WRENCHIN’ @ RANDOM YOUR VACUUM GAUGE IS YOUR FRIEND Two Essential Diagnostic Tools No Hot Rodder Should Be Without, and How to Use Them Marlan Davis hI’ve been answering read- ers’ Pit Stop tech questions for decades, explaining how to improve performance, troubleshoot pesky problems, or recommend a better combina- tion. Yet rarely do any of these problem- solving requests include information on the problem combo’s vacuum reading. That’s unfor- tunate, as [Above: Two essential diagnostic tools no hot rodder should be with- vacuum out, from left: a Mityvac handheld can tell vacuum pump for testing vacuum you a heck of a lot about an consumers (some models will even engine’s condition, without the aid in brake bleeding), and a large, easy-to-read vacuum gauge like need to invest in a bunch of this one by OTC (this model also high-tech diagnostic tools. includes a pressure gauge for even So what’s the deal on more test possibilities). vacuum? Consider an internal- [Left: Knowing how to use a combustion engine as basically vacuum gauge is the key to a giant air pump that operates diagnosing many performance under the principles of pres- problems. It aids in tuning your sure differential. The difference motor to the tip of the pyramid. It even helps diagnose problems not between normal atmospheric seemingly engine-related, such as pressure (14.7 psi at sea level a weak power-brake system. Add at standard temperature and one to your toolbox today. pressure) and how hard this “pump” sucks under various engine-management system). that needs vacuum that—at that vacuum comes from the operating conditions and states If present, the distributor least on the surface—seemingly engine. If an engine is down on of tune creates vacuum (usually vacuum advance system also doesn’t tie directly into engine vacuum, these devices may not expressed in inches of mer- depends on vacuum. Many performance. These include function properly; conversely, cury, or in-hg). The amount of emission-control devices— vacuum consumers like the if the vacuum consumers have vacuum in the induction tract at including the EGR valve, evapo- power-brake vacuum booster, a problem, it creates a vacuum various operating points is used rative control system (ECS) automatic-transmission vacuum leak, degrading proper engine to properly meter fuel (through canister purge, and PCV valve— modulator, and classic-era operation. old analog devices like the car- likewise rely on consistent and HVAC (heating, ventilation, and Erratic or low-vacuum gauge buretor, or via a MAP sensor, correct engine vacuum. air conditioning) controls. All readings indicate there’s a real as part of a modern electronic Wait: There’s even more stuff need vacuum to operate, and problem with the engine or the “Vacuum is everything from a tuner’s standpoint. It’s the key to virtually every component.” — Norm Rollings 74 HOTROD.COM/2019/AUGUST/ WRENCHIN’ @ RANDOM THE NUMBERS ADJACENT TO SOME OF THE BOXES BELOW CORRESPOND TO THE PHOTOS ON PAGE 78. Steady needle at idle. Find the I. ABNORMAL BUT STEADY Reads steady vacuum READING AT IDLE low or steady high leak. compared to nominal. various interconnected vacuum your tune-up, check vacuum- devices. On an old-school powered external device opera- 6 Steady but low Steady but high motor, you can also gauge the tion, or begin homing in on a efficiency of the tune-up by the possible internal engine issue, Spray carb cleaner around the intake-manifold–to–cylinder- engine’s vacuum output. When the Rollings tell us you can head mounting surfaces and the carburetor–to–intake-manifold 1 it comes to high performance start with just two inexpensive mounting surface. Adjust carburetor idle fuel Reading of over 18 in-hg, There is a restriction in the air specifically, understanding your devices: a vacuum gauge and mixture screws to obtain especially on a hot rod, is cleaner. Replace air cleaner or highest vacuum reading. generally way too high. install larger air cleaner. engine’s vacuum characteristics a handheld vacuum pump. If aids in diagnosing mysterious 2 Idle mixture problems—like a power-brake screws Idle mixture screws do not respond to adjustment; idle mixture quality (vacuum and setup that somehow can’t stop respond to rpm) bad. Place hand or shop rag over the carburetor air horn. the car. adjustment To get the skinny on vacuum, we visited Rollings Automotive, Idle rpm momentarily speeds up. Replace the a family-owned, full-service faulty gaskets. Some carb mounting flanges Idle rpm lack material for getting a good seal. A metal does not automotive shop that works on momentarily 3 anything with wheels. Whether plate-type gasket