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[Datsun 1200 encyclopedia]

Vacuum Leaks

From Datsun 1200 Club

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Categories: Fuel System | Engine Electrical System | Diagnoses and Corrections

Remember: Vacuum Leaks are your Worst Enemy -- they are a most common problem causing bad carburetor performance, if not the most common problem. So spend some important time on finding vacuum leaks.

NOTE: Vacuum leaks are also called 'air leaks'. 


Most Common Causes

Leaking intake gaskets and carb gaskets are not the most common problems. More often these are the causes:

  • broken hose ends
  • leaking vacuum motors (diaphragms)


When checking for vacuum leaks, don't start with the carb flange. It is far more common to leak at other places including:

  • broken hose ends
  • Vacuum Advance
  • Air Cleaner heat-flap motor (diaphragm)
  • EGR Valve (newer engines)
  • PCV valve sticking - causes part-throttle problems

Carburetor leaks

  • choke vacuum break (or Weber DGEV/DGAV choke pull-off)
  • Loose throttle shaft
  • Loose base nuts. Do not overtighteen
  • Loose adapter plate (Weber)
  • Cracked adapter plate (Weber) - caused by overtightening
  • Carburetor Dashpot (some engines)
  • Throttle opener (some engines)
  • Dashpot (some engines)

Vacuum Motors

The vacuum motors are:

  • Vacuum Advance
  • Choke Pull-off (automatic choke models)
  • Air Cleaner heat-flap Motor (some engines)
  • Carburetor Dashpot (some engines)
  • loose air jet
  • leaking brake booster
  • EGR Valve (newer engines)
  • on the carb:
    • choke vacuum break (choke pull-off)
      1008.jpgAlbum 1008
    • Throttle opener (some engines)
    • Dashpot (some engines)

NOTE: Only a few of the vacuum devices are actually powered at idle. The others only activate at certain temperatures or other conditions. So we will concentrate on other tests first.

The next section will show how to look for vacuum leaks.

Engine Off Inspection

Perform these steps before starting the engine.

Examine Hoses

Examine carefully all the vacuum hoses on the engine, especially the ends of each hose. Cut off cracked ends and refasten. This will fix or help prevent vacuum leaks at the hose ends.

The air cleaner must be removed to inspect the vacuum hoses. For instructions, see Air Cleaner Removal.

Sources of leaks include these hose places:
22830.jpgAlbum 22830

A Choke Opener hose
B PCV Valve hose
C Intake port for Automatic Transmission
D Intake port for Throttle Opener (1972-1973)

When test-running the engine without the air cleaner on, be sure to plug all the ports. In the photo one of them is plugged.

Check the Play on the Throttle Shaft

With old carbs, this is also a critical wear spot. When buying a used carb, this is the most important thing to look for.

Wiggle the throttle shaft from side to side (90 degrees to the shaft). Wear is common with old carbs. It should feel tight with no discernible play. A worn shaft makes a vacuum leak: the engine idle rough, even to the point of killing the engine at stops and it can not be tuned to idle correctly.

  • It should be tight, and if so no worries
  • If it's loose, it needs to be drilled and bushings put in. See Carburetor Repair.

Vacuum Motor Testing - Engine Off

Here are some ways to check for vacuum leaks in the vacuum motors (diaphragms) with the engine off:

Use Your Finger Method

2211.jpgAlbum 2211

Test each diaphragm. Here's an example using the choke pull-off

  1. Pull the hose off the choke pull-off
    2213.jpgAlbum 2213
  2. Use a screwdriver to push on the rod coming out of the pull-off
  3. Cover the port with your finger
  4. Let go of the screwdriver. The rod should stay pushed in
  5. Take your finger off

The rod should stay or maybe move back just a little.

  • If it moves all the way back, the diaphragm is leaking. Plug the hose until you can replace the pull-off (it only works when the engine is cold anyways -- if bad, it causes the engine to 'chug' after first starting it)

Repeat for these diaphragms:

  • Throttle opener
  • Distributor Vacuum advance
  • Carburetor Dashpot (some engines)
  • EGR Valve (newer engines starting with B210). CAUTION: This gets very hot, so only test with engine Cold!. With some EGR valves, you can push the diaphragm with your finger. With others you cannot. Alternative: You can plug the valve to see if the engine drives better. For a non-street engine, plug it permanently; it is for emissions only and affects performance negatively
  • Carburetor Dashpot (some engines)

NOTE: Newer carbs/engines/distributors may have even more diaphragms to check:

  • Distributor retard. Test in same way as Vacuum advance. For a non-street engine, plug it permanently, as it degrades performance
  • Other Emission controls

Air Cleaner Motor

Some engines have an Air Cleaner Motor. If needed, this part can be replaced OR can be left plugged. This is the hot-air flap, and is only strictly necessary in cold climates (temps close to freezing) to prevent carburetor frost-up.

air cleaner Hot Air Valve
1005.jpgAlbum 1005

It is very common for the Air Cleaner Motor to leak. You can push up on this after removing the hot air tube and check and see if it holds vacuum. This part is only needed in cold-weather operation, to keep the carb from icing over.

Alternative test: check on a warmer day that this is closing (some cold air is entering, not all hot). Plug the hose if it is not working correctly. The flap should not open all the way until 122 degrees Farenheit.

Vacuum Gun Method

Use a vacuum-making hand gun.
14074.jpgAlbum 14074

It will come with instructions and costs around $50 USD. This is the easiest, most accurate and best way to do it. Basically, you pull the hose off the vacuum motor, connect the gun, and squeeze the trigger. A built-in vacuum gauge will tell you if vacuum is holding. You can also test small emissions parts with this tool. Some parts are designed to hold a partial vacuum (a repair manual will have more details on testing these parts).

For example, let's test the Choke Pull-off diaphragm

Slide the hose off the motor and connect the vacuum gun. Pump the gun.
22831.jpgAlbum 22831

If the diaphragm can hold vaccum, it is good and not leaking. 15 to 20 inches is the normal range (-50 to -70 kpa). It should show no signs of leaking whatsoever.
22832.jpgAlbum 22832

By contrast, if you can only pump a few PSI or it immediately leaks down, the diphragm is bad and needs replacement. Temporarily plug the hose to prevent the leak until you can replace the part.
22833.jpgAlbum 22833

If you need to replace it, see Carburetor

19996.jpg 19997.jpg

Finger Method

If you don't have vacuum gauage, you can use this method:

  • Push on the motor's rod or diaphragm with your finger or a screwdriver.
  • Use another finger on the port to block the hose
  • While still blocking the hose, release the shaft or diaphragm

  • With your finger covering the port, the motor should stay fully or partly in position. This part is good.
  • If it immediately moves back to normal, it's leaking. Replace the part. Some parts can be blocked off (put a golf tee in the hose)

Parts that can be blocked temporarily and still get good performance include:

  • EGR valve. Replace the bad part to help maintain low emissions
  • Distributor advance. It will run slightly jerky blocked, but it is better than leaking 

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