IDLE MIXTURE BALANCE TEST

TM 9-4910-749-10

1-9. PCV TEST

WARNING

a. A defective PCV permits crankcase vapors to

Make sure that the par k i n g b r a k e i s s e t

escape to the atmosphere. This increases air pollu-

tion, and causes rough idling.

(3) Observe the LOW RPM scale (0-1500 RPM).

b. To perform the PCV test do the following:

RESULTS:

Too high-Speed adjusted too fast

Adjust idle speed screw.

(1) Turn the test selector to the LOW RPM

Normal-Speed correctly adjusted.

position (See Figure 5).

Too Low-Speed adjusted too slow.

Adjust idle speed screw.

(2) At norms idle speed, pull out the PCV

1-8. IDLE MIXTURE BALANCE TEST

hose assembly from the valve cover of the engine.

WARNING

(3) Place thumb over end of PCV valve while

observing meter.

P l a c e shift selector in Neutral or Park be-

fore proceeding with this test.

RESULTS: Speed drops 50 RPM or more-PCV ok.

Speed drops less that 50 RPM-PCV

a . Excessive leanness or richness causes a rough

defective. Clean or replace the PCV

operating engine, diseling or stalling at idle. A

valve and clean the PCV hose.

vacuum gauge can also be used to adjust the idle

mixture.

(4) Reconnect the PCV hose assembly to the

valve cover of the engine.

b. To perform the idle mixture balance test do

the following:

1-10. AIR FILTER TEST

( 1 ) Turn the test selector switch to LOW RPM

a. A dirty air filter causes an overrich fuel mixture

position (See Figure 5).

resulting in poor fuel economy and loss of power.

(2) At normal idle speed, observe meter and

b. To perform the air filter test do the following:

t u r n idle mixture screw(s) slowly in (clockwise) and/

or out (counter-clockwise) to achieve highest RPM

(1) Turn the test selector to LOW RPM posi-

and smoothest idle operation.

tion (See Figure 5).

R E S U L T S : Too far in - Causes lean mixture and

(2) Remove the air filter and note the RPM

rough idle. Adjust.

reading.

Normal-Runs

smoothly.

adjust-

ment needed.

(3) At normal idle speed, with the air filter in

Too far out - Causes rich mixture,

place, note the RPM reading.

rough idle, and poor fuel economy.

Adjust.

R E S U L T S : Minimal RPM Change-Air Filter ok.

Excessive

RPM

Change-Air

Filter

(3) After idle mixture is adjusted, readjust idle

dirty

and

restricted.

Replace.

speed.

NOTE

For use with waterproof electrical ignition

On many late model emission-controlled

systems, you must use Adapter, Engine

vehicles, limiter caps have been installed

Electric, NSN 4910-00-356-7504 which is

to prevent overrich idle mixtures. Refer to

a c o m p o n e n t : o f A d a p t e r S e t , E n g i n er

specific recommendations of manufac-

NSN 4910-00-348-7600.

turer.

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