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Discussion Starter #1
Ok, I've tried everything I know of to get the idle speed set properly and I'm not having success on all fronts. How it idled at 500RPM the first time I fired her back up, I have no idea.

1) I checked the floats to make sure they aren't set too high. They don't dribble ANY fuel at idle, so if anything they're set too low. This shouldn't affect idle speed.
2) The idle speed screw is ALL THE WAY OUT.
3) Both idle mixture screws are 1/2 turn out.
4) Ported vacuum and manifold vacuum ports are closed off.
5) There may be a slight vacuum leak at the carb base. Squirting starting fluid at the base of the front of the carb causes idle to blip a little bit.
6) I disconnected the electric choke to test that this wasn't a fast idle screw problem. The choke linkage is completely unhooked at the moment. I may still need to pull the fast idle lever to make sure that isn't part of the problem. A new lever was part of Holley's electric choke kit.

So far, the lowest idle speed I can get is 1200 RPM and it still runs at 1000 with the mixture screws completely in. :awkward: It was idling at 1500+ before I first played with the mixture screws (they started at 1.5 turns out) and turned the idle speed screw all the way out.

I'm stumped. Any more ideas?
 

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Make sure you dont have any timing / ignition issues before you start trying to tune it. Once you get the ignition curve set up and its firing on all cylinders when it suppose to then go back and re check the carb.
 

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Are the secondaries closed? Should be a separate screw under the carb that sets them. Has someone drilled the throttle plates so they have holes in them? Many carbs have been drilled to resolve idle slot issues. If they are drilled that might do it too. RM
 

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Discussion Starter #4 (Edited)
Timing was set at 11* BTDC with my vacuum secondary carb just a couple of weeks ago. The entire engine setup has sat idle since then and ran great with the VS 750. The only change to the entire system was the installation of the new carb last week.

The throttle plates have not been drilled.

Where is the secondary adjustment screw located on the carb body?
 

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Since it seems pretty certain you do have a vacuum leak at the carb base, I'd fix that first, then keep looking if you still need to.
 

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The secondary screw will require you to turn the carb upside down to see it. If you look, it will push on the arm that attachs to the secondary throttle plate shaft.
 

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Confirm that your throttle linkage is allowing the primary throttle blades to return to resting position.
 

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Discussion Starter #8
Real McCoy said:
The secondary screw will require you to turn the carb upside down to see it. If you look, it will push on the arm that attachs to the secondary throttle plate shaft.
Ok, I think I saw the adjustment screw, if it's the one that sits below the secondary accelerator pump lever. ??

Since I have to pull the carb to check out the gasket anyway, I'll take a peek at the secondary slot adjustment.

Thanks for the tips guys. I won't get a chance to play with it again until Sat night or early Sunday. I'll let you know what I find. :)
 

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Discussion Starter #9
DriveWFO said:
Confirm that your throttle linkage is allowing the primary throttle blades to return to resting position.
Did that. No problem there. I can't pull the throttle any further forward, and the linkage isn't keeping it from moving farther.
 

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If you take the carb off and turn it upside down, You'll see the adjusting screw on the right side. It will be under the the secondary throttle arm. It takes a small flat head screwdriver. I replace the screw with a 5mm allen bolt with a lock nut. This way I can do my idle adjustments with the carb on the engine.
 

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Discussion Starter #12
Mike Goble said:
Make sure your fast idle screw is disengaged.
I completely removed the choke assembly from the carb to rule out that possibility. There's no cam for the fast idle screw to hit right now. The choke linkage is hanging completely loose at the moment so that it can't affect throttle position.

That's what I first thought also.
 

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I'm guessing the secondary butterflies are open too far. If they are exposing any of the transfer slot on your mildly cammed setup, they are open too far. I usually start with the top of the secondary plates even with the bottom of the transfer slot (i.e. no slot exposed) and go from there. You can adjust the small screw with the carb on your perf rpm with a very small screwdriver.

When all is said and done you need at least 0.020" of the primary transfer slot exposed. Otherwise you risk a lean stumble when tipping the throttle in.
 

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I saw a carb do that once. The rear throttle plates had slipped and weren't closing all the way. It looked like someone had removed them at one time anddidn't put them back in correctly.
 
