Throttle shaft vacuum leak

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I've been chasing down a vacuum leak on my 2150 2 barrel Motor Craft carburetor and, while spraying the throttle shaft with carb cleaner on the driver's side where the shaft enters the carburetor, I found a significant vacuum leak. So, I went on line to find a throttle shaft reamer kit that includes a reamer and bushings for my 3/8 inch shaft. After talking to a reputable carb company technician (I'll leave the company and name anonymous, so I don't embarrass anyone), the tech was adamant that, when the 2150 was originally engineered, the play in the throttle shaft was built-in to the specifications of the carburetor. This simply doesn't sound right, but he works on carburetors all day and I respect his experience.

Does anyone have any direct knowledge that the engineers that designed the 2150 or 2100, for that matter, built play into the throttle shaft slot?

Thanks for any help,

Rob

 

[Rustytruck]

Throttle shaft leaks are above the butterfly's and the idle mixture transition slots. Any adjustments to the idle mixture cancels out the vacuum leak so the idle mixture keeps the leak in check. As you accelerate into other areas of activation your vacuum is dropping anyway so air leaking around the shaft is less. In comparison to the amount of air flowing through the carb it is minuscule. At the time in carburetor design I doubt it makes a big difference. It would have to be awfully bad leak. biggest issue would be dirt leaking in.

Now fast forward to fuel injection where un metered air flow through the throttle body is a big issue an air leak behind the air metering circuit means the fuel injection is adjusting its self with bad information. Plug up the air filter or have a a big leak before the air flow metering circuit the computer will adjust it out in the fuel injection circuit. Have a leak behind the Maf sensor the computer doesn't see it and the computer sees the oxygen sensor reading lean and starts adjusting the fuel injector. This may cause drive ability problems depending on how big the leak is.

 

[jckkys]

When the throttle shaft eggs out the bore, vacuum can't leak under it because both gravity and vacuum pull it down so any gap is on the top where there is no vacuum. The big problem is fore and aft movement. There is no specified clearance or play. The only diagnostic method is finding symptoms like inconsistent idle speed or mixture, whistles, etc. when the throttle plates are pulled off center. The new bushing repair is all you can do with a 2100-2150 so if you go that route, let us know if it works.

 

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I have decided, although the leak may have a limited impact as RustyTruck states, it will not do any harm to drill and bush the shaft space and therefore I am moving forward with that plan. I will most definitely report out on the results, so we can all learn from the "experiment". Should take a week or so to get the kit and do the job.
Rob

 

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I finished my throttle shaft project on my two barrel 2150. Reamed the carburetor with a reamer kit from Carb Junkies, and installed the brass bushings. Ran into a snag after finding the bushings overall inner diameter decreased after installation causing a need to hone the bushings to accept the 3/8" throttle shaft. Honing the bushing, while maintaining accuracy and consistency across the length of the shaft opening, was the challenge. I used multiple 3/8 honing strategies without over doing it. In the end, I was able to shape a nice fitting shaft throttle opening eliminating the vacuum leak. I rebuilt and installed the carburetor. Truck ran great from the first start and the original engine miss appears to be gone. Difficult to hear miss, rpm is steady and vacuum gauge is steady at 18". The mixture screws appear a little bit more responsive to adjustments as well, but that may be my imagination.

Repairing a loose throttle shaft was an interesting project and doable with the right tools and patience. It improved my idle, which was the goal. Note, I changed my 47 size jets to 50s, so, if jet size impacts idle, then that could contribute to my positive outcomes.

Rob