Keeping the Carbs

Troubleshooting Tips

Bob Mehlhoff Jan 5, 2005 0 Comment(s)

They're cool looking, set on top of your motor, and your foot gets to send them the orders. Yes, though the carburetor is synonymous with hot rodding, it certainly pre-dates that era by decades. But whether your carburetor is one of hot rodding's newest designs or a vintage '50s four-barrel, at times it can be plagued with the "just not running right" syndrome.

To help with that, we've compiled tons of carburetor troubleshooting tips taken directly from the hottest sources. Everything from the pitfalls to tuning tips, common adjustment problems, and plenty more that should get your ride running better than ever. And one of the best parts is that these are economical and easy-to-fix solutions. So whether your carb woes have your engine stumbling, idling rich, or just not making the power it used to, we aim to help.

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One of the worst enemies is dirt. It can become lodged inside the tiny air bleeds that are present on carburetors like this Demon or on Holleys. To clean them, simply spray a shot of carburetor cleaner into each of the bleed holes. If you need to remove anything more stubborn from the bleed hole, a long wire bristle removed from a wire brush and held with needle-nose pliers works great. Sorry, a paper clip is too big.

For most troubleshooting, it's best to remove the carburetor and carefully examine it as you take it apart. This one is being disassembled at Ted's Carb Service. Ted Granger tells us that the previous owner had not lubed the upper-cover screws on the secondary diaphragm unit with WD-40 during installation. Consequently, the rubber diaphragm was torn as it twisted around the screws.

A very common problem on Demons and Holleys is the maladjustment of this accelerator pump arm. For proper adjustment, there should be zero clearance on this arm with the throttle arm closed (idle position).

There should also be no clearance with the throttle simply held at wide-open throttle. There should be just a very small amount of clearance (0.020") available by depressing the accelerator arm farther down and holding the throttle at wide-open position.

To choose the right power valve for your engine, you'll first need to measure the vacuum at idle. With an automatic transmission, you should measure the vacuum with the transmission in drive, the engine idling warm, and the parking brake set. Obviously, manual transmissions should be in neutral. After you've read the engine vacuum, install a power valve that is at least two or three full steps down from your reading. As an example, if your engine vacuum reading is 12", you should install at least a 9.5- or 8.5-power valve or even a 6.5 as supplied with most kits.

If your carburetor has vacuum secondaries and a noticeable bog on full-throttle acceleration, the secondary spring may be too weak. Test and tune the performance with stiffer secondary springs that open up more slowly (as engine speed and vacuum warrants). Engine size (displacement) also affects the opening point.

Almost all threaded holes on carburetors need not be tightened down hard. A common problem found on many rebuilds is stripped threads, which almost always turns a good used carburetor into a parts bank.

The choke mechanism on this carburetor has been separated from the main body to demonstrate how the choke rod should be positioned during assembly. In this case, the rod is properly installed under the plastic lever. Dustin Granger of Ted's Carb service tells us that many carburetors come in with this rod installed above the lever, causing the choke to operate incorrectly.

When disassembling a carburetor, don't remove the staked screws that hold the throttle blades to the shaft. In other words, don't disassemble the carburetor more than it needs to be during a rebuild. These are generally only removed to replace worn throttle bushings and is to be done by an experienced carb shop.

After years of service, the throttle lever can work loose on the throttle shaft. This problem is easily solved by tack welding the shaft to the lever.

On this Edelbrock Performer Series carburetor, the float should be adjusted with the lid upside down, 7/16" above the gasket. An accurate and easy way to measure this is by placing a 7/16" drill bit underneath the open end of the float.

Next, turn the lid right side up and measure the float to provide the recommended (at the front of the float) clearance found in the instructions. This dimension is typically about 1".

Before installing the accelerator pump on an Edelbrock Performer Series carburetor into the main pump, fill the cavity half full with WD-40. Then, by hand, install the accelerator pump and depress it several times. If everything is working correctly, the pump will squirt the WD-40 into the carburetor venturis.

As with all carburetors, don't over-tighten any screws. A common problem on many Quadrajets is over-tightening these rear screws, which causes the top plate to bulge out, hanging up the secondary air door and keep it from opening properly.

Make sure to blow out all passages with compressed air. On this Holley carburetor, the airflow blown through the front of the main body travels upwards and out of the top of the air horn on the primary side. This can be monitored for even flow by placing your hand above the air horn.

Accelerator pump covers found on many Demons and Holleys are often over-tightened and warped. To check them, place a straight edge across the mating surface. If they are warped, they can typically be sanded down on a flat surface to become true again.

To check the float level on Demons and most Holleys, allow the engine to idle on a level surface and adjust the float level so that the fuel is just barely below the sight hole. You can verify this by gently rocking the car. Be careful and only adjust one side at a time.

To adjust the idle mixture on most carburetors, begin with the idle mixture screw out about one-and-a-half turns. For most musclecars and performance engines, adjust to the highest idle (rpm) or the highest manifold vacuum. Some Holley carburetors have adjustments that turn in the reverse. Watch for labels. For most carburetor tuning, you'll need a vacuum gauge and a handheld tachometer.

On engines equipped with Quadrajet carburetors, make certain that this plastic cam that raises the secondary hanger is not worn. If it is, the secondary operation will be diminished and reduce full-throttle power.

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