To see the transfer slots, with engine off, use a flashlight and look into the primary bores at the throttle blades. There is a vertical slot in the side of the venturi right above and below the throttle blades. If you have the idle speed screw set too high, the slot gets more exposed below the throttle blades than it is supposed to have and it makes the carb run really rich.
To fix it, remove the carb and drain the gas from it. Turn the carb upside down. Be careful because the accelerator pump might still have some gas in it. If you work the throttle it will squirt gas at you out the bottom of the carb. I find it good practice while draiing the carb to hold it in upright position and work the throttle until the accelerator pump is empty, just for safety.
.Look at the amount of the transfer slot that is exposed below the blades. Be sure the carb is off the choke cam which would artificially open the blades further. At the base idle position, you should only see the slot exposed enough where it looks like a square under the blades, rather than an elongated slot. If more than that is exposed, you need to set the secondary idle speed higher so the primary idle speed can be set lower.
To set the secondary idle speed higher, there is a screw only accessed from the bottom of the carb that will set the secondary speed. Open it at 1/4 to 1/2 turn increments, or just use some deductive reasoning to figure out about how much it took to close the primaries to get the slot the right size, and then open the secondaries by about that same amount. It is a trial and error thing. Set it, bolt it on and try it. Like Grabber Blu said, you do not want to have to increase the curb idle speed setting too much or more of the slot will be exposed. Doing this will improve driveability and reduce the rich smell.
It is possible to remove the secondary idle speed screw and replace it, screwing it in from the top so you have screwdriver access to it from the top without having to remove the carb. I guess Holley did not want "average Joe" tinkering with it.
Now, something else that requires attention after doing this is the linkage on the side of the carb that connects the primaries to the secondaries. Part of the function of that link (actually the only function on a vacuum secondary carb) is to hold the secondaries positively shut to their set idle speed when the carb is at idle. It should put very slight closing pressure on the secondaries when the carb is at idle. You adjust it by bending it. If you go too far, it will hold the primaries open and increase idle speed. You do not want that. Adjust it so it just barely holds the secondaries closed at idle. Every time you make a significant change in curb idle speed, you must check that linkage to be sure it is doing its job. Failure to do so can result in the secondaries being able to open a little bit and mess with your idle speed consistency.
Idle mixture screws... Start out with the screws all the way in, then backed out 1-1/2 turns. The carb should run in that position. With the transfer slot set up properly, the idle mixture screws will actually have some real effect on the way the carb idles.
You may be surprised with these changes that the power valve can be leaned out to the factory 6.5 setting which works for 90% of all cases. That will also help reduce the smell. Of course, be sure the float levels are set correctly before doing any tuning on the carb.
With my 351W in my car in years past, I had a idle smell on my car that was unacceptable. Changing the carb as discussed and setting the timing so it was not so far advanced at idle helped all but eliminate it.
What really helped the most was removing the Holley and installing a Quick Fuel carb. It is basically a Holley with some improvements that make it easier to tune.
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MustangSteve wrote:
