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carb
Jun 5, 2007 0:29:12 GMT -5
Post by DOCTOR FIELDS on Jun 5, 2007 0:29:12 GMT -5
A couple of things will cause poor idle. Idle circuit jet too small, common problem if ran with a high lift cam. You may have the idle transfer slot completely bypassed, and your idle circuit is unresponsive. I could tell you how to fix both, but i would have to kill you lol. Send it down my way if you don't get it straightend out, and i will set the idle circuit for you. Thanks, Stephen Fields
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carb
Jun 5, 2007 9:25:19 GMT -5
Post by thescottydog on Jun 5, 2007 9:25:19 GMT -5
I would do the following, in order:
1. Remove the fuel bowl lid, invert it and set your float at 15/32". Measure from the toe of the float to the surface of the lid (no gasket). Make sure both lungs of the float are at the same height.
2. Install the following on both the primary and secondary side: main jet- 160 main air corrector- 180 emulsion tube- 16
3. Set your fuel pressure at 3 psi- no less and absolutely no more. If you have more than that, get a fuel pressure regulator, Holley PN 12-704, install it according to instructions and set your pressure. Correct fuel pressure is absolutely critical and is non-negotiable.
4. Setup your idle system- instructions below (they're for a 4bbl. Holley but the principles are the same).
Most of your problems should go away.
Idle system setup- method 1
To start with, invert the carburetor and check the position of the throttle butterflies. Turn the idle speed screw to set the bottom edge of the primary throttle butterfly near the bottom edge of the lowermost transfer idle hole (you want to see about 10% of the diameter of the hole). Don't worry about measuring anything- your eyeball is good enough. Repeat the process on the secondary side. Positioning the throttle butterflies near the bottom of the lowermost transfer idle hole is absolutely critical for maximum acceleration.
Next, turn the idle mixture needle in until it is lightly seated. Excessive force here will damage both the needle and the hole and make the idle system difficult to set with any accuracy. After seating it turn it out 1 1/4 turns, which is a good baseline setting. Now you're ready to reinstall the carburetor and setup your idle system.
Find some place to tap into the manifold, attach a good vacuum gauge and position the gauge where you can see it clearly.
Start the engine and warm it up. Open the throttle by hand- don't use the idle speed screw to open it. When the engine has warmed up let the throttle lever rotate back to the idle position. If the engine dies, which is likely, you'll have to temporarily turn the idle speed screw to increase the RPM to get it to idle. Count the turns and fractions of turns so you'll know exactly how much you've opened the butterfly. Since the whole idea is to adjust the idle mixture needle to achieve the correct idle mixture, which will raise RPM and allow you to return the throttle butterfly to the position you originally set it at, you'll need to know how far you had to open it to begin with. As the engine warms up it should gain rpm, so you should be able to reduce the throttle opening at least somewhat without the engine dying. Now the fine tuning begins.
With the engine idling turn the idle mixture needle in 1/4 turn while you're watching the vacuum gauge. Give the idle a few seconds to stabilize. If manifold vacuum increases repeat the process, letting the idle stabilize after each adjustment, until it starts to decrease. If turning it in decreases manifold vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the manifold vacuum is as high as you can get it by adjusting the idle mixture needle) then the RPM should have increased to the point that you can return the primary butterfly back down to the original, correct position. As a final check give the idle mixture needle a slight turn in then a slight turn out. If any motion hurts manifold vacuum, you know that needle is set properly. At this point if the idle is stable and the engine responds quickly when you just crack the throttle, you should be good to go.
How difficult it is to find the sweet spot largely depends upon the cam profile. With a mild cam it's usually easy- there is a very definite point where manifold vacuum is the highest and a small adjustment either way will reduce it. As the cam profile becomes more radical it becomes commensurately more difficult to find the sweet spot. With a really radical race cam very often the only way to set an idle mixture screw in the proper position is to turn it in until vacuum starts to decrease, then turn it out (often a turn or two) until vacuum starts to decrease, then turn it back in until it's positioned between the two extremes.
One final note: Make sure your timing is set correctly before starting this process.
Idle system setup- method 2
If your distributor has a mechanical advance system there is another effective, though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needle as described above, attach the manifold vacuum gauge and start the engine. Allow the engine to warm up, then close the butterfly as much as reasonably possible without the engine dying. Attach a timing light, check to see how much initial ignition advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as to increase the initial ignition advance. When the initial ignition advance is increased the RPM should rise as well, allowing you to reduce the throttle butterfly opening. Simply turn the distributor to increase the initial ignition advance and continue to reduce the throttle butterfly opening until they're in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move, then adjust the idle mixture needle for best manifold vacuum. Once they're properly set if the idle RPM is higher than desired, loosen the hold down clamp and turn the distributor slightly to achieve the desired idle RPM. Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's advance curve. The first step is to attach a timing light and recheck the initial ignition advance. Let's say, for example, that it was originally 15° and now it's 22°, a 7° increase. If your total ignition advance was originally 35°, in order to keep that figure the advance curve will have to be shortened by 7°. Assuming you have a centrifugal advance system you'll have to limit how far the advance weights can move outward, which will limit the total advance. The method required will vary from distributor to distributor, so I won't get into that here, but any competent technician with a good distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can come up with a way to limit the outward motion of the advance weights, you can do the same thing using your engine as form of distributor machine. Limit the motion of the weights somewhat, make sure you have the correct initial ignition advance then check to see how much total ignition advance you have. If the total ignition advance is still too high, just continue to limit the motion of the advance weights until you achieve the desired total figure. This was posted by Earl Parker of Parker Carbs on another forum
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carb
Jun 5, 2007 9:31:37 GMT -5
Post by DOCTOR FIELDS on Jun 5, 2007 9:31:37 GMT -5
I see somebody reads 4M.net That's a tech article from Parker Carburetion off 4m. He's really good on the 5200 series carbs!
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carb
Jun 5, 2007 9:33:14 GMT -5
Post by thescottydog on Jun 5, 2007 9:33:14 GMT -5
LOL ,learn what u can where u can.
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carb
Jun 5, 2007 9:39:46 GMT -5
Post by DOCTOR FIELDS on Jun 5, 2007 9:39:46 GMT -5
Thats right! I've learned alot off 4m in the ministock tech forum. Not many (if any) topics about Chevettes, but the basics apply to nearly anything.
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