Backfire at highway speeds

[Torxx]

New 34 ADF is not as smooth running as original 28/32 carb (plugged all emission ports and used new water choke on old carb as comparison). we worked hard on the tuning of the new carb, but I get a little bog sometimes when you give a little or lot of gas (not every time) and if doing 60mph or more and floor it, it will backfire repeatedly as long as you stay in the gas. also some light popping now and again at idle or acceleration. any ideas?

[So Cal Mark]

depends where the backfire is coming from; thru the carb usually means a lean condition, out the tailpipe can be reach or ignition trouble

[mbouse]

when you redid your exhaust, did you eliminate the resonator pipe?

maybe there isn't enough back pressure.

[danaspider]

Is the vacume advance still working? Can create stumble and such.

[Torxx]

I would say that is seems to be coming from the carb. Straight exhaust to ansa muffler. I was told by a Fiat mechanic that the spiders have a mechanical advance, and the vacuum line from the distributor to the carb is to help with deceleration. Is that correct? We experimented with that vacuum line on and off, it is currently off which seemed to help with popping.

Wouldn't the fact that the car ran perfectly when we put on the old carb narrow down the issue to the new carb/carb settings / timing? Wouldn't that eliminate questions of ignition problems? Just curios.

PS I need a lower profile floor jack. The lower type springs on the car make it tricky to use this one.

[So Cal Mark]

the vacuum advance is mostly for fuel mileage-it works at light load, high vacuum situations.

[Torxx]

never say never. I made the wrong assumption that ignition issues could be ruled out. I am embarrassed to say that there is high likelihood that I had the #3 and #4 plug wires switched the entire time we have been tuning the car. I was switching out the plugs to non-platinum type, and discovered the error. That cured the backfire.