Chevrolet DB90 Series, Service Manual

Page: 105 / 242

Sec. 6M-99
1. Thoroughly warm up the engine. B e sure
choke is open.
2. Check dwell reading and adjust, if n e c e s ­
sa r y . D w ell reading on V6 engines should be 31 to
34 d eg rees. D w ell reading on 637 V8 engine should
be 28 to 32 d eg rees.
3. Adjust ignition tim ing with vacuum advance
h ose disconnected and plugged.
FUEL SYSTEM —G A S O L IN E EN G IN ES
Engine Ignition Timing
401M, 478M, 478SN . . 7JT BTDC
637 - V 8 ........................... 5° BTDC
R econnect vacuum advance hose.
4. Engine id le speed should be:
Engine Idle Speed
401M, 478M . . . . 575 rpm
478SN ...............
6 3 7 - V 8 ............
If n e c essa ry to adjust idle speed, perform the
following:
a. R eferring to figure 5, p lace a %-inch open
end wrench on diaphragm extension of vacuum idle
stop to prevent its rotation w hile adjusting plunger.
CAUTION : UNDER NO CONDITIONS
SHOULD TH E DIAPHRAGM EXTEN SIO N
B E A LLO W ED TO R O T A T E OR IN TERN ­
A L DAMAGE W IL L R E S U L T TO VACUUM
ID L E STOP UNIT.
Figure 5 —A d ju stin g Vacuum Id le Stop
8. Turn "in" idle m ixture sc r e w s equally by
rotating clock w ise to LEAN m ixture until the fo l­
low ing id le sp eed s a re reached:
Engine Idle Speed
401M, 478M . . . 525 rpm
478SN ..................
500 rpm
637 - V 8 ...............
475 rpm
b. Adjust the plunger with a 3 /8 -in c h open
end w rench until the fa ce of the plunger hex con­
tacting the carburetor throttle le v e r r e su lts in the
id le speed given e a r lie r in Step 4.
5. Adjust the id le m ixture sc r e w s. With engine
running and idling sp eed se t, turn each m ixture
sc r e w "in" until engine begins to slow down or
run unevenly, then back out sc r e w s until engine
runs sm oothly with highest vacuum gauge reading.
NOTE: If vacuum gauge is not available adjust
id le m ixture sc r e w s to give highest id le speed.
Turning id le m ixture sc r e w s "in" gives lean er
m ixture; w hile turning sc rew s "out" g iv es richer
m ixture. DO NOT turn sc r e w s in tight against
s e a ts sin c e sc r e w s or se a ts w ill be damaged.
6. With the aid of a ham m er handle or other
su ita b le driving tool Install p la stic caps on each
id le m ixture screw as shown in figu re 4. Each
p la stic cap i s to be in stalled in fuel RICH position
(as shown). Tang on cap should contact main body
o f carburetor. U se care when installing caps to
p revent bending of id le m ixture s c r e w s.
7. R eset the engine idle speed a s d irected in
Step 4.
IDLE A N D M IXTU RE ADJUSTM EN T
PRELIMINARY CHECKS
1. Thoroughly w arm up the engine. B e su re
the choke is open.
2. B e su re the carburetor is properly secu red
to the intake m anifold, which w ill exclude the p o s ­
sib ility of a ir lea k s. C arburetor-to-m an ifold b olts
or nuts should be tightened to 20-25 foot-pounds
torque.
3. Inspect the m anifold heat control valve (if
used) for freedom of action and c o rrect spring
ten sion .
4. A ir conditioning, when used , should be
turned on during the carburetor adjustm ents.
5. Check dwell reading and adjust, if n e c e s ­
sa r y . D w ell reading on V6 engines should be 31 to
34 d eg r e e s. D w ell reading on 637-V8 engine should
be 28 to 32 d eg rees.
6. Adjust ignition tim ing with vacuum advance
disconnected and plugged.
Engine Ignition Tim ing
401M, 478M, 478SN . . 7%u BTDC
6 3 7 - V 8 ........................... 5° BTDC
R econnect vacuum advance h o se.
CHEVROLET SERIES 70-90 HEAVY DUTY TRUCK SHOP MANUAL SUPPLEMENT