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Old 07-30-2005, 09:03 PM
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Performance cam with a 2 corner idle Holley DP 650

I was just reading another thread about adjusting a Holley DP, and it was mentioned that a 2 corner idle carb just doesn't cut the mustard when teamed with a performance camshaft. Unfortunately, I have that combination on my 355, and I have difficulty getting the idle to lean out to the point where it does not hurt my eyes, without causing poor off idle performance. Another anomaly I seem to have with this carb is that when I do get it tuned to an appropriate compromise between idle richness and off idle quality, I find that I have to have it idle very high to avoid having the motor die when it goes in gear from too much RPM drop. I'm guessing that my solution is to drill a couple small holes in the throttle blades to give the engine a little more air at idle, but I really do not want to do this if there is another way to get more air in, such as farting around with the secondaries. Is there a way to open the secondaries a little more on a 2 corner idle carb? Is there another trick that can be used? I plan on eventually selling this carb so I really want to avoind drilling, but until then I need to make it work.

Thanks for the help in advance.

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Old 07-30-2005, 09:05 PM
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what's your duration at .050", idle vaccum , power valve size and stall size?

I think you need a different PV & work on the pump shot! lol sounds crazy, eh?

Last edited by NXS; 07-30-2005 at 09:15 PM.
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Old 07-30-2005, 09:13 PM
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yes

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Is there a way to open the secondaries a little more on a 2 corner idle carb?
Take the carb off. There is a small screw in the base plate that opens the secondary linkage. Holley recommends it be open 1/4 turn. You can open it more but if you open it too far. Appx 1 full turn. You will not be able to get the car to idle down. Where you have an idle problem. I would start opening it until you get the carb where you want it.
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Old 07-30-2005, 09:28 PM
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Take the screw out then install it from the top, this way you can make adjustments while on the car, I usually change the screw to an allen type. Make sure your idle air bleeds are clean.
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Old 07-30-2005, 09:37 PM
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would there be any drilling needed to install the screw from the top? This would be a killer mod because it would definitely make tuning a lot easier if I didn't have to take the carb off every time I needed to adjust the secondaries.

I will check the air bleeds, but as far as I know, there is nothing in them because the carb was recently professionally reconditioned.
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Old 07-30-2005, 09:40 PM
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No drilling to do just turn the screw around, done it many times. Bob
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Old 07-30-2005, 09:50 PM
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Kewl, thanks for the tip!
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Old 07-31-2005, 12:43 AM
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The threads are tapered in the baseplate. Doing that will make the screw rock eventually. That will cause irratic idle. I run a 1" carb spacer. I can reach the secondary screw without taking off the carb.

I always set the primary throttle blades so that .020" of the transition slots are exposed. Then, I adjust the idle from the secondaries. On a 2 corner idle carb that is the best way to go. Otherwise the primary idle circuit doesn't work as well as it could.

The 2 corner carb is fine.
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Old 07-31-2005, 07:00 AM
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The threads are not tapered they are a basic SAE thread that is tapped all the way through the baseplate. The screw will not rock and will be in basically the same position but reversed so the adjustment can be made from the top, done it many times with no problems, I like an allen head design set screw though and usually change to that.

The 2 corner carbs also idle off of the secondary side through fuel transfer slots carved into the top of your baseplate, 4 corner just makes for more adjustability.

Mad Maggot, if you can't get the richness to go away by adjusting the throttle blade opening then you'll need to open up the idle air bleeds slightly or close up the idle fuel bleeds on your metering block-(the easiest way is with a strand or two of fine wire sandwiched by the gasket or epoxy them shut and redrill-just remember small adjustments in orifice size make big differences) The idle air bleed and fuel bleed size determine your supplied idle fuel ratio, the air/fuel needles on the side of the metering block only adjust how much fuel is being delivered to the engine at idle. Hope this made sense. Bob
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Old 07-31-2005, 07:28 AM
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you will need a higher stall torque converter to prevent the idle drop when you put it in gear. The power valve is another important considration, as the idle vacuum will be substantially lower with a long duration cam. Get a vacuum gauge on it and see what your power valve has on it. most often they are 6.5 in. vacuum This would be okay for an idle vacuum of 11- or 12 inches. sometimes the rupture if you have a backfire through the carb when you are setting the timing.
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Old 07-31-2005, 07:32 AM
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Supposedly the carb was just rebuilt so I'm assuming the powervalve is new, hopefully it isn't blown or yeah it would cause a really rich condition.
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Old 07-31-2005, 12:06 PM
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The threads do taper. Haven't you noticed you have to force the set screw down to get it to start in the baseplate?

Eventually the screw is going to rock. It is hard to tell at first. When the screw gets bad it will raise and lower the throttle opening just enough to screw up the mixture at idle. It will start to piss you off and will take a while to track down the cause. You can remember this now, or you can find out the hard way later. It is up to you.
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Old 08-01-2005, 06:48 AM
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I am just going to argue for the sake of arguing....

If the threads were tapered the screw would become very hard to turn in a few turns and would be loose if you ever had to back it out. I am sure it is just a machine fit thread that is nice and tight so it does not come loose. I have heard of alot of people turning over the screw.

Chris
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Old 08-01-2005, 09:47 AM
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Secondary Speed Screw.

Unless it was changed recently they threads in the baseplate for the secondary speed screw are not tappered. It's a small screw, and does not have a lot of material around it which can cause it to get loose.
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Old 08-01-2005, 07:52 PM
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That may be true about the taper, but I am sure about the screw rocking. Every time I have tried to turn the screw around it doesn't fit as easily from the top as it does from the bottom. And, when I have tried that trick, the idle becomes erratic.

Any of you that have done this should check your secondary screws. I bet you can change your idle speed by pressing down lightly on the secondary arm. If that is the case, your carb won't return to the same idle speed every time. That will make it so your mixture at idle is never correct.

Those Quick Fuel adjustable secondary arms suck too. They will rock really bad. The metal in them is weaker than the carb's throttle return springs. They get worn out quickly.
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