Let's paint a picture: You've just stuffed your new engine between the framerails of your hot rod. All the hoses and lines are hooked up. It has fresh oil. Even the screw heads on the accessory brackets are shimmed so the slots align perfectly with one another.
And with the first twist of the key, you hear nothing but the annoying whir of the starter motor before-and what seems like 10 minutes later-a "whumpf" of flame and smoke billows from the carburetor.
It's very easy to over-carb...
It's very easy to over-carb an engine, which makes accurate tuning almost impossible, as the too-large carburetor will simply dump too much fuel into the engine. Follow the sizing formula described in the main text's No. 1 tip to zero in on the right size for your engine.
With the battery worn and your nerves shot, you turn out the garage light and head into the house, hoping the carburetor will somehow right itself as it sits for the next day or so. Or maybe the carb fairy will stop by, insert the correct jets, and leave a quarter under your pillow.
With either of the miracles mentioned as likely as your hair turning back to brown, you'll have to get back under the hood and tinker with the carburetor to get your engine running. For many enthusiasts with even moderate or greater wrench-turning experience, messing with carbs remains an elusive black art-and it's hard to admit you don't know how to do it. Take it from us, you're not alone.
The variety of adjustable components on a typical four-barrel carburetor allows for an almost infinite number of tuning scenarios, but there are a few basic steps to help select, set up, and tune your carb with surprising precision. For this story, the tips are based on applications using a new Holley 4150/4160-type four-barrel (the non-Dominator type), but most are applicable to other popular carburetors, be they of the singe- or two-barrel variety as well.
Don't assume a hard-starting...
Don't assume a hard-starting or stumbling condition is strictly carb-related. Incorrect ignition timing is a common problem on rebuilt and crate engines. Break out your timing light and double-check the timing before cracking open the carburetor.
Face it, that carb ain't going to tune itself. If you want to hit the next cruise night in something other than your wife's Buick LeSabre, grab a wrench and get to work.
1. Don't supersize it. When selecting a carburetor for your engine, don't overdo it. Generally speaking, a mild-to-moderate street engine doesn't need more than a 650- or 750-cfm carburetor. To zero in on the most appropriate carb size, multiply the cubic inch displacement and maximum rpm, and divide the product by 3,456. For example: a 350 engine multiplied by a 5,500-rpm redline, divided by 3,456 equals 557, or at least 557 cfm. In that case, a 650-cfm carb is sufficient. It's OK to go a little larger than necessary, but don't choose a carb that's rated at less than the minimum requirement.
2. Timing is everything. If the first couple of starts don't produce a quick firing and idle-even if they're not perfect-don't jump to the conclusion that it's a carb-tuning problem. The hard-starting scenario mentioned at the lead of this story could very well be due to improper ignition timing. So, before cursing the carburetor as you're ripping it off the intake manifold, double-check the ignition timing. An inaccuracy of only a few degrees could produce a hard-starting/no-idle condition.
This diaphragm identifies...
This diaphragm identifies the carburetor as one with vacuum-operated secondary venturis. For most automatic-equipped, mild-to-moderate horsepower applications, it's the way to go. Save the double-pumper carb for manual transmissions and higher-horsepower engines.
Here's a simple formula: Use a vacuum-secondary carburetor on midsize and large cars with an automatic transmission. Go with a "double-pumper" type mechanical-secondary carb when using a manual transmission application or with smaller cars that have a radical cam profile or are going to see a lot of time on the dragstrip.
4. Power valve power play. The power valve is a vacuum-activated feature that's used to enrich the fuel mixture to prevent detonation or stumbling (metering rod on a Carter-style carb). They're available in different sizes to match the flow requirements of the engine. On manual transmission applications, a standard 6-1/2-inch power valve is adequate if the vacuum reading at idle is above 12 inches. For automatic applications, if the idle vacuum reading is below 12 inches, divide the number in half to determine the correct power valve size. A 9-inch vacuum reading requires a 4-1/2-inch power valve.
Access to the fuel jets (also...
Access to the fuel jets (also known as shooters or squirters) and power valve is gained by unbolting the fuel bowl (and typically some gentle prying to break the seal of the gasket). Regardless of whether the carb is a double-pumper or vacuum-secondary type, Holley four-barrels have matching jets and power valve behind the fuel bowl on the opposite side of the carb.
With the fuel bowl removed,...
With the fuel bowl removed, the power valve and jets are easy to reach. They're attached to another removable component called the metering block. The power valve is the larger, centered component, with the jets located below it.
The jets are numbered, with...
The jets are numbered, with larger numbers indicating larger-diameter orifices for the fuel to flow through. Removal and installation is as easy as screwing them in or out with a flat-blade screwdriver. When changing jets, experiment in small-size increments, as minor jet size changes can have a comparatively large effect on performance.
Changing the power valve requires...
Changing the power valve requires removal of the metering block in order to loosen it from the block.