Backfire Blues
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Freshman User
Joined: Nov 2011
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From: Durango, CO
Backfire Blues
My 74 highboy recently started backfiring through the exhaust. I thought it might be leaking collector gaskets, but that was ruled out. The engine is a mildly modified 390 with 5k miles on it. PerTronix Ignitor3 ignition & billet distributor. Edelbrock Performer intake, .500 lift cam, and a650 cfm Thunder Series AVS. I set ignition @ 10 degrees BTDC and keep the idle around 750. Since the motor's not "stock" I'm not sure if my setting are optimal or not. The rest of the drivetrain is np435 trans/205 transfer case and 4:10 Dana 44/front and Dana 60/rear. It ran fairly well untill I changed the spark plugs, during which time I separated the boot from on of the wires. The inner core looked intact, so I crimped the terminal back on. Also the inline fuel pressure gauge says I'm at 8 psi however Edelbrock says I should have 5-6 psi. Not sure if its contributing to the problem or not but in addition to the backfiring, it wants to stall at idle. Any suggestions would be appreciated as I'm still fairly green at trying to tune an engine.
Senior User
Joined: Mar 2004
Posts: 280
Likes: 2
From: Alabama
You probably need to re-look at your timing. With that setup, you might need 14-18* initial. Also check for vacuum leaks. And hook a vacuum gauge up and see what you got and how the gauge acts (needle fluctuates, bouncing around, etc...). If a wire is suspect, just change them.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
My 74 highboy recently started backfiring through the exhaust. I thought it might be leaking collector gaskets, but that was ruled out. The engine is a mildly modified 390 with 5k miles on it. PerTronix Ignitor3 ignition & billet distributor. Edelbrock Performer intake, .500 lift cam, and a650 cfm Thunder Series AVS. I set ignition @ 10 degrees BTDC. Since the motor's not "stock" I'm not sure if my setting are optimal or not. The rest of the drivetrain is np435 trans/205 transfer case and 4:10 Dana 44/front and Dana 60/rear. It ran fairly well untill I changed the spark plugs, during which time I separated the boot from on of the wires. The inner core looked intact, so I crimped the terminal back on. Also the inline fuel pressure gauge says I'm at 8 psi however Edelbrock says I should have 5-6 psi. Not sure if its contributing to the problem or not but in addition to the backfiring, it wants to stall at idle. Any suggestions would be appreciated as I'm still fairly green at trying to tune an engine.
Eddy carbs don't tolerate more than 6psi very well, you're probably overriding the needle/seat, I'd turn down the FPR to 5-6psi.
Thread Starter
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Freshman User
Joined: Nov 2011
Posts: 32
Likes: 0
From: Durango, CO
You probably need to re-look at your timing. With that setup, you might need 14-18* initial. Also check for vacuum leaks. And hook a vacuum gauge up and see what you got and how the gauge acts (needle fluctuates, bouncing around, etc...). If a wire is suspect, just change them.
Thread Starter
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Freshman User
Joined: Nov 2011
Posts: 32
Likes: 0
From: Durango, CO
I will get a FPR as soon as I get a chance. That will help with the occasional surging at idle I have I'm sure.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Stock fuel pumps put out about 7psi (+/-) sometimes you'll get a pump that plays well with an Eddy carb, other times you'll get a pump that requires the use of a FPR, it's hit or miss.
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