Need Tuning Help - Popping & Crackling Noise

[bigboi]

Well... I need some help here guys... I have had this issue for a while and just been putting it off... It surely reminds me of it when I drive the car tho... :)

I have a bunch of popping and crackling out of the exhaust... It's most prevalent when letting off the gas, however it does popple and crack a bit when cruising anywhere in the RPM range - whether it be 2,000 - 3,300 RPM.

I have a big block 496 w/ a AED Holley 750 Mechanical Secondaries. I have an MSD 6AL and a MSL distributor. I have the weights in the distributor set at 18 initial and 36 full @ 3,000 RPM.

I had the Holley jetted at 78 front / 84 rear. I thought maybe it was jetted too much in the front so I dropped the jets to 74 - I still have the problem and with the front jets at 74 the car seems to stumble when you romp on the gas.

I have a gut feeling the popping and crackling is raw gas igniting in the pipes (someone else on this board suggested that as well a while back).

What should I do to remedy this problem? What steps should I take and in what order?

Any suggestions are more than appreciated!

Brian

[Shaker455]

Hey Brian,
Sounds like your right about raw gas in your exhaust.

I suggest you take the carb apart and inspect & clean as something may be out of wack inside like a bad powervalve or leaking gasket allowing fuel to leak inside.

how many turns out is your idle mix screws?

Are you running a 10 micron filter right before the carb?
Jeff

[Carbo]

I had this same issue as well and I advanced my timing from 2* to 8* and it cured the problem. But I see you are at 18* so I don't think you'll be able to advance any more than where you're now.

[bigboi]

Jeff,

It has been doing this since me and you talked on the phone about tuning my car... Just to recap what me and you decided on - I completely took apart the carb, cleaned it with super clean, blew passages out with compressor, replaced power valve with size 4.5, jetted 78/84, replaced w/ Holley blue gaskets, 1.5 out on each of the 4 corners, butterflys adjusted evenly for idle (front and back), timing at 18/36.

It's always occurs on deceleration... But when I am cruising steady anywhere between 1,500-3,000 I can hear it slightly "blirp" in the exhaust pipes sometimes as well - It's not as loud as the deceleration popping... But it's still occuring just on a smaller level.

I am running Tank -> Tank Filter -> Holley HP150 -> 10 Micro FRAM Filter -> Carb. There are no filters up at the carb, but I "should be" ok with the 10 micron big Fram filter after the Holley pump.

Exhaust is Dougs long tubes -> 3" pipe -> 1 Chamber Flowmaster Race Mufflers dumped down before the rearend.

What is weird as well... I don't have an A/F gauge but being at 1.5 out on each corner I should be running fairly lean... Don't you think? Sometimes when the car is idling it smells rich as a pig and sometimes it doesn't (weird?).

What should I do next? It sounds like a gun popping off when I am slowing down at the stop light. I am sure people get startled as hell. LOL!

I can take apart the carb again and reclean everything like you said, but this problem was occurring after the last complete teardown/clean.

Brian

[Shaker455]

I don't care for 78/84 jetting so how are your plugs looking?

Can you email me a pic?

[bigboi]

Here is plug #2. I lowered the jetting to 74/80 and popping still occurred and it ran horrible.

Thoughts?

https://static1.pt-content.com/images/noimg.gif

[fishtail8]

One of the first things I'd have tried was screwing in a new set of plugs. For the price of em, and if they haven't been done in awhile, it's a good start. How old and in what condition are the wires in? Have you taken an ohm meter to them yet? Is there someone close by with a known good carb you can swap for an hour to see if it works?

Has it always been like this or can you remember if it started after a change of something? Time to do some thinking and small parts swappin...

[bigboi]

Yeh I grabbed a set of Autolite 3923s today at the parts store... Going to put them in tonight.

The wires are 8 months old - They are the fat Taylor wires. It has been like this for a while... I have always had issues with the carbs I have been using. I was using a 650 but it has an air leak on the base plate or something because the idle was always erratic... So I bought this AED Holley used from a guy down at the local speed shop. For the life of me I can't remember if I ever got it tuned right.

:machine: Carbs can be frustrating to say the least...

Brian

[Shaker455]

Brian,
None of your wires are laying on anything right??

I had a guy with a 572 call me last year to look the carb over and it turned out the headers burnt thru from header heat.

[bigboi]

Well... I got rid of the popping... I swapped the plugs w/ new plugs, gapped them at 30, and jetted the carb 78/80. I also dialed in the idle mixture screws a little leaner - I am at 1 1/4 out now on all 4 corners.

I do have one issue... And maybe I am just being picky. Between 1,000 - 2,200 RPM the throttle response/crispness is a little shaky and not "clean" and "tight". It feels like it stumbles a bit in that RPM range. Once I get over 2,200 tho it pulls great and sounds/feels strong.

Do you know what could be causing this? Am I just going to have to live with the stumble between 1,000 - 2,200 RPM?

Brian

[Shaker455]

It may be your squiter size

[68Formula]

http://www.stockcarracing.com/techarticles/scrp_0601_holley_accelerator_pump_adjustment/index.html

[bigboi]

Thanks for the advice and article guys... I read through the article and what I am experiencing is a slight lag/stumble when the throttle is cracked open quickly when crusing in the 1,000 - 2,200 range.

I am going to play with the pump shot volume by adjusting the bolt. Maybe I have too much fuel being delivered on the quick throttle opening causing a rich mixture thus causing it to lag/stumble.

If that doesn't solve my problem I think I might need to look at squirter size for the front. I would want to go down in size - correct? Less fuel being delivered on quick throttle opening is what I am looking for I think.

I am almost there... Just need to fix the lag/stumble. :)

Brian

[MonzaRacer]

Open your gap to .045" , oh and personally I have never had luck with autolite plugs ,they are stock for nothing. if its GM I prefer AC Delco, Motorcraft or Denso Iridiums.
Champions I rarely have good luck with in ANYTHING, heck my 8hp lawn mower has an AC Delco in it.
Motorcraft and ACs will interchange all day.
i am even hesitant in using champions in Chrysler and those are stock for them.
a nd personally I dislike street driving without vacuum advance, I can tell you the curve you have will give great pull in 1/8th or 1/4 but heck my 402 bbc ran around 58-60 degrees total at cruise with vacuum advance.
Heck that big combustion chamber could handle it unless you have 13 to 1 domes.
Good luck.
Lee Abel
AFTERMARKET PERFORMANCE