Make vacuum leaks hiss-tory

Curing deadly engine vacuum leaks

Vacuum leaks can be located with spray carburetor cleaner or a can of WD-40. If the area is obstructed by linkage or hoses, use an extension nozzle to pinpoint the area of the vacuum leak. If the engine speeds up when an area is sprayed, you are close to finding the leak.


By John Bellah

Insufficient intake manifold vacuum can be deadly to an internal-combustion engine. As a refresher, the intake stroke of an internal-combustion engine draws in a mixture of fuel and air from the carburetor at about a 13:1 ratio of air to gasoline. This creates a vacuum between the carburetor butterfly (or butterflies on a multi-throat carburetor) and the piston tops. Insufficient vacuum will reduce engine efficiency, causing a loss of power and fuel economy and rough operation, especially at idle. Prolonged vacuum leaks can eventually cause serious engine damage. There are many places vacuum can be lost, but with a few simple tools and diligent detective work, vacuum leaks can be cured for a sweet-running engine.


Leak check prep

Begin with a vacuum gauge and the vehicle’s shop or repair manual. The shop manual should specify how much manifold vacuum is specified for the vehicle’s engine. Also have access to simple hand tools in case the carburetor or manifold must be tightened. Also have access to standard or Vice Grip-brand pliers in case a hose must be temporarily blocked. A can of carburetor cleaner, an 18-inch length of 1/2-inch fuel hose and a couple of shop rags will also come in handy.

Before tackling any type of carburetion problem, the rest of the engine needs to be properly tuned. That means spark plugs and wires need to be checked, the points need to be set and operating properly and the timing — initial lead as well as vacuum and centrifugal advances — need to be properly set. If the engine has mechanical valve lifters (tappets), they must be properly adjusted as tight valves will eventually burn, causing a rough idle and low manifold vacuum. While the spark plugs are removed, a cylinder compression check will determine the condition of the valves and rings. If one has access to an ignition oscilloscope, that diagnostic tool can shortcut some of these operations. In stubborn cases, it may pay to have a knowledgeable technician scope-check the vehicle’s ignition system.


Bad Dust

BAD Dust® Containment Systems

Reusable, modular containment bags designed to shield airborn dust and friable materials created by treating surfaces with media-blasting equipment or other mechanically operated hand held tools.


Auto Paint Specialists

Tower Paint™. APS/Tower Paint
The Original Custom
Color Spray Can Leader
2120 West Nordale Drive
Appleton, WI. 54914



With the engine cold, check the manifold heat-control valve for proper operation. Also called the “heat riser,” this often-ignored and neglected item can also cause a multitude of carburetion issues. Many people find the heat riser to be an “out of sight, out of mind,” component. However, when the engine is cold, a properly operating manifold heat-control valve channels exhaust heat through passages under the carburetor for proper warm-up in cold weather. After warm-up, the valve should divert the hot exhaust gasses through the muffler(s) and tailpipe(s). If the heat riser is stuck in the cold position, the results can be vapor lock and fuel percolation in the carburetor float, resulting in hard starting when the engine is warm. Extreme cases can cause manifold and carburetion warping or even cracking. Occasionally, the passage under the carburetor can become clogged with carbon deposits, a common occurrence I’ve seen in Chrysler small-block V-8 engines.


If you suspect internal leakage, pinch off the suspected hose with a pair of pliers or Vice-Grips. If the idle significantly changes, you are close to the solution.

A rough idle only when stopped at a stoplight may be a defective power brake booster. Pinching off the vacuum at the power brake booster may help in isolating the problem.

Airing out the issues

There are various signs and symptoms of vacuum leaks, some subtle and others not so subtle. One obvious sign is a loud hissing sound under the hood, sometimes accompanied by a whistle during deceleration. Another sign and symptom is a rough idle. This is when a vacuum leak can be tricky.

A rough idle that only shows up when stopped at a stoplight could be a defective power brake booster. Often, the plastic check valve will fail, giving an obvious hissing sound. Replacing the valve should cure that problem. If the problem is in the power brake booster, use Vice Grip pliers to clamp shut the hose. If that solves the problem and there is a noticeable change in idle quality, you are on the road to success.


Gauge Repair & Restoration – Bob’s Speedometer has been repairing and restoring gauges since 1929 – For over 80 years. No other gauge shop in the world can make this claim. We service most brands and types of gauges – speedometers, tachometers, clocks, oil, fuel, amp, volt, pressure, and a host of others – for all American makes/models & years, and most foreign, no matter the vehicle – Automobiles, Boats, Buses, RV’s, Fire Trucks, Tractors, Motorcycles – Anything with a gauge!
Free estimates



If idle quality is poor, a “shade tree” method of placing a shop rag over the carburetor intake may indicate a vacuum leak, as the reduction of intake air will make the idle mixture richer, offsetting the leak. If the idle speed and quality improves, there is a leak — somewhere.

From the 1960s and thereafter, vehicle manufacturers used manifold vacuum to operate various accessories: heater and air conditioning controls, remote trunk locks, door locks, hidden headlamp doors, power brake boosters, radio antennas, emission-control devices, transmission modulators, and so on. This is where pliers or Vice Grips can come in handy. Start by clamping shut the main vacuum hose at the manifold. If there is a noticeable change in idle speed or idle quality, you are on the track to tracking down the problem.

