If the rods/pistons stay down at idle, they will stay down at cruise speed because the vacuum is higher.
This will be my last post on this thread because I see my advice [ posts 26 & 43 ] of 40+ yrs years on these carbs is being ignored.
For the umpteenth time, do the idle screw test in post #43 because you may well be still on the idle cct at cruise & THE MET RODS/ MAIN JETS WILL HAVE NO AFFECT IF THIS IS THE CASE, SO CHANGING THEM DOES NOTHING. And it needs more idle timing.
Here is my last piece of help:
T slot is checked from under the blades. Carb would normally have to be removed each time to check this. A pain in the a**. To check without removing the carb....with carb on the bench, set t slot to 0.040" [ ideal/perfect ] with speed screw. Using vernier calipers, measure from screw boss to head of screw & record it. Now open blades for a 0.060" gap. Measure again & record. With carb on the car, you can check your gap. More than 060 would require bypass air [ only after correct idle timing is established ]. Less than 040 is OK & no corrective action is needed. Your surging problems are quite likely T slot related. I would also set the valve lash to 0.030" which will improve idle vac.
