Jetting help

[eddieb1965]

My carb I just bought has 160 primary and secondary jets on the drivers side it has 158's on the passenger. I can see the thought here ,but is this standard? I get a flame out of my driver header on the deacceleration after reving it up.I'm in the process of tuning this carb and this is my starting point.I'm pulling the plugs today for a look,but I'm relatively sure I'm fat and will need to lean it up a bit. I have a jet kit on the way. Thanks for any thoughts on this,Eddie

[Earl Parker II]

It possible that your butterflies are too far open at idle, exposing too much of the transfer idle slot to manifold vacuum. When you rev it up then close the throttle, vacuum goes to the moon and a lot of fuel issues from the exposed slot. You often see this phenomenon on the track when the throttle is closed on corner entry.

Below are two idle system setup methods:

Idle system setup- method 1

To start with, make sure your timing, fuel pressure and float levels are set correctly.

Invert the carburetor and check the position of the throttle butterflies. Turn the primary idle speed setting screw to set the bottom edge of the primary throttle butterflies about .015" from the bottom edge of the transfer idle slot. 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 transfer idle slot at curb idle is absolutely critical for maximum acceleration.

Next, whether you have two or four idle mixture needles, turn them all in until they are lightly seated. Excessive force here will damage both the needles and metering block and make the idle fuel mixture difficult to set with any accuracy. After seating them turn them out 1 1/4 turns, which is a good baseline setting. Now you're ready to reinstall the carburetor and setup your idle system.

Before you start the engine, examine the primary side of the throttle body. Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port connects with a passage in the throttle body that 'sees' manifold vacuum. Remove the plug, attach a good vacuum gauge to the port and position the gauge where you can see it clearly.

Without touching the carburetor, turn the engine over until you have pumped fuel into the bowls. Open the throttle a few times then start the engine. If it dies, which is likely, you'll have the turn the primary idle speed setting screw to increase the RPM to get it to idle while it's cold. Since throttle butterfly position is critical, count the turns and fractions of turns so you'll know exactly where you're at. The whole idea is to be able to return the throttle butterflies to the position you originally set them at. 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, pick one of the secondary idle mixture needles and turn it 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 each time, 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) repeat the process with the other secondary idle mixture needle.

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.

At any rate, once the secondary idle mixture needles are adjusted the idle speed should increase to the point where you can adjust the primary throttle butterflies even closer to their initial setting. After you do that, repeat the whole process with the primary idle mixture needles. Presumably you'll be able to pick up enough idle speed by optimizing the idle fuel mixture that you can close the primary throttle butterflies down to their original position near the bottom of the transfer idle slots. As a final check give each 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.

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 needles as described above, attach the manifold vacuum gauge to the vacuum port and start the engine. Turn the primary idle speed screw to increase the RPM, again taking note of exactly how much you have to turn the screw to open the throttle butterflies enough for the engine to idle while it's cold. Allow the engine to warm up, the close the throttle butterflies 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 needles 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.

Let me know if you need more.