OK, there is a leak, but where is it? Find a quiet place to work away from outside noises. If your hearing is not what it used to be, enlist a helper. A hissing noise from under the car indicates a leak at the transmission modulator, or the hose leading to it from the intake manifold. If leaking heat and/or air conditioning components can be heard inside the car, from under the dash, enlist that 18-inch fuel line to pinpoint the leak.

If the leak hasn’t been found, find a can of carburetor cleaner or even WD-40. Using an extension tube, spray around all of the intake manifold joints and gasket surfaces. If the spray is sucked up and idle increases, the problem may have been located.

On older engines, problems that the engineers who wrote the repair manuals have never dreamed of may appear. I have occasionally encountered warped and cracked carburetors and manifolds when I operated my tune-up shop. On one occasion, I encountered a porous intake manifold, which was very tricky to find. Emission controls, such as Exhaust Gas Recirculation (EGR) valves, can also cause numerous carburetion problems. On mid-’60s vehicles with Air Injection Reactor (A.I.R.) systems where air is pumped into the exhaust manifolds, the carbon vanes inside the pump will occasionally disintegrate. This causes carbon chunks to block open the diverter valve and lean out the mixture. Clamping off the hoses will reveal if that is the issue.


Northwestern Auto Supply

An old time parts store since 1946 with 4 buildings full of old inventory of antique and classic parts for the quality minded owner who wants to avoid off shore products. Engine, chassis, ignition, and brake parts shipped world wide. Machine shop and babbiting also. 800-704-1078

Sullivan’s Garage

From Model T, A to Early V8 Ford Reproduction parts, supplies, books and manuals. Stocking over 3000 parts, from nuts and bolts to grilles, gas tanks, running boards and more.
Daily worldwide shipping. Secure 24×7 online shopping at .

Monthly specials, Model A shocks, speedometers many street rod items. 1-855 – 785- 5988.
1-855- SULLY -V8



Often, the plastic plug of the power brake connector will pop uot causing a massive vacuum leak.

Experiencing vacuum leak first-hand

I recently encountered an extremely frustrating issue with my daily driver. After many thousands of miles of faithful service, I noticed a slight hesitation in accelerating from cruise and made a mental note that a carburetor overhaul was due. When the idle became extremely rough with an accompanying whistle from under the hood, this indicated the carburetor was in serious need of attention or replacement, especially since I was able to wiggle the primary throttle shaft fore and aft.

Installing a standard rebuilding kit at home was not an option due to the worn throttle shaft/bushing (standard carburetor overhaul kits do not have replacement bushings to seal leaking throttle shafts). A master carburetor kit has the bushings, and even though I am skilled in tune-up procedures, I did not have the equipment in my home shop to change out carburetor bushings.

I chose a nearby carburetor rebuilding firm that promised to completely rebuild the unit with a two-hour turnaround, flow-test it afterward and guarantee its work. When I dropped off the carburetor, I pointed out the seriously loose primary throttle shaft.

While the rebuilt carburetor improved matters, the problem was still present. I removed the carburetor and it appeared I was using the wrong gasket as the carburetor flange and manifold did not completely line up. Changing out gaskets didn’t help much, and in checking with the carburetor manufacturer, it strongly suggested using a specific heat-shield/spacer. As I suspected, the previous spacer was porous, so I replaced it and still no change.

Then I suspected the carburetor base was warped, as confirmed by my straightedge, and I spent a couple of hours trueing everything to be smooth and flat, but still no change. Spraying both the right and left sides of the carburetor base with carburetor cleaner showed major leakage, and in desperation, I resorted to using silicon sealer on the base gasket. That seemed to solve the problem until I stopped at the next traffic signal and the car barely idled.


Several carburetor gaskets will appear to be correct, however, an incorrect gasket will cause vacuum leaks and possibly block vital sensing ports.

Thoroughly frustrated, I went home, and broke out a third can of carburetor cleaner. This time I used an extension nozzle to get past the linkages and hoses and meticulously sprayed the area. There was a very slight reaction when I sprayed the area with silicon. Assuming the carburetor was properly rebuilt with the bushings replaced, I determined that couldn’t be the problem. Nevertheless, leaving no stone unturned, I aimed the stream of carburetor cleaner directly on the secondary shaft. Bingo! Intake manifold vacuum sucked in every drop.

I removed the carburetor and headed back to the carburetor shop and explained my situation to the owner. He has been rebuilding carburetors for more than 50 years and told me this was all but impossible. Secondary butterflies on a four-barrel carburetor are rarely activated, certainly far less than the primaries which constantly open and shut.

Nevertheless, when his technician opened the unit, it was evident that when in a certain position, the secondary throttle shaft became sloppy, letting in huge amounts of air past the secondary butterflies and on into the manifold. This caused the silky smooth V-8 to idle like a cement mixer. The carburetor shop quickly replaced the secondary bushings, which permanently fixed the problem and we all learned something that day.


An example of an incorrectly installed gasket.

This is another example of the wrong gasket for a carburetor. Note that use of this gasket would cover the tiny vacuum sensing port.


If you liked this article on fixing vacuum leaks, you might be interested in our other restoration products:

More Resources For Car Collectors:



2 thoughts on “Make vacuum leaks hiss-tory

  1. Georgevette

    I have the hiss noise on the driver side of the carburetor. Do i need to change the gasket or loosing up the 4 screws? Its a rochester Quadrajet 4bbl


This site uses Akismet to reduce spam. .