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Troubleshooting Tips
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.
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 Carburetor 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 carburator shop.
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