I re-fitted a K&N stub stack I used to use.  After changing to a different filter assembly, in order to fit taller spacers a couple years ago, it didn't fit with the new air cleaner base, so I have been running without it.  I took a closer look and figured out what I needed  to do to make it fit.  I don't see how that could cause this, but it's all that I changed before hand.

During one  part of the the test drive to evaluate things, I accelerated hard onto an interstate style highway, it has on and off ramps, held it at about 3000rpm for a half mile, to the next exit.  When I got to the exit, it would not idle below about 2100-2200rpm.  I did what I could in the moment to see if the pedal was stuck and it did not seem to be.  It ran fine on the way home.  It just would not idle below that rpm.  When home, I turned it off to gather tools to investigate and, of course, when I start it up, everything is back to normal.  I've been on a couple test drives, trying to duplicate things, and everything remains fine

It's about a 1400rpm increase over the normal idle speed so the only explanation I can come up with is that the throttle was held open some how.
If it were a vacuum leak that large, it would run terribly but it seemed fine, other than the high idle.  I also don't know of vacuum leaks that  just fix themselves.  I've checked all the gaskets  and vacuum hoses and caps the best I can for evidence of some very odd failure but don't notice it.
Timing can affect rpm, but I don't think it can affect it that much, and again, it doesn't usually fix itself.  I checked timing and all looks normal.
The primaries are operated mechanically, but the secondaries are vacuum.  I do not feel any bind if I operate the primaries or secondaries, pedal linkage attached, by hand and there is nothing that I can see that might cause it.  If I'm not mistaken, if the secondaries stick partially open, it can hold the primaries partially open, which might result in a situation in which vacuum is being applied to the secondary diaphragm, and may not release until the engine is shut off, eliminating the vacuum source.  Is that plausible?

At this very moment, it's running great, possibly better than ever.


Lawrenceville, GA

The only I would suggest that the cause may be is you happened to hold the throttle at the  one spot where it binds. Could be a  combination the the linkage and the position of the engine at that particular rpm.  A fluke? , probably yes.  I suggest you go over all the linkage and lube it where necessary.

If it was around 3000 rpm, can you duplicate it while the car is stationary?


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

I’ll suggest looking at the secondary side of the carburetor where the throttle shaft could be sticking thus preventing the secondary blades from re-seating completely after opening.  If you don’t use the secondaries very often, then some tightness in the carb where the secondary throttle shafts resides could result in what you experienced.  I see this high idle situation on the dyno on occasion after making a full throttle pull and if the engine is still running, then I have to put pressure on the secondary linkage to get the idle back to where it belongs.  Most of the time, simply cutting off the engine and re-firing the engine finds the idle speed restored just like in your case.

Lorena, Texas (South of Waco)