Engine Exhaust - Popping & Crackling

[bigboi]

Ok... This may be a dumb question... But I want to get some feedback from you guys.

I have a 496BB w/ Holley Double Pumper 750 Carb. Timing is 18 Initial / 36 Full / Kicks in full at 3,000. Jets are 70F / 84R. Idle mixtures are 2 turns out on all 4 corners.

Exhaust is 3" pipe to 1 chamber flowmasters dumped before rear axle.

I have A LOT of popping & crackling when I let off the gas and the engine RPM winds down. I had the idle mixture screws about 2 1/2 turns out each and the popping & crackling was less. 2 1/2 turns was too rich tho so I turned them down to 2 turns out each corner - Now the popping & crackling is a lot.

Is this bad? Is this going to cause problems? From my understanding it's either cold air coming back into the exhaust and/or not enough fuel getting mixed in causing the popping sound.

Thoughts?

Brian

[Mommy Wagon]

Idle mixtrue screws do nothing once you are out of that circut. Fiddling with them in search of your problem won't assist, as your problem does not occur at idle, correct?

How old is your engine?

Is it a hydraulic flat tappet cam?

Was your cam degreed when it was installed?

When was the last time you went through your distributor?

How old are your ignition wires?

[chicane67]

The popping and crackling have to do with your mufflers and their chambered design itself. And... especially with the fact that you are running the mufflers with mere turn-downs... and not going up and over the axle and out the back... also plays in on this.

As you speculated... it does have a lot to do with colder air getting back into the mufflers. The mufflers chambered design creates a fair amount of negative pressure when it is cancelling a wave inside the muffler... and with the short length of the turn downs... it is pulling in fresh O2. It too can also be influenced by the mixture... somewhat. Not a lot... but in the proper conditions, the incomming fresh air will re-ignite inside the muffler... leading to the popping. When you get to just under stichometric... ie, on the lean side... it will become more predominant.

It used to be very predominant with the early Flowmaster mufflers. When I did the system on my 67 back in 1989 with the original 17 inch cased 3 chambers... and the 'true x-pipe' design (not the overly simplified design that most are use to seeing now-a-days)... even with the system going up and over and out the back... it sounded like someone lit off a brick of firecrackers in the mufflers. In that peticular system... it sounded cool... and worked very well.

Will this damage anything ? No... not really. But... be sure that you inspect your mufflers over time to make sure that you are not cracking the welds on the muffler. No... this isnt why the mufflers crack... the newer Flowmasters ('94 and up) just crack at the welds I believe from the thinner material and more likely from not being supported well enough under the chassis.

[Vegas69]

Idle mixtrue screws do nothing once you are out of that circut. Fiddling with them in search of your problem won't assist, as your problem does not occur at idle, correct?

How old is your engine?

Is it a hydraulic flat tappet cam?

Was your cam degreed when it was installed?

When was the last time you went through your distributor?

How old are your ignition wires?

Have to disagree. A holley does use the idle circuit with the primary circuit. by leaning out his idle mixture screws he is slightly leaning out his mixture especially under decel when the throttle plates are closed. My custom motorcycle with 3 and 4 foot pipes does the exact same thing and the idle mixture absolutely effects it as well. The S&S super also feeds the engine in the primary circuit. Moral of the story is install some tail pipes and make sure your jetting is spot on or live with it.

[Mommy Wagon]

The ammount of fuel your idle circut provides is minimal. It does not effect throttle once your circut has transitioned.

We can agreee to dissagree, but if you find that using your idle screws helps with the use of other circuts in throttled operation above idle then you should look at other circuts as the culprit, and not assume that adjusting your idle mixture should help at 1000+ rpm.

You may also get popping with a burnt valve.

You may also get popping with a collapsed lifter.

Ruling out everything except mufler design can be detrimental, I am not ruling this out though, as this does occur. The mention of how long the popping has been happening has not been mentioned, was it fine before the exhaust install? Has it always happend? Was anything else changed that could accociate this problem?

I had a similar issue when I had purchased a vehicle once, the owned had relayed that the engine was tired and just wanted to get rid of it. All that was required was to back the timing off. The facory markings were off, and had WAY too much advance. Cylinder presseure was good, bleed down was good, oil pressure was good, just to far advanced.

[chicane67]

Well.. as I see it...

