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Popping/backfire through exhaust on loaded downshift

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melsman

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Discussion starter · #1 ·
Hello All,

I've done a lot of searching on this forum and others on this topic, and I know there have been a lot of answers, but I'm not sure I have the same issue. Or at least, others that I've read have not described the same symptoms.

I have a '68 Camaro with a pre-81 GM Goodwrench crate engine (350ci - made in Mexico type) with Hooker headers, Flowmaster exhaust (2.5"), an Edelbrock 1806 Carb, Mallory HEI (nothing fancy), mild cam, and Accel wires. Timing is set at 6* BTDC and have a strong spark. No discernible exhaust leaks. Muncie M-21 tranny.

I have strong, smooth acceleration under load, and smooth no-load deceleration (clutch in, load off engine - decelerates quietly and smoothly). However, if I downshift and decelerate under load (engine braking), I get all kinds of popping and backfiring through the exhaust.

This is a recent occurrence, and not associated with any engine work. I'm guessing that the solution is probably a simple one that's right under my nose, but I'm kinda at my wits end.

Any ideas?

Melsman
 
#2 ·
I don't think you have enough initial timing - 12 degrees is a good starting point. Move your distributor counterclockwise to advance the timing.

Do you have the vacuum advance hooked up ? It should be hooked up to manifold vacuum.

See if you can do this without the engine pinging and go from there.
 
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Discussion starter · #3 ·
Thanks for the input, Beth.

Yes, I have the vacuum connected - until I time it, at which point I plug the vacuum line and set the timing at idle.

Speaking of that... are you talking about 12* ATDC? You said to "advance" the timing from the current 6* BTDC. That doesn't seem correct to me, but I'm no expert.

Melsman
 
#4 · (Edited)
Agree you need more Initial Advance also if you have pin holes before the mufflers the unburnt fuel in the pipes, with the throttle closed, will super heat from the inrush of air and ignite - a good way to blow the mufflers too ;o)

Sometimes using a 1 quart Vacuum Reservoir "T'd" into the Vacuum Advance hose can help eliminate this condition.

Stock Initial is approx 8 degrees BTDC. Generally a SBC will take 12 to 14 degrees BTDC Initial with some modifications - Every engine is different. When setting Initial Advance (which is where an engine will start and idle by turning the distributer) ensure the Carb is setup with throttle plates in the idle circuit, the Secondary Plates set per Mfgr's Specs and Idle Air Screws approx 1 1/8th turn off seat. Disconnect all Vacuum sources such as Vacuum Advance, Tranny Modulator Sx, Power Brakes and PCV - The last thing you want is a Vacuum leak when establishing Initial Advance.

Once you have establish the initial advance write it down for future reference and take a Vacuum Reading. The Power Valve and Distributer Vacuum Pod should have approx 1.5 to 2 Hg's below the engine's Vacuum at Idle. If you note the Timing with Vacuum Advance haunting on the Damper it probably means you have Too High of a Vacuum Pod for the Engines Vacuum at Idle or you have a Vacuum leak - This is why you plug all Vacuum Sources to ensure there are no Vacuum leaks when establishing Initial Timing for a particular Engine.

Now connect the Vacuum Advance and slightly adjust the Throttle Screw and Idle Air Screws to compensate for the Vacuum Advance. The Timing at idle should be approx 10 or 12 more (say 24 degrees) at idle with full vacuum advance and the idle air screws set approx 3/4 turn off seat.

Drag Racers are not that concerned about Initial Timing and concentrate on the Total and getting it all in at a specific RPM but for the Street, Initial Timing is a MUST.

I also recommend that you use Manifold Vacuum to the Distributor Advance Pod and Plug the Carburetors Port Vacuum source. Port Vacuum is for SMOG Engines.
 
Discussion starter · #5 ·
Hi Z15CAM,

Thanks for all of the info. I need to do more reading and research to completely understand everything you wrote. Thanks for taking the time!

