750 cfm Holley Carb Throttle Sticking at Idle... Only when Running! - Team Camaro Tech
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post #1 of 15 (permalink) Old Jan 5th, 13, 11:21 AM Thread Starter
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Cameron
 
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750 cfm Holley Carb Throttle Sticking at Idle... Only when Running!

Hello,
Its been a while since I've posted here, nice to see the forum is still running strong.

Anyway, I've got a 69' with a 400 small block with an M22 and its got a 750cfm, dual line, vacuum secondary carburetor that will not idle correctly. It never has in the last several years. It idles too high because the throttle sticks a little bit at or around the idle position. I can quickly jab the throttle and sometimes it will return to the stop, but then you can feel the sticky spot when you press the pedal down again. It does this with and without the gas pedal linkage hooked up.

The weird part is, it only does it when the engine is running. If you shut the engine off and watch the throttle arm, you can literally watch it return to the stop. With the engine off, the linkage works just fine. I removed the carb for inspection this morning and I could find nothing wrong. I'm baffled as to what could be causing this. Anyone got any ideas?

Thanks,
Cameron

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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post #2 of 15 (permalink) Old Jan 5th, 13, 11:34 AM
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Steve
 
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Two things come to mind, vacuum pull off to the choke and maybe vacuum opening of the secondaries.

69 SS, 350/300 h.p., M-20, 12 bolt, matching numbers.


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post #3 of 15 (permalink) Old Jan 6th, 13, 07:50 AM Thread Starter
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

I had some similar thoughts. I just haven't had a chance to look into them yet. Does a Holley have chock pull off?

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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post #4 of 15 (permalink) Old Jan 6th, 13, 08:11 AM
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Steve
 
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Not sure about your carb....look for a small diaphram with linkage to the choke plate.

69 SS, 350/300 h.p., M-20, 12 bolt, matching numbers.


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post #5 of 15 (permalink) Old Jan 6th, 13, 09:25 AM
DjD
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Vacuum secondary Holley's have a venturi (small passage) in the primary throttle bore that leads to the vacuum secondary diaphragm. As air moves through the primary throttle bore it creates vacuum in the venturi which controls the opening of the secondary. I can't see this causing the throttle to stick.

Does your carb have a choke? What kind? Electric, Divorced or Manual?

You've eliminated the throttle linkage, how about air cleaner base interference with the throttle? Also what about a bent throttle shaft or one that is worn to the point that the engine running causes the throttle blades to stick even though operation is smooth when not running....

...Dennis

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post #6 of 15 (permalink) Old Jan 6th, 13, 09:27 AM
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kevin
 
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

this might help.
this 750 came with the engine when i bought it from united racing.
it has no choke started and idled great even when the tempeture was in the mid 30s

there is a bread tie around the wire that holds the linkage open
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post #7 of 15 (permalink) Old Jan 7th, 13, 09:35 AM Thread Starter
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

This is the carb that I've got, or at least the most recent version of it. Mine is about 10 years old. Its only got 10 or 15k miles on it though.

http://www.holley.com/0-80508SA.asp

So, I've got an electric choke. I looked at the primary throttle valves when I had the carb off and all seem fine. Everything worked very freely. I did not check for excessive wear in the throttle shaft bores. I should have some time after work tonight so I hope to be able to investigate this further.

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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post #8 of 15 (permalink) Old Jan 7th, 13, 07:46 PM Thread Starter
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Well, I tried a couple of things tonight. I did unhook the secondaries to see if that made a difference......it didn't. I also reoriented the throttle return spring so that it pulled opposite of the gas pedal linkage. Again, no change.

I removed the carb again to try to figure out what is going on. I did figure out that the secondaries were catching on the baseplate gasket a little. So, I replaced the thick insulating gasket with a regular one from Mr. Gasket. I tried pushing, pulling, twisting, and whatever else I could think of doing to the primary throttle shaft to get it to bind, and I just couldn't get it to, no matter what I did.

I also played with it some more while the engine was running. Its just weird....with the engine running, the throttle linkage is really rough feeling near the idle position, but then I shut the engine off and its as smooth as silk. I even called Holley and they were clueless. They couldn't even make any suggestions. I'm baffled.

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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post #9 of 15 (permalink) Old Jan 7th, 13, 07:53 PM
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Motor mounts loose or trans mount worn causing the engine to torque irregularly cracking open the throttle blades by moving the linkage? Most original linkages will hit the valve covers...

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post #10 of 15 (permalink) Old Jan 8th, 13, 06:53 PM
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Possibly to much mechanical advance in your distributor. Try heavier springs and see if that helps.
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post #11 of 15 (permalink) Old Jan 8th, 13, 07:08 PM Thread Starter
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Cameron
 
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

I haven't noticed any engine movement when idling verses the engine not running. I'm running Energy Suspension Polyurethane mounts.

I did check to see that the problem existed with the gas pedal linkage unhooked..... it did. That should eliminate the possibility of any motor mount problems causing this... at least, I think it does. I'm beginning to wonder if I know anything.

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
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AutoGear M22 4 Speed
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post #12 of 15 (permalink) Old Jan 9th, 13, 03:09 AM
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

You said electric choke, is it wired correctly? Is the motor grounded to the frame, and the body grounded to the frame via factory ground straps or equivalent? I've seen some weird stuff happen when things are not grounded properly...

The only Mustang I'd ever own is a Fender.

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post #13 of 15 (permalink) Old Jan 9th, 13, 06:38 AM Thread Starter
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Cameron
 
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Yes, it has an electric choke. The motor is grounded to the body, and back to the battery. The choke has its own ground wire, also running back to the car's body. Then it gets 12v from switched power.

Thanks for the ideas..... keep'em coming.

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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post #14 of 15 (permalink) Old Jan 9th, 13, 06:06 PM
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Just a thought but you said it only happens when the car is running. It could be one of the primary blades could be sucked by engine vacuum just enough to make it stick on the side of the venturi hole. It could be the smallest piece of dirt on the plate or a piece of gunk on the idle slot. Is the carb set with the idle slot showing .020?(should look square) Idk just a guess. Let us know if you find out.
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post #15 of 15 (permalink) Old Jan 9th, 13, 08:42 PM Thread Starter
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Re: 750 cfm Holley Carb Throttle Sticking at Idle... Only when Running

Mike,
That sounds like a good one to check more into. I've tried to wiggle the throttle arm around while rotating it to see if maybe vacuum is moving the throttle shaft around causing it to bind. Its always worked smoothly when i've tried this though.

The transfer slots are showing a little at idle. My memory says that they are showing a little more than .020", but I really don' recall.


It looks like I'm going to have to put this little project on the back burner for a while. Other priorites have arisen. Thanks for all the help. If I ever figure out what is going on, I'll report back.

49 Chevy 3100 Project
Acro Sport II Biplane
69 Camaro
400 Small Block
AutoGear M22 4 Speed
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