Marvel-Schebler TSX-458 carburetor choke shaft seal

[dbotos]

Don't know how much vacuum the choke bore sees, but for less than a buck, the o-ring is cheap insurance.

 

[rsallen]

ahem, insurance against what exactly?
A sloppily bored carb body?

 

[dbotos]

Unfiltered air going through the carb and into the engine (i.e. sucking in dirt/dust).

 

[rsallen]

Okay fair enough I guess.

The shaft shouldn't be loose enough to worry about it though....... even if the air filter isn't maintained.

That said, as it isn't on the vacumn side of the carb operation I just wanted to know the thought process behind putting a seal on a choke shaft.
Thanks.

 

[dbotos]

During a naturally-aspirated engine's intake stroke, the highest pressure is in the atmosphere. Anywhere along the intake path, the static pressure is lower because you trade pressure for velocity. So if you had any holes/leaks along the intake path, it would want to suck in unfiltered air (or rather, the higher pressure in the atmosphere would push it in).

 

[namyessam]

Heres what I don't understand,
Why does the choke shaft even need a seal?
I understand a throttle shaft needing a reasonable seal, but a choke shaft? As long as the choke is free to do it's thing and isn't falling apart what is the big deal?
If it's too sloppy, sleeve it, put a little grease on it, keep it lubed now and then. It's not like it has to seal against a great vacumn pull. And in fact it designed to open a bit with some vacumn.

What am I missing?

seal out dirt maybe?

 

[rsallen]

That's why it's called a vacumn. And it's all under the throttle plate side.
Other than from the volocity column of the air flowing from the other side of the throttle plate via the air filter there is no vacumn, ergo you get a metering of gas through a sized jet set in the middle of the column.
But there is no vacumn per se on the atmospheric (the filtered) side of the throttle plate.

 

[rsallen]

Myself, I'd worry more about bad sealings between the air filter and the carb.
If the choke shaft fits so loose as to be a concern, I'd drill the carb body and sleeve the shaft properly.

Randy