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A momentary flat spot accompanied by a backfire is much more likely due to not enough fuel being supplied quickly enough by the accelerator pump (or clogged passageway). I took note of your answer to my post regarding the vacuum retard--that is--you wrote that the vacuum "advance" is disconnected. Is it advance?-- or retard? If a water treatment doesn't cure your dieseling it might do you well to have a working "retard timing" type distributor installed. If your distributor is actually a "vacuum advance" type then the vacuum should get hooked up. Another thought occurred to to me with your last post--is your mechanical advance working properly? Can you (with the motor off) twist the rotor further in the direction of rotation (counterclockwise) about 15-20 degrees and when released it snaps back to normal position?
BTW for general information---when I had my water injection hooked up--water (actually washer fluid) was supplied by an old (new part) Plymouth vacuum controlled heater valve (high vacuum--valve shut) hooked to manifold vacuum (I have a few of those - NOS valves if anyone wants to play with this type thing). Water got sucked into a ported vacuum spot--metered through a small carb jet (about a #85) inserted into the feed hose. It would use about a half gallon of water per tank (15 gallons) of gasoline and it completely eliminated pinging.
These days (10-15 percent ethanol gas) with the motor freshened up and despite a very high compression ratio it does not ping or diesel-- now with the SU carbs - and the water injection disconnected. -- Dave