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Well...I'd say the secs aren't an issue, and you shouldn't change the screw setting for them, unless the carb was assembled incorrectly. that secfs screw is there NOT to supplement idle quality, but to make sure the secs plates don't BIND in their bores when the plates are closed. This is their ONLY function. To use them any other way is a crutch and INCORRECT. To correctly reset the screw, turn it out until the plates are closed in the bores and feel "sticky", then to the zero point, then 1/8th turn, no more.

Now, for the real problem, air leak. I have found some carbs and manifold carb pads, even those that should "match" can get leaks from small passages under the carb, because the carb pad cannot fully support the carb to pad gasket, and therefore leaks small leaks into one large one. To fix this, use two gaskets and an Edelbrock plate, p/n 2732. This plate seals the manifold to it, then gives the area under the carb to support sealing the ENTIRE gasket to the carb, no droops, no leaks.

Also, I'd suggest either taking the front metering plate OFF the vac secs carb and putting it onto the double pumper, replacing the 650 front plate, OR, drill and tap both the idle restriction and air corrector jet areas on the 650, and use 6/32 Allen set screws to make the carb both air corrector and idle jet adjustable. Start with the orifice sizes the stock vac secs metering plate has, go from there. Use a hand help pin vice NOT A DRILL PRESS) to bore the Allen set screws, and you can get aluminum set screws from McMaster-Carr.

Front metering blocks in DP's are set for the more radical engine packages, a milder setup will need adjustment to a lesser setting, right around what a vacuum secs carb utilizes.

Also, beware the emissions carbs. Emissions calibrated carbs use things like reverse idle systems, not with fuel adjustment screws, but with air adjustment screws, their richest setting is with the screws turned all the way IN onto their seats, and even then, they can't get richer until the metering block is modified.

These emissions type air screws differ in appearance to the regular Holley fuel screws. The regular fuel screws use a screw slot that has the same diameter as the threads. The air screws use a screw end that appears to taper larger towords the threads, and has serrations on the oouter part of the taper. Although the air and fuel screws will interchange into any metering block, it isn't the screws that would be the main issue, but the type of metering block they came in stock, emissions (air adjuster) or performance (fuel adjuster).

The DP should have fuel screws, although there are those very few specialty LIST number DP's that are emissions cerified for a very small number of factory "performance" engines, and use the reverse idle systems. Case in point, replacement carbs for Q-Jet, called the "Spread Bore". ALL vacuum secs Spread Bore carbs use emissions reverse idle systems, asa they are designed to replace a stock emissions Q-Jet. The "performance" Spread Bores are all mechanical secs, NOT emissions certified and use the stock performance fuel screws.
 

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Discussion Starter #16
Thanks for the info IM. It's a bit over my head to make those kinds of mods to my carb at the moment. I'll save it off for when I'm feeling ambitious though. :)

I took the carb back off this morning, reinstalled the choke housing paying special attention to binding and linkages, and checked the secondary to bore clearances (they were spot on as IM suggests).

I took a look at the carb gasket and noticed that it didn't appear to be sealing properly based on the marks on the gasket from the carb base plate and fuel marks. I flipped the gasket around when reinstalling it this time.

I had much better luck this time when firing her back up. With the idle speed screw all the way out and the mixture screws 1/2 turn out, she didn't want to run at all unless I had my foot into it a bit. (this was actually good, letting me know I was in the ballpark) I moved the mixture screws in another 1/2 turn and set the idle speed screw a bit higher and she's purring again at 650 RPM for idle speed.

It looks like my entire problem was what in retrospect appears to be a huge air leak.

Thanks guys. Based on your input, it looks like I can safely get my car to the NW Nova Club meeting on Sunday. :D I'll just play with idle mixture a bit more to bring vacuum up to its best point and I'm ready to run.
 

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that secfs screw is there NOT to supplement idle quality, but to make sure the secs plates don't BIND in their bores when the plates are closed. This is their ONLY function. To use them any other way is a crutch and INCORRECT.
Well, I guess Holley must be mistaken, this is from their own tuning instructions:

If the idle speed is slower than recommended, turn the screw clockwise to speed up the rpm. If the idle speed is too fast, turn the idle
screw clockwise to slow down. This adjustment should be made to both the primary and secondary screws in equal amounts, so that the
throttle plates are opened the same amount.
I always use both sides, just like they were meant to be used. Demons even have an easily accessed screw on top for that purpose.
 

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I'm no expert but I know what worked for me!

Opening up the secondaries a little allowed me expose less transfer slot on the primary side. Didn't want to drill holes!
 
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