I have a 1961 348. The code that should be on the right front is not there, possibly ground off during a rebuild. The dip stick is on the passenger side which I think some truck engines were, don't know for sure. I'm having a problem with intermittent backfire above 1500 RPMs. I'm running an Edelbrock 1406 carb. After installing the carb I drove the car three times, about 50 miles, through our development not exceeding 30mph then the backfire began. Plugs show the mixture was way rich, black. Have adjusted as lean as I can but still have the backfire. Don't know what the timing should be, running at 8 deg btdc now. Thinking I should just run it for a while and hopefully burn some carbon out? Any advice please.
348 questions
- Thread starter mike348
- Start date
Did it backfire before you installed the edelbrock?
Are you running the stock distributor?
Have you checked timing while increasing RPMs?
Where is your vacuum @dvance hose connected?
Try putting more timing in it and see how it likes it, it’s probably not enough at 8 initial.
I don’t know how you adjusted the carb, if you mean idle/air screws or jetting and spring Or all the above.
Edelbrock doesn’t like more than 5 or 5 1/2 psi. it will push fuel past the needle seat and cause rich condition.
Do you have the booklet that came with the carburetor, lots of good tips on tuning it.
Hopefully this gets you started, others smarter than me will chime in and be a big help.
I think that you have 2 or 3 things going on, start with checking the timing curve with your light.
Took me 20 minutes just to type this, you probably have it straight by now!
Are you running the stock distributor?
Have you checked timing while increasing RPMs?
Where is your vacuum @dvance hose connected?
Try putting more timing in it and see how it likes it, it’s probably not enough at 8 initial.
I don’t know how you adjusted the carb, if you mean idle/air screws or jetting and spring Or all the above.
Edelbrock doesn’t like more than 5 or 5 1/2 psi. it will push fuel past the needle seat and cause rich condition.
Do you have the booklet that came with the carburetor, lots of good tips on tuning it.
Hopefully this gets you started, others smarter than me will chime in and be a big help.
I think that you have 2 or 3 things going on, start with checking the timing curve with your light.
Took me 20 minutes just to type this, you probably have it straight by now!
Thanks guys, some good info here. Ignition is electronic no cracks on the cap. Float level is per specs and vacuum is connected to the small port on the right side of the carb. Adjustments were made with the screws. I'll check the other things mentioned over the weekend and let you know!
Disconnect the vac. advance and plug the carb port, reset the timing to 8-10* btdc. reconnect the vac. advance. Verify your mech. advance is working correctly. Total timing should be 32-34 deg. (approx. 2800-3000 rpm) mechanical + initial. A dial back timing light is very helpful.
OEM vac. advance is plumbed to manifold vac.
Are you blowing black smoke? Back fire (loud bang) from the exhaust at constant throttle? The black smoke is going to be carb related but the bang from the exhaust is usually ignition.
I'm running that 1406 on my boat as it is a marine carb, as equipped, the jetting is very close to perfect for the 348/350 at sea level
OEM vac. advance is plumbed to manifold vac.
Are you blowing black smoke? Back fire (loud bang) from the exhaust at constant throttle? The black smoke is going to be carb related but the bang from the exhaust is usually ignition.
I'm running that 1406 on my boat as it is a marine carb, as equipped, the jetting is very close to perfect for the 348/350 at sea level
Don beat me to it. 
Turning the idle mixture screws on the base of the carb only adjusts rich/ lean at idle and tip in throttle. Jets, rods, and springs adjust rich/lean everywhere else. Condition of plug wires and plugs? Lots of things to verify. Do you have another coil you can swap as a test?
All of those issues remind me of the hell I went through many years ago when putting 2-4’s on my 61 348 w/powerglide and stock rear ratio! Bottom line had to put the distributor on a Sun machine then build a limiting device on the vacuum advance. Had to completely go through the new carbs to “tune” them to the very accurate Eldebrock tuning guide. Then cooling system issues! I’m having nightmares thinking about those several months. But when it was all done it ran perfect even started very nice in cold with no chokes. Yes I even had to change to the hottest range plugs I could find.
Robert
Robert
Ist, it is a big block...please don't tune it like a small block....timing initial should be 15-16 degrees not 8, that is retarded to much for a 348. Depending on your cam, probably 35-36 ant WOT. if you are running vacuum advance, which port are you using, manifold or port....some engines like one or the other. if manifold, be sure advance is disconnected to set initial. I think your engine will like more timing if you have not tried that yet. AND that being said, is just my opinion on many 348-409 engine configurations have had now and in the past.