might be the solution here. Quadrajet carbs are known to sometimes increase. Vacuum Some improvement, Car stalls Car you need to get a handle on leak through a loose or worn primary throttle increases to but vacuum is still out. Spray- continues shaft. Shaft bushing kits are available (requires the normal lower than desired. out air running. complete carburetor disassembly). range for Check ignition timing. bleeds with You have your vehicle: Add more initial lead carburetor a vacuum Stop, you’ve to achieve highest cleaner. leak. 5 7 Engines with found the vacuum, then back off Vacuum is now a separate problem. about ½ in-hg. Check idle circuit. On standard carbs Engines without a separate valley cover. within acceptable valley cover: There can still be range. Stop, the with the usual two idle mixture Idle screws screws, a big cam may result in a leak out the bottom (blind problem is solved, Idle screws side) of the intake into the but you may also now respond don’t unresponsive mixture screws and/ respond, or require turning in the idle speed engine valley. Pull the PCV need to refine the to adjustment, valve out and cap the breather centrifugal advance vacuum improves. but car does screw too far. Adjust the secondary not smell stop screw to increase rear blade side of the system. Spray carb Vacuum is curve amount Stop, you’ve found cleaner through the hole into and rate if you’ve the problem. rich. angle, then back off the primary- still low but side idle screw on the throttle shaft. the crankcase. steady. added in a bunch more initial timing (see “How Much Idle screws don’t respond to adjustment. Car also Motor speeds up Motor does not speed up Advance Do You 8 Need?” sidebar on smells rich, sounds loaded up, and/or belches black page 84). exhaust smoke at idle. Disconnect all The intake gasket vacuum hoses and Idle mixture Idle mixture screws remain is leaking out the plug all vacuum screws can now nonresponsive. The carb in its bottom. Replace. ports, or clamp Jumped or be adjusted. Stop, present state won’t work with all vacuum hoses Valve lifters incorrectly you’ve found the the rest of the combo. You shut. adjusted too tight. installed timing problem. have several choices. chain. Fix and repair as needed. Remove and inspect power valve (PV) located in the primary metering block on a Holley-style carburetor. Adjustable Net-lash Motor does not Motor speeds up. valvetrain valvetrain speed up. There Reconnect each Check the Wrong PV Blown (ruptured) PV 3 3 may be a serious vacuum hose one at a last-connected Drill a ⁄32- to ⁄16- Have a specialty Get the proper internal engine time until motor slows vacuum hose for inch hole in both carburetor shop high-end carb for back down. primary throttle trick-out the the application. At issue. cracks and holes. Your Holley-style blades. existing carb. a minimum, a high- carb’s power 4 end carb (such as a Holley HP or Ultra) Properly Determine and obtain the valve (PV) should open at half the If PV is correct for should have a four- adjust lifters. proper-length pushrod. the application, corner idle system engine’s normal Bad hose found. idle vacuum check to see if and replaceable air it both holds bleeds. Replace with number. For new hose. example, if a vacuum and opens big-cam engine when vacuum idles at 13 inches, drops to the value use a No. 6.5 PV. stamped on the valve. If it doesn’t, Otherwise, the Original hose checked None of the above fully resolves the issue. You may have a PV may open replace the PV. Vacuum not system-wide internal engine issue. Hold engine speed steady fully improved: good and/or vacuum prematurely and still doesn’t improve at 2,500 rpm for 15 seconds, then rapidly release the throttle. flood the motor. Continue testing Vacuum should momentarily increase a couple of inches. If it remaining vacuum after replacing hose. doesn’t increase or pulls down to zero, you’re looking at worn circuits as neces- Test vacuum consumer rings, cylinders, or valve seats; confirm with a cylinder sary one by one the hose connects to cranking compression test. Also see diagnostic tree II until issue is found using handheld vacuum on pages 80–81 and tree III on page 82. and resolved. pump. Replace with Still not right? new unit if device won’t Check the ignition. hold vacuum. 76 HOTROD.COM/2019/AUGUST/ HOTROD.COM/2019/AUGUST/ 77 WRENCHIN’ @ RANDOM THE BOX NUMBERS ABOVE THE PHOTOS CORRESPOND TO THE DIAGNOSTIC TREE I BOXES ON PAGES 76–77. See box 1 See box 2 See box 3 Norm Brandes A B B A [If the idle mixture screws don’t [Sometimes just spraying the air bleeds with carb cleaner helps free up respond, place a shop towel or idle system response.