Popping in Exhaust

[lukecp]

While my truck is running, from 1000-3000 RPM, i hear a small "pop" coming out the exhaust. It souds pretty normarl, just vroooooooom-pop-vrooooooooom. It has done this before, but giving it a tune up stopped the noise. I gave it a tuneup about 3,000 miles ago, and now its doing it again. I posted about this before, and someone said to check for an exhaust leak. I can't find one anywhere, and i have replaced both the flange and collector gaskets on my headers. The distribuotr cap and rotor are both fine (i inspected them yesterday), and the plug wires look fine. I use Accell Super Stock plug wires on the truck. I had problems with one wire touching the headers, getting burned and making it miss. I just replaced the wire, and that wasn't the problem. I'm going nuts tring to figure this out. I'm thinking that maybe it is the plug wires. Any ideas on what would cause this?

[dion]

I had a simular problem, I thought it was a loose baffle but my muffler guy said it was a miss in the engine, carb, plugs and wires were all good. Turned out to be a loose header bolt.

[mikep]

running a little lean will often cause the popping your'e experencing. All your ignition might be up to snuff but the mixture wont be rich enough to fire the cylinder occasionally. Could be a slightly plugged jet in the carb if you have ruled out exhaust leaks. Other causes could be over advanced timing and spark tracking inside the distributor cap. I'd begin by really inspecting for exhaust leaks though.

[toms68cst]

Double-check your spark-plug wires. On two occasions now I've had that popping problem. I even had a brand new wire that turned up bad. I do a quick check on mine by seeing if I can get a timing light to fire at each spark plug.

[lukecp]

Well, i went and put the timing light on each of the cylinders, and all 8 caused it to flash. I dropped the timing to 8 degrees advanced, instead of the 11 i was running and it still does it. The noise disappears above 2200 rpm, unless i rev the engine and let off the gas, and then it pops as it is winding down. Also, it pops pretty loud if i am going 1000 rpm, and then go up to 1500 or so. Is there any way to check for exahust leaks besides simply liseting for them?

[crazy longhorn]

Take a lenght of heater hose, & put it up to your ear,& just run the other end around the suspected problem areas. If you have a leak, you will find it. good luck.......crazy al (aka 69 longhorn)

[mikep]

Does it only pop when accelerating or upon deceleration? what type carburetor? Have you done a compression test on all the cylinders?

[lukecp]

It dosen't really pop when accelerating, just crusing at low speeds around town (below 2200rpm), and when i let off the throttle at any speed it will pop as the motor is winding down. The carb is a Edelbrock 1406. The jets are stock, but i have changed the metering rods to be 1 stage rich in the power mode. I haven't done a compression test yet, the motor is still pretty new (13xxx miles on rebuild), but who knows. If i still cant find out what is doing it, i will check the compression.

[mikep]

Check for vacuum leaks as well. manifold bolts, carb bolts, vacuum lines and the like. Deceleration is a hi vacuum time and any vacuum leak will really show itself at that time as a lean condition and subsequent popping in the exhaust.

[lukecp]

Well, me and a couple friends found out what the problem was on my truck. Turned out that the carb bolts were kinda loose, and it caused a small vaccum leak around the base gasket. I yanked off the carb, inspected the gasket (looked OK, but not great...it's proalby gonna get replaced), and put the carb back on. Now, it dosen't pop. I also re-adjusted the idle so it isn't so rough anymore. Thanks for the help!