If you have an engine that’s a heavy breather at idle because of a large camshaft or engine displacement, chances are you’ve come face to face with an idle adjustment issue. To keep the idle up, one might crank the primary throttle plates open to compensate. This practice is fine in minute increments, but if the throttle plate is opened too much, the idle circuit is bypassed and the chance for stable idle is lost.
First, we’ll address how to properly set your idle. Holley sets idle mixture and speed from the factory on a wet-flow bench to give a solid baseline, but chances are by now things have been moved around. To start from scratch, lightly seat the idle mixture screws then back them out 1.5 turns. You will have two or four mixture screws located on each side of the metering blocks depending on whether you have a two- or four-corner idle carburetor model. There is a tapered needle on the opposite end of the mixture screw that interrupts fuel flow to the engine. Keep in mind that turning these screws in all of the way cuts fuel off from the idle circuit and backing them all the way out is full rich.
To attain a healthy idle mixture, attach a vacuum gauge to a manifold vacuum source. Adjust the mixture screws evenly by alternating between the two or four screws in 1/8-turn increments. The goal is to achieve the highest manifold vacuum reading possible.
If the engine seems to be running rich at idle and the idle mixture screws aren’t clearing it up, there are a couple of things to check. The first is the power or enrichment valve. This valve is a gateway from the fuel bowl into the main circuit. It’s opened when the vacuum drops to the number stamped on the valve under acceleration. However, if vacuum at idle is lower than the vacuum rating stamped on the valve, it will add fuel to the mix when you don’t want it. The rule of thumb is to use the power valve that opens at half the idle reading. For example, an engine with 10 in-Hg (inches of mercury) idle will call for a 5.0 power valve. If the engine makes 12 in-Hg at idle or more, the 6.5 valve that comes in most carburetors will be sufficient.
With idle mixture set and the correct functioning power valve in place, it’s time to look at where the throttle plates lay on the walls of the venturii. When the throttle plate is closed no more than .020-inch of the transfer slot should be exposed on the primary side. To assess this exposure without a measuring device, the slot should appear as a perfect square. In other words, the slot should only appear as tall as it is wide. If there is too much exposed, back the primary idle speed down until the proper amount of the transfer slot is visible, and then crack open the secondary throttle plate using the secondary throttle plate adjustment screw sometimes located on the underside of the throttle plate on the intake manifold mounting surface. This adjustment should not uncover more than .020-inch of the transfer as well.
You may be asking why are we avoiding the transfer slot? The transfer slot is what transitions us from the idle to the main circuit. As the throttle is opened and the transfer slot is exposed to manifold vacuum, fuel is drawn in through the transfer slot circuit. If this happens at idle rather than acceleration, it will cause extreme richness and poor drivability.
When all idle mixture and throttle plate position adjustments are exhausted, we must move on to modification. The root of the problem is that there is plenty of fuel but not enough air at idle. We must somehow allow air to pass into the intake manifold without engaging the main circuit. An easily performed but more difficultly reversed modification is to drill holes in the primary throttle plates. Holley’s technical department advises this is a last-resort modification and that starting small and moving up slowly is the smartest way to go about it. Holley recommends starting with a 3/32-inch bit drilling carefully into the mid-point between the center and the outer edge of the plate on the front half. Being careful not to spread shavings into the engine or the carburetor, clean the surfaces well before reinstalling the carb onto the engine.
If drilling isn’t for you, there is an excellent option. Holley’s new all-aluminum Ultra HP carburetors feature an idle-bypass valve. It allows air to flow through the main body to the underside of the carburetor separately, rather than the air moving past the throttle plates. The adjustment is located under the air cleaner stud and can be manipulated with a flat-blade screwdriver. Upgrading to this style of carburetor allows more flexibility for idle adjustment without altering the throttle plate and transfer slot relationship. Just because an engine needs more air at idle, doesn’t mean it needs more air at wide open throttle, so fixing the idle problem with a bigger carb may be more detrimental than helpful. The new Ultra HP brings 30 new features to the table for the same price as the original Ultra HP.
Now you can ensure you’ve got your Holley carburetor set up just right for your engine without damaging the carburetor or the eyes and lungs of people nearby.
By Liz Miles
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