Bronco II 4WD V6-171 2.8L (1985)
Air/Fuel Mixture: Adjustments
The idle mixture screw is protected by a cap which prevents tampering during normal tune up procedures. If it is evident that idle mixture requires
adjustment, remove cap as outlined under ``Tamper-Resistant Cap Removal'' in the appropriate carburetor section.
*** UPDATED BY TSB #84-11-20
V6-171 Engine
1.
Apply parking brake and block wheels.
2.
Check that timing is set correctly.
3.
Remove air cleaner.
4.
Disconnect wiring harness from feedback solenoid located in rear of carburetor.
5.
Remove PCV valve from rocker cover grommet and position aside so that crankcase vapors will not be drawn in.
6.
Disconnect evaporative purge line from intake manifold spacer at purge solenoid.
7.
Install adapter orifice (part of tool No. T75L-9600-A) on 1983 - 84 models, or purge line tester (tool No. T84T-9600-A) on 1985 - 86 models, into
end of purge line that is still connected to intake manifold spacer.
8.
Disconnect idle speed control motor from wiring harness as follows:
a. Connect suitable tachometer and start engine.
b. Manually hold throttle open above 1000 RPM and push idle speed control plunger until it retracts fully.
c. When plunger is fully retracted, release throttle and quickly unplug connector. Make sure that throttle lever is resting on speed screw before
continuing with procedure.
9.
Install air cleaner assembly, but do not connect the flexible fresh air tube to the air cleaner duct and valve assembly. Using a propane enrichment
tool, Rotunda Tool T75L-9600-A or equivalent, insert the tool hose approximately 3/4 of the way into the duct and valve assembly. If necessary,
secure the hose with tape.
10.
Locate crankcase ventilation hose running from oil filler cap to air cleaner and disconnect at air cleaner end, allowing hose to vent externally
during mixture check.
11.
With transmission in Neutral, operate engine at 2500 RPM for 15 seconds before each mixture check.
12.
With engine idling at normal operating temperature place transmission in Neutral. Gradually open propane tool valve, and observe engine speed
gain, if any, on tachometer. When engine speed reaches maximum and begins to drop off, note amount of speed gain. Make sure that propane
cartridge stays in vertical position. If engine speed will not drop off, check propane cartridge supply. If necessary, repeat steps 11 and 12 with new
propane cartridge.
13.
Compare measured gain to specified gain of 160 - 240 RPM.
a. If measured speed gain is within specification, proceed to step 15. Carburetor must be removed from vehicle to remove tamper-resistant caps.
Refer to ``Carburetor Section.'' With tamper-resistant caps removed, turn both mixture adjustment screws clockwise until fully seated. Then
turn both screws back out 2-3/4 turns (counterclockwise). If fuel mixture adjustment is determined to be necessary, adjust mixture according
to reset gain of 210 RPM.
b. If measured speed gain is greater than speed gain specification, turn mixture screws counterclockwise (rich) in equal amounts, and repeat
steps 11 and 12 until measured speed rise meets reset RPM gain specification. When adjusting mixture screws, do not exceed 3-1/4 turns
from fully seated position. If more than 3-1/4 turns of adjustment are required, an internal carburetor malfunction is the probable cause.
c. If measured speed gain is lower than speed gain specification, turn mixture screws clockwise (lean) equally, and repeat steps 11 and 12 until
measured speed gain meets reset specification. When adjusting mixture screw, make sure that screws are no less than 2-1/4 turns from fully
seated position. If adjustment is less than 2-1/4 turns, an internal carburetor malfunction is the probable cause.
14.
Reinstall tamper-resistant caps.
15.
Reinstall PCV valve in grommet.
16.
Connect wiring harness to feedback solenoid and idle speed control motor.
17.
Remove adapter orifice from purge line and reconnect line to solenoid.
18.
Remove test equipment, reconnect all system components, reinstall air cleaner, and tighten air cleaner nut(s).
