Old School Ignition Fixes: Half a Dozen Little Tune Up Tips Part 2
Wayne Scraba
Click Here to Begin Slideshow In our last issue, we presented a half dozen little ignition tune-up tips. They’re all “old school” tech that has worked for decades. This time around, we’ll do the same thing for Holley carburetors. And yes, plenty of folks still use carburetors on their race cars, musclecars and hot rods. Although they might not quite have the finesse of a modern EFI system, they can still get the job done. When set up right, you’ll find Holleys (and Holley derivatives) can prove rock solid and reliable too. Best of all, they’re simple and adjustable. That could be the reason some have trouble with them (turning the wrong knobs to accomplish the wrong things). For more, check out the accompanying little tips:
Jet Extensions: While a Holley carburetor is mounted inline (normal orientation), fuel runs away from the secondary jets as the car is accelerating. For street applications this isn’t an issue, but if you drag race your car and it hooks, then the back jets can become uncovered. Obviously, this creates a tuning problem. The solution is a set of jet extensions (Holley sells them, and so do Moroso and others). I prefer the Holley extensions because they screw on. If you add jet extensions, you’ll need to notch the floats or install pre-notched nitrophyl floats (the easiest and best solution). These floats are designed to clear the extensions.
Power Valve Block Off: On the secondary side of the carburetor, if you chose to use a jet extension, you’ll quickly discover there’s no way to extend the power valve. The solution here is to replace the power valve with a block off (plug). Holley sells them. When a block off is installed, you must increase the secondary jet numbers by 6 to 10 (or so) steps.
Eyeball Float Height: If the floats have been replaced and/or you messed around with the needle and seat assembly, the floating setting will be incorrect. It’s easy enough to obtain a baseline float setting by flipping the bowl upside down (as shown in the accompanying photo) to establish a baseline. Compare the top of the float with the bowl site plug. It doesn’t have to be 100% exact either. You can fine tune with the engine running. The idea here is for fuel to just trickle out of the sight plug. If you drop the float level slightly, you can actually lean the air-fuel mixture. Raising the float level slightly (high) does the opposite – richening the air-fuel mixture. Keep in mind you can go too far in either direction!
Baseline Idle Mixture Screws: In order to obtain a baseline on a new or rebuilt carburetor, idle mixture screws should first be turned in and lightly seated. Do not overtighten! Next, turn each idle mixture screw out 1.5-turns. With this setting, the engine should start. You can fine tune (preferably with a vacuum gauge attached) with the engine running.
Simple Choke Fix: Let’s say you have a street-strip car that never sees cool weather. Basically, it’s a weekend driver. You probably don’t need a choke, particularly if has a Holley carburetor. You can remove the linkage (not a big deal) or you can disable it. Simply allowing the linkage to flop around isn’t a great idea because there’s always a chance the choke can be stuck closed at precisely the wrong moment. There’s an easy mod for hand choke, over the speed-shop counter Holleys: Simply cut a coat hanger to length and use it as a solid rod to keep the choke fixed at wide open. Depending upon the carburetor, you might have to use a second piece of the coat hanger as a shim, allowing the clamp on the choke linkage to tighten.
Secondary Stop Screw. If you have engine that is unresponsive to idle mixture adjustments, considering opening the secondary throttle blades slightly. This will allow more airflow through the engine. The secondary stop screw in located on the baseplate (you have to flip the carb over). Turn the screw approximately 1.5 turns. The secondary throttle blades will open slightly. By doing this, you can effectively close the primary throttle shaft, which will cover up the idle transfer slot. As a result, the idle mixture screws will become more sensitive. This is particularly useful in engines with big, long duration camshafts.
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You can see the brass jet extensions located on the secondary metering block on this Holley XP double pumper carburetor. Jet extensions are often fitted to the secondary side to compensate for fuel rushing toward the back of the bowl on acceleration, which in turn uncovers the jets. Obviously, on the front end of the carb, fuel rushes toward the jets on acceleration, so no extension is needed or desired.
When you extend the jets, you’ll discover there is no way to add a power valve extension. The solution is to increase the jet numbers and install a power valve plug, as shown here. The text offers more insight.
This is a notched float, which is necessary when you add jet extensions. But the real point here is, it’s easy enough to set Holley floats dry. With the bowl inverted (shown here), you just have to eyeball the height in relation to the sight plug on the side of the bowl. In this case, the float is a little too high.
Setting the choke up so that’s permanently “open” is easy. Just cut a section of coat hanger to length, and insert it as follows. The text offers more information, but it’s dirt simple.
If you flip a Holley carburetor upside down, you’ll often find this little screw. This is the secondary throttle stop adjustment. If the stop screw is turned in, it’s possible to open the secondary throttle slightly. This in turn will allow you to decrease the opening of the primary throttle. In turn, this will provide for more sensitive adjustments for the primary idle mixture screws. And once again, there’s more info in the text.
