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Is it safe to assume there is a vacuum-leak when the car hesitates to respond to pushing the throttle down for example, to pass? What does this inquiry fall under in the repair manual? Thanks in advance!!

Or your accelerator pump isn't working that great.

It’s been so long since I’ve driven one at speed but isn’t there a throttle rod to the Ultramatic that might be out of adjustment?

thanks, I was just looking at one on ebay

BigKev thanks!

Ultramatics are not known for fast takeoffs particularly in H range but a throttle rod adjustment issue is not likely the cause of stumbling and there is absolutely no vacuum associated with the transmission that could leak.

I would suggest looking at a the accelerator pump first and rule out any issues there before going farther. It is easy to do by removing the air cleaner and looking down the throat of the carb. Make sure there is fuel in the carb float bowl and then as you have someone push the accelerator to the floor (key off), You should see a good strong squirt of gasoline spray thru the jets. If there is barely a trickle suspect the leather plunger seal is damaged or perhaps the check valve ball in the supply passage at the bottom of the pump chamber is leaking.

A throttle rod adjustment issue will often show up if the engine tries to run away on takeoff. The high range clutches depend on feedback from the throttle pressure to determine how hard they are squeezed together. If they are not being squeezed tightly enough to pass engine torque thru them they will start to slip and the engine will race. Another typical symptom is the transition to direct drive will come in way too soon or too late. If you are immediately jumping into direct drive somewhere well below approximately 20 mph at a typical thottle opening that could cause a stumble. Slipping high range clutches almost always result in overheating and burning and soon they loose ability to clamp entirely so the car mostly sits when you try to move in H.

In a stock 51 the only vacuum not switched on or off would be in the vacuum section of the fuel pump and tubing and hoses to the wiper motor or to the distributor advance. Cracked hoses and leaking seals and gaskets are common issues with wipers and a frequent source of vacuum leaks. A cracked and leaking vacuum diaphragm in the advance cannister is also a known issue and if timing is not appropriate for the engine speed there could be some stumbling. In some cases a warped intake manifold will also result in vacuum leaks at the manifold gasket to block joints.

Thanks, I will add these to my checklist, appreciate all the feedback folks!