Here’s the latest reader question, along with my reply!
Mike asks: I know you’re an old school carburetor guy so I figured you’d be the right guy to ask about putting spacers under a carb. I’ve read and heard about doing this as a way to get more power out of the engine. Is it true? Are there any downsides?
My reply: It is sometimes true – and there are downsides, including the possibility of screwing up the geometry of the throttle/kickdown linkage and air cleaner/hood clearance relationships. 
The general idea behind spacers is to increase the velocity of airflow and thereby create more power; however, depending on the engine and factors such as how it is cammed as well as the flow characteristics of the existing/factory intake manifold, a too-high spacer can result in worse airflow and reduced power. This is why people usually experiment with them. The problem is that without a dyno, it is very hard to objectively quantify the results as the differences are usually small (maybe 3-5 horsepower – maybe). What sometimes does change noticeably is the way the engine feels/responds. It may seem more – or less – so. It may feel a little stronger at lower – or higher – RPM.
It’s fun to experiment!
And no real harm can come from it. Just be sure to make sure the throttle arm/kickdown adjustments are right and don’t close the hood hard before you have established the clearance relationships!
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Agree that it takes experimenting to find out.
In general (but not always) an open spacer works better on top of a dual plane and 4 hole spacers work better on single planes. Some times the only benefit from a spacer is that it insulates the carb from manifold heat. Have also had to retune power enrichment due to a spacer before (power valve on a holley/springs on an edelbrock).