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Throttle stuck open * now fixed*


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The car is a 1960 Rambler Ambassador 327 four barrel with Overdrive. A well restored car but museum stored.




Background: My good friend bought this car at an auction at a huge discount based on what had been spent on the restoration. It truly was a good car but it did have some problems. The main one was that instead of the correct Holley 4 bbl carb it had a huge double pumper Holley on it. Incredibly overcarburated it ran rich beyond belief, it barely idled and was flooded at every start.  Without a suitable correct Holley to install, I put on a 1960s FoMoCo Autolite 4 bbl. We are Ford people and this type of carb is available, reliable and doesn’t balk at sitting for long periods.




I recently took it to a Rambler event and beyond the typical plugged fuel filter, the only issue was that the Overdrive (Borg Warner electrical type) did not seem to be engaging regularly. For those who are unfamiliar with these systems you accelerate to a speed above 28 MPH and lift your foot and ‘feel’ it shift into OD. For whatever reason on this car you can hardly feel the shift into OD and the engine is so quiet that even that the noise isn’t even much of a giveaway.  One way to check that the OD is engaged is to push the pedal to the floor where the linkage activates the kickdown switch and it shifts instantly into direct drive. You can then release your foot to see if it fell back into OD. On this car sometimes you could feel it and sometimes not.


Returning home after the event on an LA freeway at 9:00 PM I wanted to see if it was truly in OD so I floored it. It did kick down, BUT when I lifted my foot the car kept accelerating (!) Fortunately traffic was light so I continued driving (too fast) until I could find a place to pull over. Yes, during this time I bent down and tugged on the gas pedal but that did nothing (still accelerating!) and all I accomplished was to separate the pedal from the rod (!) So, I turned off the key and coasted to the shoulder. Because the OD needs electrical power turning off the key also shuts off the OD which allows the freewheeling to come on, and you lose all engine braking. Scary.


I get out, check under the hood and nothing is wrong. The linkage is fine, the spring is in place and the linkage is not binding. It snaps back every time. I restart it and it is idling fine.  I continue home but now being careful to not press the pedal down over the halfway mark.

Next day I check and recheck ALL the linkage. I add additional springs (just for good measure) and with each test drive, it SOMETIMES ‘sticks’ open and sometimes returns to idle speed(?)  I make more adjustments and further checks and during subsequent test drives it continues sticking open, BUT not EVERY time I mash the throttle. Hummmm. . . . . .


In the garage, I rev it up and try everything I can think of, but I cannot get the throttle to hang open. But it does commonly (but not every time) hang open during the test drives resulting in uncontrolled acceleration. I was baffled, hours of frustration before I discovered the problem.


Ford-Autolite four barrel carbs use vacuum opened secondaries. The vacuum is drawn from a small tube in the primary intake and routed under the lid and into a chamber with a light spring and a diaphragm. There are many places for the vacuum to leak (especially if the body/lid is warped) and it is very typical for these secondaries to not open at all. When they are open, they are closed again by a little rod on the primary plate linkage that rubs against a cam area on the secondary plate linkage. All this is tucked up under the choke mechanism and you can hardly see it.



Well turns out the little pin snapped off. When you were accelerating fast, the secondaries did open but they were not being mechanically closed. The primary throttle plates (activated by the pedal linkage) were slamming shut, but the uncontrolled acceleration was coming from the still open secondary throttle plates. (who woulda thought?) 


Why couldn’t I replicate the problem? In my garage tests I never held the throttle linkage open long enough to open the secondaries (that is a lot of RPM when the engine is not actually driving anything). When I had uncontrolled acceleration during test driving, I turned off the key, coasted to the side of the road, and without any vacuum, the secondaries were being closed by the little spring in the diaphragm chamber. Nothing was wrong when I opened the hood. 


The Repair: I drilled and taped the pin hole in the linkage for a screw that would reliably, mechanically close the secondary throttle plates and I had no more problem.







Uncontrolled acceleration from having open secondary throttle plates, who would have guessed? But if you see the 4 barrel design as not just one carb, but instead as being really two different carbs then it makes more sense.


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Glad you found the issue!😲 And made an easy repair.

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