backfire
- Thread starter jerdan
- Start date
Tinker
Well-known member
I'm noooo expert. Pre and post fires are usually based on fuel. Too much or too little from the carburetor. I'd guess since you changed carburetors you may have excessive fuel igniting in the exhaust.
A popping exhaust could be a few things like valves etc. But being you just changed out a carburetor and it ran great before?
A popping exhaust could be a few things like valves etc. But being you just changed out a carburetor and it ran great before?
drolston
Well-known member
Just to be clear, when you say "backfire", you mean in the exhaust, not popping back through the carb.
In either case, basic flathead diagnostics for such behavior starts with a vacuum gauge, What are the indications at idle? Next would be to drive it pretty hard for a few miles and then shut it down in the garage without idling very long. When it cools down enough to work on, pull the plugs and look for ones that might be wet or sooty. Then with the throttle and choke blocked open, run a compression test on each cylinder.
Report what you find. and we can better advise where to look for the problem.
In either case, basic flathead diagnostics for such behavior starts with a vacuum gauge, What are the indications at idle? Next would be to drive it pretty hard for a few miles and then shut it down in the garage without idling very long. When it cools down enough to work on, pull the plugs and look for ones that might be wet or sooty. Then with the throttle and choke blocked open, run a compression test on each cylinder.
Report what you find. and we can better advise where to look for the problem.
Flathead Fever
Well-known member
I know that popping back through the carb can be caused by a lean condition, burned intake valve or a timing issue. During the carburetor years, that use to be one of the questions on the ASE engine certification tests. “What would cause an engine to spit back through the carb”? The only correct choice was a lean condition.
I do not exactly understand what is igniting that lean mixture in the intake? The only ignition source would be from an intake valve opening. Maybe the split second there is valve overlap it would allow the hot gases leaving the cylinder through the exhaust valve to ignite the lean mixture coming in through the intake valve.
It does mention below that spark plug firing at the wrong time, when the intake valve is open, would cause the mixture in the intake to ignite. That could happen if you ran your plug wires to close together. The magnetic field from one firing plug wire can induct a current into a plug wire alongside of it and cause a spark in that cylinder while the intake valve is open. I always thought the way Ford bundled the plug wires tightly together, inside a metal tube was a bad idea as far as cross firing is concerned.
I found this video where they tried to cause crossfire between two plug wires, and it didn't work. Year's ago I did see a video where it did cause a spark in the second wire. I guess depends on the coil output and the type of plug wires. What is interesting, is how this video shows the magnetic field created around the plug wires and why you should keep any wiring away from the plug wires.
https://www.youtube.com/watch?v=v_bCX7FrUrI
If you had two plug wires mixed up and one fired a cylinder on its intake stroke that would cause the mixture in the intake to ignite. Check your firing order.
I borrowed this from another site.
What causes popping through carb?
Fuel To Air Mixture Is Too Lean
Too much air and not enough fuel causes backfires to occur in the intake manifold. The exploding mixture then vents through the carburetor. Improper carburetor adjustments or vacuum leaks can cause this condition.
Also, what causes backfire out of carb?
"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up.
Likewise, what causes engine popping? If your engine is getting more fuel than it needs, a rich fuel to air ratio is the result. When your car has leftover fuel in the exhaust and the cylinders, that fuel explosively burns and creates a loud popping sound. Specifically, here we mean delayed timing, which causes the backfire.
Secondly, what causes engine backfire on acceleration?
Engine backfires can be produced by a vacuum leak, bad timing, problems in the ignition system, a faulty sensor, an exhaust leak, or some other system fault. The backfire is produced when unburned fuel ignites inside the intake or exhaust manifold instead of a cylinder.
Is backfiring lean or rich?
Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.
I do not exactly understand what is igniting that lean mixture in the intake? The only ignition source would be from an intake valve opening. Maybe the split second there is valve overlap it would allow the hot gases leaving the cylinder through the exhaust valve to ignite the lean mixture coming in through the intake valve.
It does mention below that spark plug firing at the wrong time, when the intake valve is open, would cause the mixture in the intake to ignite. That could happen if you ran your plug wires to close together. The magnetic field from one firing plug wire can induct a current into a plug wire alongside of it and cause a spark in that cylinder while the intake valve is open. I always thought the way Ford bundled the plug wires tightly together, inside a metal tube was a bad idea as far as cross firing is concerned.
I found this video where they tried to cause crossfire between two plug wires, and it didn't work. Year's ago I did see a video where it did cause a spark in the second wire. I guess depends on the coil output and the type of plug wires. What is interesting, is how this video shows the magnetic field created around the plug wires and why you should keep any wiring away from the plug wires.
https://www.youtube.com/watch?v=v_bCX7FrUrI
If you had two plug wires mixed up and one fired a cylinder on its intake stroke that would cause the mixture in the intake to ignite. Check your firing order.
I borrowed this from another site.
What causes popping through carb?
Fuel To Air Mixture Is Too Lean
Too much air and not enough fuel causes backfires to occur in the intake manifold. The exploding mixture then vents through the carburetor. Improper carburetor adjustments or vacuum leaks can cause this condition.
Also, what causes backfire out of carb?
"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up.
Likewise, what causes engine popping? If your engine is getting more fuel than it needs, a rich fuel to air ratio is the result. When your car has leftover fuel in the exhaust and the cylinders, that fuel explosively burns and creates a loud popping sound. Specifically, here we mean delayed timing, which causes the backfire.
Secondly, what causes engine backfire on acceleration?
Engine backfires can be produced by a vacuum leak, bad timing, problems in the ignition system, a faulty sensor, an exhaust leak, or some other system fault. The backfire is produced when unburned fuel ignites inside the intake or exhaust manifold instead of a cylinder.
Is backfiring lean or rich?
Not only can a rich air/fuel ratio cause a backfire, a mixture that doesn't have enough gasoline can cause a backfire, too. A "lean" mixture is one that doesn't have enough fuel, and too much air.
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FlatheadTed
Well-known member
Quote I replaced the original carb,
It sound like it went OK with org carb ,he just wants to know what jets to use on the ecg ,I dont know but what ever # on there now go bigger
It sound like it went OK with org carb ,he just wants to know what jets to use on the ecg ,I dont know but what ever # on there now go bigger
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scicala
Well-known member
It would be nice to know from Jerdan if it's a carb or and exhaust backfire. Two different causes.
Don't change the main jet sizes. Carbs are calibrated to give a good mixture ratio based on the amoint of air going through them. No such thing as a higher air flow carb being too rich because the engine is smaller than the carb was calibrated for.
If it's a carb backfire, I would check out the 1 1/16" carb you put on. Mainly the accelerator pump circuit. Sometimes there is slop in the pump link, where the throttle can open slightly without moving the pump rod, causing a delay. Or possibly something else wrong in the pump circuit.
Also check everything in the carb if you have it off.
Sal
Don't change the main jet sizes. Carbs are calibrated to give a good mixture ratio based on the amoint of air going through them. No such thing as a higher air flow carb being too rich because the engine is smaller than the carb was calibrated for.
If it's a carb backfire, I would check out the 1 1/16" carb you put on. Mainly the accelerator pump circuit. Sometimes there is slop in the pump link, where the throttle can open slightly without moving the pump rod, causing a delay. Or possibly something else wrong in the pump circuit.
Also check everything in the carb if you have it off.
Sal