Air/Fuel Mixture: Adjustments

Fig. 1 Propane enrichment specifications:

Refer to Propane Enrichment Chart, Fig. 1, when adjusting.
1. Apply parking brake, block wheels, disconnect automatic brake release, if equipped, and plug vacuum connection.
2. Connect suitable tachometer to engine.
3. If vehicle is equipped with hot idle compensator (HIC), ensure HIC is in closed position and remains closed throughout test. Idle vehicle on kickdown step of fast idle cam for three to five minutes to ensure HIC is closed.
4. On 1983 models except V8-302 engine with 2150A-2V feedback carburetor, proceed as follows:
a. Disconnect fuel evaporative purge return hose, if equipped, and plug connection.
b. Disconnect fuel evaporative purge hose, if equipped, at air cleaner and plug nipple.
5. On all vehicles, disconnect flexible fresh air tube from air cleaner duct or adapter, then, using tool T75L-9600-A or equivalent, insert tool hose approximately 3/4 of the way into duct or fresh air tube. Secure hose with tape as necessary.
6. On vehicles equipped with thermactor systems, revise dump valve vacuum hoses as follows:
a. On dump valves with two vacuum fittings, disconnect and plug hoses.
b. On dump valves with one fitting or combination air bypass/air control valve, except vehicles with feedback carburetor, remove hose at dump valve and plug hose. Connect slave hose from dump valve vacuum fitting to an intake manifold vacuum fitting.
c. On V8-351 engine equipped with 4180C-4V carburetor, disconnect and plug vacuum hose to dump valve.
7. On 1984 - 85 models with feedback carburetor, disconnect battery ground cable for at least 10 seconds.
8. On all models with feedback carburetor, proceed as follows:
a. Disconnect electrical connector at engine coolant temperature sensor.
b. Disconnect and plug hose from manifold absolute pressure (MAP) sensor.
c. Disconnect and plug hose from carburetor vacuum operated throttle modulator (VOTM).
d. Connect vacuum pump to manifold port of MAP sensor, evacuate to at least 20 inches Hg. and hold.
e. Start engine. Leave all vacuum hoses attached to air cleaner assembly when moving assembly to perform carburetor adjustments. The air cleaner assembly must be in place when measuring engine speeds. Prolonged engine idling may cause catalyst overheating and excessive underbody temperatures.
9. On all vehicles, ensure that ignition timing is set to specifications.
10. Check engine curb idle speed or A/C-OFF RPM, adjusting as necessary. Remove PCV valve from grommet and allow PCV to draw underhood air during idle fuel mixture check.
11. With transmission in neutral, run engine at approximately 2500 RPM for 15 seconds for each mixture check.
12. With engine idling at normal operating temperature, place transmission in specified gear, gradually open propane tool valve and watch for engine speed gain, if any, on tachometer.
13. When engine speed reaches a maximum and then begins to drop off, note the amount of speed gain. The propane cartridge must be in vertical position. If engine speed will not drop off, check propane cartridge gas supply. If necessary, repeat steps 11 through 13 with a new propane cartridge gas supply.
14. Compare measured speed gain to specified speed gain.
15. If measured speed gain is within specifications, proceed to step 20. If carburetor assembly is equipped with a tamper resistant idle adjustment feature, the carburetor must be removed from the vehicle. If idle fuel mixture adjustment is necessary, adjust mixture according to reset RPM specification.
16. If measured speed gain is zero and minimum speed gas specification is zero, proceed to step 19.
17. If measured speed gain is higher than specification, turn mixture screw(s) counterclockwise in equal amounts while simultaneously repeating steps 10 through 13 until measured speed rise meets reset RPM specification. After final adjustment, proceed to step 20.
18. If measured speed gain is lower than speed gain specifications, turn idle mixture screw(s) clockwise in equal amounts while simultaneously repeating steps 10 through 13 until measured speed rise meets reset RPM specifications. After final adjustment, proceed to step 20.
19. If there is no RPM rise and the minimum speed gain specification is zero RPM, perform speed drop test as follows. Speed drop test information is specified with the artificial enrichment requirements whenever minimum speed gain specification is zero RPM.
a. While observing tachometer, adjust mixture screw(s) clockwise by number of turns specified for speed drop test, noting drop in engine speed.
b. If measured speed drop is equal to or drops off by more than the speed drop specification, return mixture screw(s) to position before adjustment and proceed to step 20.
c. If measured speed drop is less than specified minimum speed drop specification, leave mixture screw(s) in adjusted position and repeat steps 10 through 19.
20. Install tamper resistant feature for carburetor idle fuel air adjustment.
21. Install PCV valve in grommet.
22. On vehicles equipped with feedback carburetor, proceed as follows:
a. Connect electrical connector to engine coolant temperature sensor.
b. Connect hose to MAP sensor.
c. Connect hose to VOTM.
d. Check curb idle speed and adjust to specification.
23. Remove all test equipment, reconnect all system components and install air cleaner.