How to get Choke to open faster? Big Cold Start Problem

Discussion in 'Carter' started by Corellian Corve, Apr 5, 2006.

  1. Corellian Corve

    Corellian Corve Well-Known Member

    Hey folks. Have a bit of a problem with the AFB on my '66 Skylark.

    I can't seem to get the choke to open fast enough to keep the car running well in the mornings.

    I've made sure the pulloff is set correctly. I have the choke cover (which does not have a mark) set just to the point that it sets the fast idle cam when I press the gas when the engine is cold.

    The problem is that the choke doesn't open fast enough and the car dies (repeatedly) as soon as I start driving and come to a stop. It will do this until the choke opens up fully which takes a while.

    I know the problem is in the choke because if I physically open the choke myself the car runs fine.

    If I set the choke cover to lean and get the car running well, it doesn't have any tension on it and won't set the fast idle cam when cold.

    There are 2 rods that affect the choke I can see - the rod from the choke cover housing on the passenger side, and the rod on the drivers side that connects to the fast idle cam. I'm not sure how adjusting these affects the choke.

    Bottom line - what's the correct way to adjust the choke so the spring has enough tension to close the choke and set the fast idle cam, but is loose enough to open up really quickly?

  2. carbking

    carbking carburetion specialist

    A common cause of the choke opening slowly is a plugged vacuum line. The choke operates because of a tiny vacuum passage from the passenger side primary throttle bore. This passage causes the choke piston to work against the tension of the spring, and also causes hot air to be pulled into the choke housing to relax the spring.

  3. Corellian Corve

    Corellian Corve Well-Known Member

    I'm probably not articulating it quite right.

    I don't think the choke is opening abnormally slow (e.g. there is a blockage), it's just opening slower than the car would like.

    Basically, I can't get the right balance.

    If I set the choke to fully close and properly set the fast idle cam, then it's too rich.

    If I lean it out to get it running well, then it doesn't have enough tension to set the fast idle cam.

    It's really sensitve. I mean 1 or 2 notches either way really makes a difference.

    - One notch leaner than the centerline and it runs great but no fast idle
    - One notch richer and it sets the cam and starts great, but runs crappy.

    What I need to figure out is how to set it leaner so it runs right, but still engages the fast idle cam. :Do No:

    Thanks!!! :TU:
  4. Buford

    Buford Old guy member

    I always set the choke to close lightly, then set the fast idle by bending the the link on the driver's side that pulls the stepped cam into place. Sometimes the tube running through the exhaust manifold is bad, and lets exhaust into the hot air tube leading to the choke. This lets "crap" build up, and eventually plugs the small vacuum passage to the choke. Good luck! Frank
  5. Corellian Corve

    Corellian Corve Well-Known Member

    Thanks Frank.

    I'm pretty diligent about keeping things clean. I just replaced/cleaned out the choke tubes, and just removed and cleaned the choke housing and lubed the linkage to make sure everything is working smoothly. I was afraid the issue might have been what you mentioned (gunk and buildup).

    I'll try playing with the rod on the drivers side and see if I can get that working right.
  6. Corellian Corve

    Corellian Corve Well-Known Member

    Quick question - which way do I want to adjust the rod in order to get the cam engaged?

    If I'm making choke "looser" - do I lenghten the rod to get it to engage earlier or shorten it?
  7. carbking

    carbking carburetion specialist

    First adjust the choke. This is easiest with a helper. The ambient temperature should be between 65 and 70 degrees F. Engine must be cold. Loosen the 3 screws holding the choke cover. Have the helper move the throttle to approximately the midpoint of travel and hold. This will assure that the choke lockout is off. Now rotate the choke cover so the choke plate is open. Now reverse the rotation of the choke cover and rotate slowly until the choke plate just touches closed. Tighten the three retaining screws, the choke is set.

    Now, locate the fast idle adjustment screw. It is located on the under side of the throttle linkage, and contacts the fast idle cam.

    Push the throttle to the wide open position and release. This resets the choke linkage and the fast idle linkage. Turn the fast idle adjusting screw clockwise. You should be able to observe the throttle linkage moving as the fast idle screw should engage the fast idle cam. I do not have specs at this location, but as a general rule, COLD fast idle should be between 1600 and 2200 RPM (set with the fast idle adjusting screw). Once the choke has warmed and the choke plate is vertical, the fast idle screw will no longer contact the fast idle cam, and curb idle is maintained by the curb idle screw (or the big air screw if used).

    When starting a cold engine, do NOT keep your foot on the footfeed!!!! Push the footfeed completely to the floor one time, this sets the choke. Pump the footfeed a number of times (experience will tell you how much, each engine is different), remove your foot from the footfeed, and start the engine. Engine RPM should immediately go to the cold fast idle spec.

  8. Corellian Corve

    Corellian Corve Well-Known Member

    Thanks Jon this is very helpful!

    I've somewhat gone through those steps and here's where I could use your assistance.

    Bascially, if I set the choke per your instructions, which enables the proper allignment and settings of the fast idle cam, once I step off the fast idle, the car dies.

    If I move the choke to 1 or 2 notches leaner, the car runs great but will not activate the fast idle cam.

