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Any AFB gurus here??

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BU1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BU1 Quote  Post ReplyReply Direct Link To This Post Topic: Any AFB gurus here??
    Posted: Oct/03/2022 at 7:37pm

 Having an odd issue with an 68 AMX AFB where the throttle is slow to return to idle. With the motor off and the linkage disconnected, the springs on the carb are enough to close the throttle just as they should. But with the motor running, the throttle is noticeably a bit stiffer to open, and the carb springs now will no longer close the throttle. RPM's will hang wherever they are when you let go of the butterfly. You have to manually push the throttle closed. What would cause the primary throttle shaft to act tight with the motor running???    
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#1 1968 AMX Rally Green 343 Z code 4spd since 1975 #02642
#2 1968 AMX Rally Green 290 N code 4spd since 2019 #02959
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmaher94087 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2022 at 8:50pm
This sounds like a warped throttle shaft.  It probably occurred with a backfire through the carb.  Replace the throttle shaft.
Mike
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BU1 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2022 at 9:01pm
Originally posted by mmaher94087 mmaher94087 wrote:

This sounds like a warped throttle shaft.  It probably occurred with a backfire through the carb.  Replace the throttle shaft.

Don't think so.  Warped shaft should be sticky at all times, no?? This one only sticks with motor running. 
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#1 1968 AMX Rally Green 343 Z code 4spd since 1975 #02642
#2 1968 AMX Rally Green 290 N code 4spd since 2019 #02959
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2022 at 10:17pm
What if the motor is running but the throttle cable is disconnected?  That scenario was not covered.
May be a problem with the throttle cable.
Bill O.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mmaher94087 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2022 at 10:21pm
The 4-speed, AEB, has a dashpot to slow the closing of the throttle but beyond that, I can't think of anything that would cause the throttle to hang up.  I'm back to a warped throttle shaft.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Oct/03/2022 at 10:59pm
The throttle body of any carburetor can warp and create a misalignment of the throttle shaft bore. Overtightening the mounting studs usually causes it. The warp can change as the carburetor heats and cools during operation. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mixed up Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2022 at 1:34am
check all the linkage on the chock side and have you had carb off recently were gasket might be a trouble
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sweatlock Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2022 at 6:08am
Don’t laugh but you can have the AFB carburetor base gasket mounted back to front - the hole diameter between the primaries and secondaries on a Carter AFB are very close to being the same size. If you mount the gasket backwards, the secondaries will stick. 

Edited by sweatlock - Oct/04/2022 at 6:18am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BU1 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2022 at 7:32am
Originally posted by troutwilly troutwilly wrote:

What if the motor is running but the throttle cable is disconnected?  That scenario was not covered.
May be a problem with the throttle cable.

 68 has throttle linkage, no cable. Linkage is disconnected as stated in my original post.
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#1 1968 AMX Rally Green 343 Z code 4spd since 1975 #02642
#2 1968 AMX Rally Green 290 N code 4spd since 2019 #02959
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BU1 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/04/2022 at 9:37am
[QUOTE=mmaher94087]The 4-speed, AFB, has a dashpot to slow the closing of the throttle . QUOTE]


 FTW!!!!!! I thought when I installed the carb that I had the dashpot adjusted so it would not come into play. I was wrong! It was contacting ever so slightly at the very end of throttle travel. 

 But, the throttle still hangs with the engine running and the throttle linkage disconnected. Turn the engine off, and you can watch the throttle close on it's own. Never seen a carb do that. It's all good though, as the throttle returns properly now with the linkage connected.  Cool
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#1 1968 AMX Rally Green 343 Z code 4spd since 1975 #02642
#2 1968 AMX Rally Green 290 N code 4spd since 2019 #02959
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