4100 carb question

[MRadke]

This is on a stock 352 in a 66 Galaxie. 

Idles about 650 rpm after a brief warm up without the air cleaner, but jumps to about 800 rpm without the lid on the carb.  With the air cleaner on the idle will quickly turn choppy and wants to stall.  Both of the screws holding the boosters in place were loose, as was the center stud, all were tightened and all of the vents in the cover were checked and blown out with compressed air.  When the lid is removed with the engine running, there is an audible rush of air into the carb and the vacuum on the lid can be felt.

With the lid on the carb, the idle quickly gets choppy and wants to stall, without the lid, the idle picks up and smooths out.

The air filter is clean, and there is a pertronix kit installed.

I plan to swap the plugs today.

I can't figure out why there is so much vacuum on the lid and why it runs so much better with the lid off. 

Any ideas?

Mike

[machoneman]

Check the clearance of the choke blade, while open, to the bottom of the lid. Is the choke's blade getting partially closed by the lid? Then, if only the lid is off (bottom still on the carb) does it runs well? Have seen the bottom of an air cleaner assm. hit some choke linkage parts (i.e. wrong air cleaner for your carb) when it's screwed down. Look for witness marks on the paint/chrome.

[Heo]

Must be some restriction in the air cleaner snorkle or
something wrong with the air filter.
sometime rats build nests in the snorkle
Remove filter and reinstal the lid and test if problem
goes away
You should not have that high vacum in the aircleaner
at idle

[cjshaker]

You said this was a stock 352, but is it a stock air cleaner housing? I would be surprised if a stock air cleaner housing is causing this problem UNLESS the housing snorkel is plugged somehow as Heo mentioned. A stock housing should clear a 4100 without problems unless something has been modified. Did you add a manual choke? Sometimes the aftermarket choke set-ups can cause interference problems with stock or aftermarket housings.

That air cleaner is definitely restricting airflow for some reason. Good advice from Bob and Heo. I have seen aftermarket drop bases hit choke linkages also like Bob mentioned. And if it is clearing ok, then the top may be way too close to the air horn causing some serious restriction or actually closing the choke plate some.

Either way, get it figured out quick. If it's that bad at idle, then it's going to be much worse while driving.

[MRadke]

I may not have explained it correctly.  With the stock air cleaner housing off, if I remove the lid from the carb while running, the idle will pick up 100 RPM and smooth out.   The vacuum that I notice is under the lid of the carburator.

[machoneman]

Hmmmh! Did you mean to say below that "with the stock air cleaner ON, If I remove the lid (top)....."

Clarification needed as one has a bottom (the formed steel lower housing with the air intake snorkel ) and a round top piece, or lid. Is that right?

I may not have explained it correctly.  With the stock air cleaner housing off, if I remove the lid from the carb while running, the idle will pick up 100 RPM and smooth out.   The vacuum that I notice is under the lid of the carburator.

[Lenz]

Maybe a picture of this setup would help if you can post one?  I too am having some trouble sorting out possibilities for the restriction issue based upon your description.  My former 390 started out running rich because the lid on a 1" tall oval air cleaner assembly was literally sitting on the choke butterfly.  It wasn't closing the choke but it had about the same affect as running 1/2 choke on a hot engine.

[Heo]

I Think he is talking about the lid on the carb
you can run the ford carb without the lid...
atleast the 2 barrel
Dont remember if the 4 barrel is like that
but i dont see how to get any sucktion on
that lid?? maby if the bowl vent is plugged

[Heo]

I Think he is talking about the lid on the carb
you can run the ford carb without the lid...
atleast the 2 barrel
Dont remember if the 4 barrel is like that
but i dont see how to get any sucktion on
that lid?? maby if the bowl vent is plugged

[MRadke]

Heo - that is exactly what I'm talking about.

The lid of the carb itself is what I removed while running.  The air cleaner assembly was sitting in a corner of the garage next to my fire extinguisher.

I have blown out the bowl vents with compressed air.  Is the vacuum in the bowls what activates the power valve?

[machoneman]

Think Ed's site can help.

http://www.network54.com/Forum/88781/thread/1260281683/Autolite+4100+1.08+new+issue

[MRadke]

Thanks Bob, I've got a few things to look at just from the post in your link.  I think I may be in need of a set of floats before I go too far.

Mike