Autolite 4100 carburetor sticky secondaries

[Urgefor]

My apologies if I missed a previous topic that covers this particular issue, but my search of this forum didn't come up with anything useful.

In a nutshell, the secondaries on the 4100 I'm using on a 390 open easily and smoothly if I have the engine off and operate the primaries and secondaries by hand but if the engine is running the secondaries are difficult to open.  I have even tested with the engine running and the rod that attaches the secondaries to the arm and vacuum diaphragm disconnected.  I assume this indicates wear on either the shaft or the carb housing itself and when vacuum is being applied to the closed throttle plates with the engine running, the end result is sticky secondaries.  Any information or insight would be appreciated.

[winr1]

How far in is the secondary stop screw turned in ??  .( just curious )

With the linkage dissconected and engine running, it is difficult to open the secondarys by hand ??


Ricky.

[Urgefor]

The secondary stop screw is turned in just enough the keep the secondaries from binding when closed.

Correct.  With the linkage disconnected, engine running and opening the primaries enough to allow the secondaries to open a bit, they are difficult to open by hand.  Smooth as silk if the engine is not running and creating a vacuum in the intake manifold.

[winr1]

Should have asked in my previous post

Does the mill idle smooth ?? ... does it run fine on the primarys ??

Is the exhaust stinky/gassy,burn your eyes or fine when idling ??

If you screw the mixture adjustment screws all the way in, does the engine die or try to die ??

What is the vacuum reading at idle ?? ... Try slowly closing the choke at idle and see what happens ...


I have never seen this happen and am very curious to learn why ( Used a bunch of 4100s over the years and still have around 10 left )



Ricky.

[shady]

If it operates freely with the engine not running it is ok. With the engine running, vacuum holds it closed until the primaries are at wide open, then the secondaries are easy to open. I think you're looking for a problem that doesn't exist. BTW with a properly operating vacuum secondary carb. you will hardly feel the secondaries open. Just seems like one big 2bbl.

[1964Fastback]

If it operates freely with the engine not running it is ok. With the engine running, vacuum holds it closed until the primaries are at wide open, then the secondaries are easy to open. I think you're looking for a problem that doesn't exist. BTW with a properly operating vacuum secondary carb. you will hardly feel the secondaries open. Just seems like one big 2bbl.

Huh.  I never knew that about the vacuum holding the secondaries closed until the primaries were fully open.  I learned something new.  I guess one final test if you were worried about binding would be to bring the engine up to normal operating temperature, shut it off, and verify the secondaries still work okay and aren't binding from heat expansion.  But like Shady says, it sounds like you are good to go.

Pat

[Urgefor]

Should have asked in my previous post

Does the mill idle smooth ?? ... does it run fine on the primarys ??

Is the exhaust stinky/gassy,burn your eyes or fine when idling ??

If you screw the mixture adjustment screws all the way in, does the engine die or try to die ??

What is the vacuum reading at idle ?? ... Try slowly closing the choke at idle and see what happens ...


I have never seen this happen and am very curious to learn why ( Used a bunch of 4100s over the years and still have around 10 left )



Ricky.

It idles as smoothly as expected for the camshaft the engine is using and runs perfectly fine on the primaries.  Start and idles fine.  Starts from a stop (toploader 4 speed) and accelerates just fine as well.

Exhaust is fine when idling.  No excess unburnt fuel.

I have adjusted the idle mixture screws.  They operate as expected.

I've used a number of 4100s throughout the years as well.  I've seen this happen one other time many many years ago.  Didn't bother looking into fixing the issue back then, just swapped with another 4100 to have working secondaries.  I don't have spares these days and really don't want to get rid of this particular carb since it is the factory carb off a complete drivetrain (engine to rear diff).

[Urgefor]

If it operates freely with the engine not running it is ok. With the engine running, vacuum holds it closed until the primaries are at wide open, then the secondaries are easy to open. I think you're looking for a problem that doesn't exist. BTW with a properly operating vacuum secondary carb. you will hardly feel the secondaries open. Just seems like one big 2bbl.

I'll look into everything else in relation to operation of the secondaries.  Diaphragm, cover gasket, etc.  I put a fresh kit in it a few years ago but it has been sitting on the shelf since.  I'll post back if I find anything.

[Urgefor]

Huh.  I never knew that about the vacuum holding the secondaries closed until the primaries were fully open.  I learned something new.  I guess one final test if you were worried about binding would be to bring the engine up to normal operating temperature, shut it off, and verify the secondaries still work okay and aren't binding from heat expansion.  But like Shady says, it sounds like you are good to go.

Pat

Checked for binding due to heat expansion as you noted.  They work fine with the engine off and at operating temp.

[Urgefor]

Thanks for all the input.  Long story short, the diaphragm for the secondaries wasn't quite lined up properly to allow vacuum to be pulled on it and open up the secondaries.  After getting it in place properly and testing with a vacuum hand pump, took it for a road test and they are working as expected.  Thanks to all again.

[allrightmike]

You can test secondary diaphram function; with the engine not running, open the primary butterflys and with an air gun blow air past the brass vacuum sensing tube in the RH venturi creating a low pressure area thus mimicking full throttle operation and the rear butterflys will pull open if everything is in order.