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.



V6-232

1. Apply parking brake and block wheels. On vehicles equipped with a vacuum parking brake release, disconnect and plug vacuum hose from parking brake actuator.
2. Connect a timing light and tachometer to the engine. Use a dampened tachometer with no greater than 20 RPM divisions. Digital tachometers are not recommended for this propane enrichment procedure. For high energy ignition systems, use an approved tachometer such as the Rotunda 20362 or equivalent.
3. Remove and plug vacuum lines to the distributor.
4. Start and operate engine until normal operating temperature is obtained. Turn off all accessories. Do not allow engine to idle for extended periods. Exhaust system overheating may cause excessive underbody temperatures.
5. Set initial timing to specification. Engine idle speed must be at or below specified curb idle speed.
6. Connect vacuum lines to distributor. Leave all vacuum hoses connected to the air cleaner when moving the assembly to perform carburetor adjustments. The air cleaner must be installed onto the carburetor when measuring engine speeds and during propane enrichment procedure. An excessively dirty air cleaner element could cause wrong propane readings. If necessary, replace air cleaner element before performing propane enrichment procedure.
7. Disconnect evaporative emission purge hose from air cleaner. On vehicles equipped with fuel evaporative purge valve signal vacuum hose, disconnect hose as follows:
a. Trace purge valve vacuum hose from purge valve located on purge canister to the first point where the hose can be disconnected.
b. Disconnect hose at that point, then cap open port and plug vacuum hose. Do not disconnect vacuum hose at purge valve, to prevent possible damage to the purge valve.
8. On vehicles equipped with fuel evaporative purge return hose, disconnect hose from engine and plug connection.
9. Position idle mixture limiter cap(s) in the maximum rich position. Idle mixture screw(s) without limiter caps must be turned inward until lightly seated, then turned outward 11/2 to 2 turns. On carburetors with more than one mixture screw, the screws must be turned in even amounts.
10. Check and adjust curb idle speed to specification. Operate engine at 2500 RPM for 15 seconds before each propane adjustment.
11. Disconnect PCV valve hose from air cleaner and plug air cleaner hose.
12. On vehicles equipped with Thermactor System, note the following:
a. For dump valves with two vacuum fittings, disconnect and plug hoses.
b. For dump valves with one fitting, disconnect and plug hose from dump valve. Connect a ``jumper'' hose from dump valve vacuum fitting to an intake manifold fitting.
13. Plug propane tool T75L-9600-A into air cleaner evaporative purge nipple. If vehicle is not equipped with a purge nipple, disconnect flexible fresh air tube from air cleaner duct or adapter. Insert end of propane tool hose approximately 3/4 of the way into the fresh air tube or duct. Secure hose with tape, if necessary. Position propane bottle in a vertical position with gas valve on top. Hold gas valve down throughout the propane check with engine idle speed constant. Operate engine at 2500 RPM for 15 seconds, and check curb idle speed with transmission in drive if equipped with automatic transmission, or in neutral if equipped with manual transmission. Adjust curb idle speed to specifications, if necessary, before introducing propane into air cleaner. Slowly open propane valve on the side of the propane tool until engine speed reaches a maximum and then begins to drop. Note maximum increase and compare the gain to specification. Gain should be 10 - 30 RPM.
14. If engine speed increases to a maximum, but will not drop with both the propane tool plunger down and side valve fully open, note the following:
a. Check propane bottle gas supply, replace bottle and repeat procedure, if necessary.
b. If engine speed now reaches a maximum and then begins to drop, repeat step 13.
c. If engine speed still does not drop, remove valve from propane tank.
d. Using compressed air, reverse flush the valve to remove dirt that may prevent full propane flow.
e. Repeat step 13. If engine speed still does not reach a maximum and then begins to drop, check propane side valve. The valve should be fully open after 1 - 2-1/2 turns counterclockwise. Replace valve, if necessary and repeat step 13.15.
15. If propane gain is within specification, proceed to step 19.
16. If propane gain is not within specification, note the following:
a. Adjust idle mixture according to RESET RPM specification. Reset specification is 20 RPM.
b. Remove limiter caps from carburetor idle mixture screws. Use tool No. T75L-9500-A or equivalent, to prevent screw damage. To adjust mixture with tamper-resistant idle adjustment caps, remove carburetor from vehicle and idle mixture shield from idle mixture adjustment screw.
17. If speed increase is lower than specification, note the following:
a. Lean the mixture without propane by turning mixture screw(s) clockwise. Lean the mixture while watching the tachometer curb idle speed drop the number of RPM needed to approach the speed gain RESET specification.
b. On carburetors with more than one mixture screw, turn the screws in equal amounts.
c. Reset curb idle speed to specification.
d. Compare propane gain to the RESET specification. Operate engine at 2500 RPM for 15 seconds.
e. Repeat idle mixture adjustment procedure and curb idle adjustment procedure until correct RESET propane RPM gain is obtained. Proceed to step 19.
18. If speed increase is higher than specification, note the following:
a. Richen the mixture without propane by turning the mixture screw(s) counterclockwise.
b. Richen the mixture while watching the tachometer curb idle speed increase the number of RPM needed to approach the speed gain specification.
c. On carburetors with more than one mixture screw, turn the screws in equal amounts.
d. Reset curb idle speed to specification.
e. Compare propane gain to RESET specification.
f. Repeat idle mixture adjustment procedure until correct RESET propane RPM gain is obtained. Proceed to step 19.
19. For carburetors requiring limiter cap(s), install new service (blue) limiter caps in the maximum rich position. Use correct length caps, two types are available. Pre-soaking the limiter cap(s) in hot water will soften the material and reduce the effort needed to install it. For carburetors equipped with tamper-resistant idle adjustment screw(s), install a new plug over the idle mixture adjustment screw(s).
20. Check engine idle speed and idle speed gain to ensure idle mixture screws were not disturbed during limiter installation.
21. Switch engine off and remove propane equipment. Install purge tube and fresh air duct as required. Remove all plugs from evaporation purge valve vacuum hose and connect vacuum hose. Connect fuel evaporative purge valve return hose to engine, if equipped.