if you block the secondaries completely from opening does this stop the bog? or is it just when they open? I'm assuming the float level/line size/psi (some of which you have already mentioned) are OK & no vac leak anywhere (that can be hard to pin down). & you are getting a real healthy squirt of sufficient duration? You mentioned that it is immediate, what about duration & checking in your driveway might be different then under load/wot so I asked about the overall fuel delivery capacity.