    I need to figure out how to properly adjust the choke 1 or 2 notches leaner for proper driving and still get the fast idle cam to activate. It appears that can be achieved by bending the rod on the drivers side, I just do know the right process to do that.
  9. carbking

    carbking carburetion specialist

    With the choke plate closed, the fast idle adjustment screw should be in contact with the high "step" on the fast idle cam. Bend the rod to achieve this setting. The fast idle cam should have distinct "steps". If the fast idle cam is worn, these steps will blend into a curve, and it will be difficult to hold the fast idle when driving.

  10. awake13

    awake13 Well-Known Member

    The AFBs' choke is actuated by a heat tube running from the exhaust manifold to the choke housing. In my opinion you could fabricate the same type of set-up using brake line or copper tubing. These tubes are noted for plugging the housing with carbon so if you are running a heat tube check and clean the housing.

    If you haven't replaced the choke spring and it is original you will not get the desired response from the choke. There is also an electric conversion but this actuates purely by time not tempreture.

    As previously stated the small screw under the circular cam controls fast idle and it is here that you should be controling your fast idle speeds in relation to choke position.
  11. 67COUPE 340-4V

    67COUPE 340-4V Well-Known Member

    Hi, I know when I had an electric choke, you had to turn the key to the on position, then push the pedal to the floor, then turn the key to start it and keep it running. I have a manual choke now and it's great. My car will start after sitting for a couple weeks in the dead of winter(-20) and start up and run on the first try. Then you control how fast you want the car to warm by the choke lever. Not sure if a conversion is an option. I did the same change on my 70 Chevelle an it ran great in the winter years ago. Good luck. Mark
  12. Schurkey

    Schurkey Silver Level contributor

    I respect carbking, but I'm going to disagree (well, maybe "enhance" is a better word) a bit with his advice.

    In my experience, perhaps 4 out of 5 "choke problems" are due to two interrelated adjustments. Note that there are four typical adjustments that need to be correct on a choke: Choke coil position, choke pull-off amount, fast idle cam syncronization, and the WFO unloader. The unloader causes the fewest problems. The two most troublesome are the first two, choke coil and pulloff.

    That is, the choke coil is adjusted too lean, as partial compensation for a choke pull-off that's adjusted too rich.

    The first thing I would do with your set-up is to verify that the hot air really is hot enough. With the engine fully warm, if you touch the hot air tube leading from the exhaust to the choke housing, you will be instantly blistered. This tube shouldn't be "warm", or even "pretty hot". I mean, it should be SCREAMING hot.

    Once you have verified that the choke coil is actually getting the BTUs it needs to release properly, adjust the choke coil one or two notches richer, but adjust the choke pull-off so it cracks the choke blade open a touch more.

    From here, I'm going on generic carb advice. I'm more of a Rochester person than a Carter person.

    There should be an adjustment (perhaps by bending the linkage) that will syncronize the fast idle cam rotation to the position of the choke blade. You want the SECOND HIGHEST step of the fast idle cam to be aligned with the fast idle screw when the choke blade is opened by the choke pulloff. So you have to do this adjustment AFTER adjusting the choke pull-off. From your description, it sounds like the fast idle cam is rotating "off" too soon in relation to choke blade position. So, choke fully shut, you're on the high step. Engine starts, pulloff cracks open the choke blade, you touch the throttle and the fast idle goes to the second step. From there, the fast idle rotates to lower and lower speed as the choke blade opens based on the opening rate of the choke coil.

    The fourth choke adjustment is the unloader. At WFO, the choke blade should be forced open just a bit more than the choke pulloff would open the choke blade. This will help you clear a flooded engine.

    You damn sure do NOT need a manual choke to fire an engine reliably at -20F, given an engine in good condition, having a stock- or mild cam, and winter-blended fuel.
  13. Corellian Corve

    Corellian Corve Well-Known Member

    Thanks all. I think part of the problem Schurkey you identified. For some reason, when I hit the gas after startup, the choke wasn't going to the second step it was going all the way off.

    I pulled the carb last night and measured all the adjustments. One problem I noticed right away was that the choke piston adjustment was way off. According to spec, when the piston was 1/8 in from the top of the bore, the choke blade should have been 7/32 from the air horn; it wasn't. @ 1/8 from the top the the blade was fully closed. So I lengthened the rod so the blade was open 7/32 (it was QUITE a bit shorter than it should have been).

    At that point I then started the advice from above. I adjusted the choke coil to the point JUST where it fully closed the choke blade. That didn't get me close to alligning with the mark on the fast idle cam, so I had to bend that rod to get it propery alligned.

    The choke pulloff has always been set to spec at 7/16.

    At that point things appear to be working repeatedly. I would re-set all the linkages, pull the throttle lever, and the choke would snap closed and allign perfectly to the center mark.

    Put the carb back on the car and it started right up. I hit the gas and noticed right away that the idle stayed higher than normal so when I looked at the carb I noticed it was set on the 2nd step of the fast idle cam. That was already better behavior than before.

    I haven't had a chance for extensive testing but so far it appears to work better. I'll be paying attention during regular driving to see how things go.

    This is all much easier with the carb off the car!

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