And Beth... it finally dawned on me what you were trying to tell me... Sometimes my brain ain't so quick...

Melsman
 
#6 ·
Intial will not make any difference to the backfire...its what is in the curve at the rpms you are doing and what is in the VA...as the engine is on full vaccuum.
Intial means squat other than how well it idles and NOx emissions
upping thwe intial could over advance the normal running causing detonation etc
What is the total?
intial +cent and intial +cent +VA

Backfire is normally a lean mixture or sevier scavenging.
 
#8 ·
Sounds like idle mix is a bit lean. Once the throttle closes, that is what is supplying the fuel. A bit too lean of a mix and you get incomplete burn so the unburned fuel gets into the hot pipes and pop pop pop.

Used to happen all the time on a carbed bike whenever changing out to a new set of pipes even after installing new jets. Another half turn out on the idle mix screws used to cure it. I was always too lazy to use the four dial synchronizer.
 
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#9 ·
steptoe said:
intial will not make any difference to the backfire...its what is in the curve at the rpms you are doing and what is in the va...as the engine is on full vaccuum.
Intial means squat other than how well it idles and nox emissions
upping thwe intial could over advance the normal running causing detonation etc
what is the total?
Intial +cent and intial +cent +va

backfire is normally a lean mixture or sevier scavenging.
Click to expand...
straight-line-69 said:
vacuum leak
Click to expand...
bingo!!!!!

Vacuum leak!
 
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Discussion starter · #10 ·
Thanks for all of the responses, folks! I appreciate it.

I played with the Initial Advance, and found that the engine liked running about 18* BTDC(!) - good idle, good start, and no pinging. And no more popping on loaded deceleration.

Melsman
 
#12 ·
I'm lost as well.
 
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#13 ·
I don't see how it is physically possible to get a lean pop out of the exhaust from the lack of fuel? that would mean what? The cylinder is igniting while the exhaust valve is open? You would def. hear that through the motor as detonation. I am confident that it is un burned fuel going through a hot exhaust, and the exhaust is probably a little too big and open for the power you are at. If it's a lean condition, why don't stock trucks with ******* pipes do it on de cel? Assuming the truck is computer controlled here, as an example.
 
#14 ·
I don't see how it is physically possible to get a lean pop out of the exhaust from the lack of fuel?
Click to expand...
That what everyone says when they 1st come across it...detailed explantion in older post.
 
#15 ·
Steiner said:
Sounds like idle mix is a bit lean. Once the throttle closes, that is what is supplying the fuel. A bit too lean of a mix and you get incomplete burn so the unburned fuel gets into the hot pipes and pop pop pop.
Click to expand...
So when the throttle closes, the last bit of the secondary circuit is getting an incomplete burn from going back to the idle circuit...? I'm still confused a tad.
 
#16 · (Edited)
ace's68 said:
So when the throttle closes, the last bit of the secondary circuit is getting an incomplete burn from going back to the idle circuit...? I'm still confused a tad.
Click to expand...

An excessively lean mixture can burn incompletely because it takes longer to burn off or it can misfire. So, unburned fuel can make it into the exhaust. Since the idle mix is what it providing fuel during a throttle closed decel, a richer idle mix is usually the fix. A lean misfire will actually show up rich on an AFR gauge.

Here is a very good explanation of it from the Mikuni. I haven't had a carbed bike for several years but when I did, screwing out the idle mix screws always fixed it with new pipes. Just jetting was never enough. Also with my EFI system on the Camaro, I had to turn off the decel fuel shutoff to keep it from popping occassionally. If I had computer controlled timing it would help.
http://www.mikuni.com/tg_backfires_in_exhaust.html
 
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#17 ·
Steiners onto it...
Seen the old Charlie Chaplin movies...the old cars pop pop pop down the road (even if subtiles lol) that because those old carbs where notorious for wearing out the butterfly bushes...espec vintage english because of the Gov restriction of the time as to bore to stroke ratios.
 
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Steptoe
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