GMC 1976 ZEO 6083 Maintenance Manual page 280

NOTE :
Does not respond promptly
GENERAL
Poor fuel quality.
Vapor lock . (11)
Late ignition timing .
MECHANICAL
Accelerator pump linkage adjustment
correct.
Accelerator linkage faulty or improperly
adjusted .
GENERAL
Cold engine and choke too lean .
Loose carburetor mounting bolts. (9)
Loose intake manifold bolts. (9)
Incorrect timing and dwell.
Vacuum leaks (hoses etc.) . (9)
IGNITION
Leaking distributor cap may cause
to occur in cylinder on intake stroke .
Two crossed spark plug wires may also
cause backfire through carburetor .
GENERAL
Late timing .
Burnt exhaust valve.
Air cleaner restricted .
Air leak in exhaust manifold
MECHANICAL
FLAT SPOT (SAG, STRETCHINESS)
or pipe .
Late valve timing .
Worn or broken exhaust valve spring.
Tight exhaust valve.
Choke stuck closed .
when throttle is opened quickly.
backfire
FUEL
Low fuel pump pressure .
Accelerator pump piston or diaphragm
Accelerator pump valves leak or passages
restricted .
Float level incorrect.
Defective fuel pump .
Carburetor defective or improperly set.
Fuel filter plugged.
Dirt in carburetor jets .
BACKFIRES, POPPING BACK OR SPITBACK THROUGH
CARBURETOR (SUBDUED EXPLOSION IN INTAKE
MANIFOLD)
FUEL
Lean mixture .
Dirt or water in fuel .
Faulty accelerator pump .
MECHANICAL
Leaky or sticky intake valve .
Weak or broken intake valve
Faulty heat valve.
Plugged heat crossover passage.
Improper camshaft timing .
Improper valve lash .
AFTER-BURNING OR MUFFLER EXPLOSION (BACK FIRE)
NOTE :
A subdued put-putting at the exhaust tailpipe may be due to leaky ex-
haust valves which permit the mixture to finish combustion in the muffler. If
exhaust pipe or muffler is red hot, better let it cool, as there is some danger of
setting the vehicle on fire . Most likely to occur when mixture is lean .
ENGINE 6A- 1 5
spring .
IGNITION
Intermittent open circuit in primary .
(Ammeter needle swings further away from zero
when generator is charging.)
Intermittent short in primary. (Ammeter
swings toward zero when generator is
charging .)
Short in coil or secondary coil wire.
If just a couple of explosions are heard
and then no more for a time (even for days)
the trouble may be due to a gradually failing
condenser .
FUEL
Carburetor flooding .
leaks.