Author Topic: Choke/high speed idle cam detail and time lapse graphics  (Read 2027 times)


  • Oracle Forum Member
  • *
  • Posts: 1869
  • Fort Worth, Texas
      • http://www.301garage.com
 One often asked question has to do with choke functions
and idle control.

 As we know the choke is electrically heated and since it's
a bi-metallic, it changes shape with heat. The change in shape
is what pushes the choke flap open.

 Simple enough, and easy to see.

 But what makes the whole choke function complex is it's
programed way to step down it's function. This aspect is
called the Hi-speed idle cam. It is a mechanical feature
that relaxes the choke flap function by the release of the
bi-metallic spring tension.

 Qjets live in a terrible habitat... extreme heat and cold,
oil mist, and gasoline seepage. Put it all together and the
device eventually does not function correctly.

 When gasoline "mostly" evaporates it leaves behind a gummy
residue that keeps linkage from working smoothly. One of
the more important functions that gets gummy is the high
speed idle cam.

 Ok, but how would a person know if the linkage is even working.

 When totally cold the Qjet needs to make sure the engine gets
an extra rich fuel mix until it starts to warm up. But the thing
just isn't on/off simple.

 There are basically 4 steps to the degree that the choke functions.

 1) Full running choke

 2) Medium running choke

 3) Low running choke

 4) No choke

 To illustrate each of these 4 points, I've done two time lapse
sequences. One on the stepped cam/choke side and the other on the
throttle side.

 These show what you should be able to see on your carb. Keep in
mind that I have adjusted the timing to illustrate the function,
but each point should be noted.

 First is the choke/idle cam side showing all 4 points of operation.

 Next is first a still to show what we are focused on. the effect
of holding the throttle open somewhat during warm up.

 And then the same four points of idle cam movement as seen from
the throttle connection side.

 Choke plate 4 point movement graphic:

 cool huh? <grin>

« Last Edit: May 10, 2012, 01:56:22 PM by jjr »
1979 10th Aniv 400/4spd
1980 Black SE 301NA
1980 Indy Turbo Pace 301T
1981 Turbo 301T
1981 Black SE 301T
1981 Daytona Pace 301T


  • Jedi Council
  • Oracle Forum Member
  • *
  • Posts: 18807
Re: Choke/high speed idle cam detail and time lapse graphics
« Reply #1 on: May 31, 2012, 08:17:29 PM »
That is awesome!
I want to be like Paul Kenny when I grow up
Joe Bays
1977 Y82 4-speed (red interior)
1978 Skybird 350
1978 Redbird 305
1979 Redbird 301 4-speed
1992 Lincoln Mark VII LSC/SE (red)
1970 Datsun Pickup
1953 Chevy pickup (converted 4x4)
1988 T-bird Turbo Coupe

Re: Choke/high speed idle cam detail and time lapse graphics
« Reply #1 on: May 31, 2012, 08:17:29 PM »
You can help support TAC!