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popping out the exhaust

95486 Views 13 Replies 9 Participants Last post by  Guy Hiltz
got an SBC 350. speed demon 650 carb. all brand new wires, coil, dist. its timed correctly. it pops out the exhaust throughout the rpm range. at idle, at slight acceleration, at full throttle (if it doesnt crap out when i step on it which it usually does) isnt this a sign of a misfire rather than a carb issue if it pops all the time? i turned the lights off in the garage with it running and saw a plug arching on the header. so i wrapped it in lineman's tape and it went away but the pop still persists. part of the boot had been melted. and i dont see any other arcs anywhere. what else could it be? the engine is very responsive on takeoff (doesnt hesistate) but still pops and isnt smooth like its suppose to be
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could be a burnt valve..... also could be a vacume leak as well i hada small manifold vacume leak one time on my '77 350 and it made it pop throughout the rpm range and idle roughly....
the motor has less than 2000 miles on it. total rebuild, so it cant be a burnt valve.
another senario could be if you ran it with open headers right after you built the engine in cold weather...that could have bent a valve and is not seating properly but thats unlikely none the less its still possible if you did run it uncapped in cold weather
Doc here, :pimp:

First thing I'd look for, is the dreaded 5 and 7 plug wire switch..

That will cause bad idle and popping under acceleration..

Easy enough to screw up for Pros and Novices alike..because 5 and 7 are the ONLY TWO plug wires next to one another at BOTH ends..the head and the Dizzy..So hand over hand them to be sure..

The other offender might be a slightly too tight exhaust might want to check that..try a Vacuum gauge , it should be 17 to 21 In Hg on the gauge,AND steady (no rapid fluctuations)

Low Vacuum might be a bad carb baseplate gasket..use some carb cleaner to check , Where it speeds gotta leak..make sure the air cleaner is on, or you will get false triggers down the throat from overspray..

If it was just on decel, I'd say header leaks but you say it pops in both I'd say no to that but a quick look won't hurt either.

Doc :pimp:
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I am not sure I understand exactly when the eng is popping out the exhaust. Is it under heavy load or just under normal driving. I am assuming it is popping all the time, an exhaust backfire is usually ign related. Are you running an internal coil (hei) or external. I would check your coil wires and make sure you have a good 12 volts (if running an HEI or 9 volts if running a std coil after the balast resistor) and by some slim chance have them wired backwards. Also If you are running an external coil get another coil wire and try it again.
Other possibilities are if an intake manifold bolt was a little to long and bent a push rod when it was tightened. (the bolts by the heat crossover) Or the valves could possibly be overtightened. One time i did have a sbc popping out the exhaust because a lobe had worn off the cam (hyd lifter motor)
its a petronix flamethrower II ignition...just bought it. msd 8.5mm wires.

engine puts out approx 425hp so my vacuum is in the 14-15 range and it's steady. the engine pops under normal driving and under heavy load. i had a motor pop from a worn cam as well but everything is new on this one. the car sat for a while and never did this, i started the car at least twice a week for maybe 15 minutes just to keep the battery some-what charged. one day i start it and it just starts popping randomly. thats the strangest thing about it. thats when i saw the arc in the wires and thought i'd found the problem but it didnt seem to change anything even though the arc went away after i put lineman's tape on it.
It could be that your plug wires are ruined,or maybe not making real good contact.If they are really close to the headers they could have burnt the boot itself and this could be causing your problem.Boot protectors are a good investment to protect your wires from header burn.
As already asked your positive you don`t have any wires crossed?
Lastly, it doesn`t matter the engine is new, did you use a flat tappet cam?
If so I hope you used diesel oil, flat tappet cams were going flat right after break in was becoming a very common problem recently and nobody knew why, then the bomb was dropped, big brother Gov. Had the oil company`s remove Zinc as well as other anti wear angents that are critical to cam break in. So now it`s recommended that diesel oil be used in the break in period then after the break in change the oil and filter back to regular oil. If you didn`t use a flat tappet cam but a roller, still check the valve train.
Ditto on the 5/7 plug wire swap. Its bit me on a few occasions. Tripple check.
It's possible you have a weak valve spring.
I had a simular problem on a small block Chevy, & thru alot of parts at it.
I finally pulled plug wires one at a time, until I found the offending cylinder.
I changed the valve springs for that cylinder, (while you're at it change the seals also) & it has ran fine ever since.

Have you pulled the plugs yet?

A Rich condition can backfire (or afterfire) out of the exhaust. The carburetor may be too rich, or you may have a situation where you're plugs have gotten killed. A little dirt or debris in the carburetor can cause the carburetor to run overly rich if this just started.
turns out it was the plug and wire on one cylinder, it was arcing and the boot was melted. i fixed it but the problem is it keeps doing it after a day or so of driving it. i've replaced it twice so far.
I had the same problem earlier this year.What was happening in my situation was that the plug boot was touching the header flange and burning/shorting and not making 100 % contact.My fix was to take the boot off,bend the terminal to clear the flange,wrap this with some electrical tape,cover this with a boot protector.If you arent familar with boot protectors,they are a high heat flexible fiber type slip on cover,Taylor,Accel,MSD,or Jegs make/supply these.After I fixed mine as I described I had no more problems,no more popping/backfiring or burnt plug boots!
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