It ain't timing. vac leak? EDIT & when you say the can is disconnected you mean the can ain't being used at all OR that you are checking (initial+mech) total with it disconnected/plugged (which is proper procedure). MORE EDIT I missed that the hesitation is gone. "flat" as in just a lack of power? yes you need more timing and when (RPM) is it all in at? may need mixture and or fuel delivery volume checking. is this part throttle or WOT or both?