The adjusting needle simply controls the amount of fuel emulsion which is allowed to reach the port. Contrary to popular belief the mixture needle does not control idle circuit air/fuel ratio. Although this may simply be a matter of wording, there is a difference between the mixture in the idle circuit itself and the mixture that ultimately reaches the intake manifold. The idle circuit calibration is determined by the diameters of the idle feed restrictions and the idle air bleeds. A feed restriction is nothing more than a metering jet for the idle system and the air bleed serves as an airflow regulating device. As the air/fuel emulsion exits the idle port it is mixed with small amounts of intake air that flows past the throttle plates. Varying the throttle plate opening (with the idle speed screw) increases or decreases both the amount of air that is allowed to flow into the intake manifold and the amount of pre-mixed air/fuel emulsion discharged at the idle port. Since these two idle mixture components are both effected by the degree of throttle opening, the ratio of intake air to idle mixture air/fuel is not drastically altered by changes in idle speed. Turning the idle mixture screw varies the volume of air/fuel emulsion discharged into the manifold, not the ratio of air/fuel in the emulsion. Therefore, when a satisfactory idle cannot be achieved by tweaking the mixture screws richer... or leaner... it can be assumed that the carb's idle calibration is not within the range demanded by the particular engine combination.

This... would then become an adjustment in the idle well restrictors... which have no real effect in the transition characteristics between the idle circuit, into the accelerator circuit... and then into the main circuit. To solve the issue that is speculated above... one would need to make changes to the idle air restrictions in the metering block... and to the air bleeds at the mouth of the venturi. (All hail the HP series carbs.)


The burnt valve possibility is allowable. But... the frequency of it actually popping, I am pretty sure, would be less than what is actually occurring. However, to verify this... if you are running a PCV valve... would be to cover the PCV valve with your thumb with the engine running at idle. If this increase's the frequency of the popping occurrence, while at idle... vice that of running at speed and decelerating as described... I would definately say it's a burnt valve.

The collapsed lifter possibility is also allowable. But... it would give you audible signs before you even put the car in gear to drive forward.

I only approached this from the obvious. A one chamber Flowmaster and turn-downs. This is all too common with the specific set-up. Most of all timing issues are more prevalent on acceleration. Whereas this example was pretty much isolated to deceleration.

Brian... I would get a vacuum gage on the manifold and tune each idle mixture screw to maximum idle vacuum first. This will establish a much better baseline condition in the carb by itself without being influenced by the exhaust system. Then take it for a drive... and get back to us. Then make any adjustments to any other circuit from there.

[bigboi]

How old is your engine?
New - Only about 1,500 street driven miles on it.

Is it a hydraulic flat tappet cam?
Hydraulic Roller

Was your cam degreed when it was installed?
Yes, Engine Builder degreed it in

When was the last time you went through your distributor?
Just recently. Replaced advance bushings/springs

How old are your ignition wires?
New - Big Fat Taylor Wires

I have not logged on the board for some time... I have just been driving the car like it is - popping and crackling.

I have to say... I am truly impressed with everyone's knowledge and input!

The engine is new and probably has 1,500 miles on it. I went through the distributor and tuned the carb the best I could (shaker455 helped me a lot). Timing is 18 at initial and 36 full (full kicks in at 3,000 or 3,200).

I don't have the popping and crackling unless I let off the gas and the car is decelerating. I honestly believe it's the muffler design pulling the cold air back in - I just wanted to validate my thought by asking you guys.

I don't have the popping/crackling during acceleration... Now, once in a while I do have the occasional "ping" when I am cruising steady at 1,500 - 2,500rpm. I have a slight feeling this is probably due to timing (pre ignition is what I think it's called) - Could this be true?

Thanks again for all the time you have taken to reply to my post! It's appreciated!

Brian

[Vegas69]

If you have a ping under partial throttle you may need to change up your distributor springs to bring the advance in a little later. What springs are you running?

[bigboi]

If you have a ping under partial throttle you may need to change up your distributor springs to bring the advance in a little later. What springs are you running?

I am using 2 light blue springs w/ black bushing. I have my initial (at idle) set at 18. With the bushing/spring combo I have it should put me at 36 @ 2,500 RPM.

I could go 1 Silver (Heavy) and 1 Silver (Light) w/ the Black Bushing. This would put me at 36 full advance at 3,000 RPM.

Thoughts?

[Vegas69]

Actually just getting ready to play with it this afternoon. Black bushing is going in for more initial timing. I am going to see which springs are in the distributor and go from there. Have been recommended a silver and blue light but seems a little radical on 91 octane gas. My main goal is as much initial timing as possible for a better signal to the carb.

[MonzaRacer]

thats raw fuel lighting in exhaust, try getting air bleeds dialed in you may have some minor fuel curve issues. Had several carbs I had to retune to decrease this problem

[bigboi]

thats raw fuel lighting in exhaust, try getting air bleeds dialed in you may have some minor fuel curve issues. Had several carbs I had to retune to decrease this problem

Yeh I kept thinking about this problem I am having... And I think it might be raw fuel igniting... I know I am running rich as hell because my clothing stinks to high hell when I am done driving the car around.

I need to get the mixture dialed in right on this thing...

Brian