Check Dwell Variation; Check And Adjust Ignition Timing; Adjust Idle Speed And Mixture - Chevrolet 10 1971 Series Chassis Service Manual

ENGINE 6-8
Fig. 15— Setting Point Dwell (Typical V8)
V8 ENGINES
• With engine running at idle, raise the adjustment
screw window and insert an Allen wrench in the
socket of the adjusting screw (fig. 15).
• Turn th^^fSs^ihg screw as required until a dwell
pi
reading 30° is jobtained. A 2° variation is allow­
able forlwear> , / '
• Close acwrSs cover fully to prevent the entry of
dirt into the distributor.
If a dwell meter is not available, turn
NOTE:
adjusting screw clockwise until engine starts to
misfire, then turn screw one-half turn in the
opposite direction to complete adjustment.
IN LINE ENGINES
• Remove distributor cap and recheck point setting.
If dwell is still not within specifications check the
distributor as outlined in Section 6Y.

Check Dwell Variation

Slowly accelerate engine to 1500 rpm and note dwell
reading. Return engine to idle and note dwell reading.
If dwell variation exceeds specifications, check for worn
distributor shaft, worn distributor shaft bushing or loose
breaker plate.
Check and Adjust Ignition Timing (Fig. 16)
1. Disconnect the distributor spark advance hose and
plug the vacuum source opening.
2. Start engine and run at idle speed (See tune up chart
in Specification section.)
3. Aim timing light at timing tab.
The markings on the tabs are in 2" incre­
NOTE:
ments (the greatest number of markings on the
" Before" side of the "O" ). The "O" marking
is TDC and all BTDC settings fall on the " Be­
fore" (advance) side of "O" .
4. Adjust the timing by loosening the distributor clamp
and rotating the distributor body as required, then
tighten the clamp and recheck timing.
5. Stop engine and remove timing light and reconnect
the spark advance hose.
W INDOW
"H EX" TYPE
WRENCH
SCREW
Fig. 16— Ignition Timing Marks
Adjust Idle Speed and Mixture (Fig. 17)
10 Series Vehicles with Gasoline Engines
1. With engine running at operating temperature, choke
valve in fully open position, parking brake on and
drive wheels blocked - adjust idle speed as follows
(See " Tune-Up" Decal Figure 18):
All carburetors are equipped with idle
NOTE:
mixture limiter caps (fig. 17); the idle mixture is
preset and " locked in" by these caps - no
attempt should be made to adjust mixture. Do
not remove mixture screw caps.
If the Combination Emission Control
CAUTION:
solenoid screw (fig. 17) is used to set engine
idle or if the solenoid is adjusted out of limits
as specified in Section 6M, a decrease in engine
braking may result.
• 250 and 292 Cu. In. (Single-Barrel Carburetor)
Disconnect " Fuel Tank" line from Evaporation
Emission vapor canister.
Disconnect the distributor spark advance hose and
plug the vacuum source opening.
Adjust carburetor speed screw to obtain 550 rpm
with manual transmission in neutral; 500 rpm with
automatic transmission in drive. DO NOT ADJUST
SOLENOID SCREW.
If the Combination Emission Control
CAUTION:
solenoid screw (fig. 17) is used to set engine
idle or if the solenoid is adjusted out of limits
as specified in Section 6M, a decrease in engine
braking may result^
Reconnect " Fuel Tank" line to vapor canister and
reconnect distributor spark advance hose.
« 307 Cu. In. (Two-Barrel Carburetor)
Disconnect " Fuel Tank" line from Evaporation
Emission vapor canister.
10-30 CHEVROLET TRUCK SERVICE MANUAL