Back firing through the exhaust-HELP!!! - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 6 (permalink) Old Oct 18th, 18, 4:04 PM Thread Starter
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Back firing through the exhaust-HELP!!!

My daily broke down so I had to dig my Nova project out of the garage for a daily for a week or two. I drove it a few days last week to work and it was back firing through the exhaust and just running rough in general. Its a stock bottom end '76 350 with double hump heads, a cam of unknow size and a edelbrock intake with a holley 600dp. Its got a new MSD hei with new plugs and wires. I rebuilt the carb yesterday and replaced all gaskets, pumps and power valve. I took it around the block and it seemed fixed. Well today, I took it on a 25-30 trip to enjoy the car and weather and it ran terrible. Back firing through the exhaust on the slightest load and surging under part throttle. On the way back, I stopped at the local supermarket for some adult beverages and the 4 miles back home, the car ran great!!! What am I missing here???? I know my way around a toolbox but this has me flustered!! Any ideas?? Thanks!
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post #2 of 6 (permalink) Old Oct 18th, 18, 4:59 PM
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Re: Back firing through the exhaust-HELP!!!

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Originally Posted by wes-in-nc View Post
My daily broke down so I had to dig my Nova project out of the garage for a daily for a week or two. I drove it a few days last week to work and it was back firing through the exhaust and just running rough in general. Its a stock bottom end '76 350 with double hump heads, a cam of unknow size and a edelbrock intake with a holley 600dp. Its got a new MSD hei with new plugs and wires. I rebuilt the carb yesterday and replaced all gaskets, pumps and power valve. I took it around the block and it seemed fixed. Well today, I took it on a 25-30 trip to enjoy the car and weather and it ran terrible. Back firing through the exhaust on the slightest load and surging under part throttle. On the way back, I stopped at the local supermarket for some adult beverages and the 4 miles back home, the car ran great!!! What am I missing here???? I know my way around a toolbox but this has me flustered!! Any ideas?? Thanks!
Sticking vacuum advance, cracked vacuum line?

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post #3 of 6 (permalink) Old Oct 19th, 18, 3:15 AM
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Re: Back firing through the exhaust-HELP!!!

Bad tank of gas?

How long was it parked?

David

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post #4 of 6 (permalink) Old Oct 19th, 18, 6:38 AM
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Re: Back firing through the exhaust-HELP!!!

Backfiring through the exh [ through the intake. different story ] with the slightest load sounds more likely to be electrical.
Check that the HEI is getting 12v or more with the engine running.

Double check initial timing [ with vac adv disconnected ]. If it checks out, do a resistance check on all the leads. Try another set of spark plugs, then another coil/module.
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post #5 of 6 (permalink) Old Oct 19th, 18, 8:48 AM
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Re: Back firing through the exhaust-HELP!!!

Exhaust leak, maybe? A little fresh air getting into your exhaust can complete combustion a little more to give you a backfire.

Rick

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post #6 of 6 (permalink) Old Oct 19th, 18, 9:48 PM Thread Starter
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Re: Back firing through the exhaust-HELP!!!

Team, Thanks for all the replies!! @Pioneer4x4 , its done this with 2 completely different HEI distributers. @davewho1 , the car sat for 30 plus years but I put in a new tank and its a fresh 10 gallons…..ethanol junk though. I used the AC Delco Green shield hose but I needed gas and couldn't make it to my local "Pure" gas station, lol. @Gtogeoff , that might be something I need to look into. When I switched to HEI, I did not verify that a true 12 volts was coming though the lead that I used. @B52bombardier1 , the exhaust system is a new 2 1/2 system from Summit. It has lots of leaks. The system had terrible fitment issues. I think the kit for a Ford truck got sent to me, lol.

At first, I thought was the Holley carb but now I think its something electrical. I drove the car to work today and not a single issue. The carb is rich but that's a different issue. Thanks again for all the suggestions!!
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