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Engine RPM sticking at 2000 rpm

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tomasont View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomasont Quote  Post ReplyReply Direct Link To This Post Posted: Sep/21/2019 at 9:43pm
Hey Chris,

Did you ever get this issue resolved?  I'm really curious how this turned out. 

Later,

Todd

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Greyhounds_AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2019 at 9:40am
At this point I still have the problem.

I replaced the distributor as it was obviously getting sloppy, replaced the throttle pedal cable with a Lokar, replace the cable bracket with a nice stout aluminum billet piece, added a pair of stronger return springs, and had the brake booster rebuilt by Booster Dewey.

I also tore down the carb and checked/adjusted everything there.

Last night I figured out what is happening, although it's something I've never seen before. When the car is shut off I can run the throttle by hand and it snaps back closed tightly every time.

But if the car is running, the throttle stays where ever I leave it, and doesn't return to the closed position. If I twist it open say 5-10 degrees and get the engine up to 2000 rpm and then let go of the throttle shaft it stays right there. If I do the same with the car shut off it snaps back tightly closed every time.

It's like the differential pressure across the throttle blades forces the shaft into the base plate hard enough to create enough frictional force that the return springs can't overcome it. I suppose I could add more return spring force, but it would be really crappy to drive then. With what I have it's already tending to act very "on/off" in nature instead of smooth. With a stick I really want the smoothest throttle action possible.

I believe the root cause is the aftermarket primary throttle shaft I used during the initial rebuild. I'm guessing it didn't have the correct anti-friction green coating that the real Holley parts have. I'll probably either replace the base plate completely with a new one from Holley or replace the entire carb if I stumble onto a good deal somewhere.


Edited by Greyhounds_AMX - Sep/28/2019 at 9:45am
1968 AMX 390 w/T5
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6PakBee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2019 at 9:56am
Originally posted by Greyhounds_AMX Greyhounds_AMX wrote:

At this point I still have the problem.

Last night I figured out what is happening, although it's something I've never seen before. When the car is shut off I can run the throttle by hand and it snaps back closed tightly every time.

But if the car is running, the throttle stays where ever I leave it, and doesn't return to the closed position. If I twist it open say 5-10 degrees and get the engine up to 2000 rpm and then let go of the throttle shaft it stays right there. If I do the same with the car shut off it snaps back tightly closed every time.


Hmm.  Does this happen with the engine cold (both conditions) or the engine hot (both conditions).  I know you know what I'm driving at, thermally driven expansion interference.  But that is a long shot, I would think the baseplate would expand more than the shafts, ie the clearance would open up.
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2019 at 11:37am
It doesn't seem to ever change, cold or hot. If I drive it to a cruise night when I pull into the parking lot I'm still fighting with the thing to get the revs to come down.
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PHAT69AMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2019 at 3:06pm
Did not read it all.
Throttle Blades have tapered edges and specific top & bottom, front and back edges.
Was that small detail observed when replaced ?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/28/2019 at 11:01pm
Yep - numbers down on the butterflies. This is really crazy, and not something I've ever seen or heard of happening before.
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