plug was fowled. swaped out distr, plug wires, compression check was 120 like the others. added a edelbrock adapter plate between the performer intake and the edelbrock 1406 carb. she ran good about halfway down the street then started popping again. Fuel pressure seems to be ok at idle.
Motor is new and the heades have been worked twice now. I give up..... Thinking maybe intake valve stuck open or to tight? stock heads with the stock oem cam in it still.
Motor is new and the heades have been worked twice now. I give up..... Thinking maybe intake valve stuck open or to tight? stock heads with the stock oem cam in it still.
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Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
Backfiring is a symptom of a lean condition, never rich. It can also be caused by a weak spark, or inadequate timing.I think he should check timing first, and fuel level in the fuel bowls too.
Drifting
Joined: Apr 2007
Posts: 1,970
Likes: 45
From: Cameron Park CA
This page shows how the gauge reads with different engne problems. http://www.secondchancegarage.com/public/186.cfm
Melting Slicks
Joined: May 2000
Posts: 2,305
Likes: 5
From: Orlando,Fl,USA
I had a problem with the ignition module at the base of the HEI' wire was shorting out due to break. I couldn't get the engine to rev past 4000 rpm. It would start to breakup like crazy at that rpm. It ran fine at low rpm's.. Put new one in and problem was solved.
Burning Brakes
Joined: Jul 2008
Posts: 923
Likes: 37
From: Green Bay Wisconsin
