Backfires and Poor Throttle Response

[Husker45]

Hello all,

I’m nearing the end of a decade long journey to build a cafe racer from my father-in-laws 1962 CB77. Unfortunately for the past couple of months I am unable to get it to run properly. Some background on the bike;

Big bore kit
Sasche Electronic Ignition with new dyna coils, wires, and spark plugs
Pod Filters
After market mufflers

It starts, but after a short period the left cylinder begins to backfire upon shutting the throttle. Right side does not.

Left exhaust runs 50-100 degrees cooler than the right side.

On partial throttle rpms will stop at 2000, pause and then surge to 4000-6000.

Carbs rebuilt with new float bowls. Float height set per the Loud Mouse method. Stock jet sizes. Needle clip second indent from bottom. Changing position of air bleed screws has n0 impact. Note, all passages cleaned and checked during carb rebuild.

I swapped all electronic and coil components right to left with no change.

I have searched the forum for a similar problem, but if I missed the right post I apologize.

I am out of ideas. Thanks for taking the time to read this.

Husker45