missing or defective thermostat, causing low operating temperature
clogged intake manifold exhaust crossover passage (for heating intake manifold)
engine misfire caused by bad spark plug, ignition wire, coil, dist. cap or rotor
late ignition timing
bad vacuum advance unit (common in old cars)
stuck centrifugal advance mechanism in distributor (needs oil occasionally)
bad tension adjustment of centrifugal advance springs
defective oxygen sensor (causes rich mixture)
defective coolant temp. sensor for engine computer (causes rich mixture)
worn timing belt (may cause rich fuel mixture in fuel injected cars with a MAP sensor)
incorrect timing belt installation (timing marks should line up)
sticking or poorly adjusted carburetor choke (causes rich mixture)
dirty air filter on an engine with a carburetor (causes rich mixture at high power)
clogged or defective PCV valve
incorrect valve clearance (especially if too tight)
high performance camshaft installed (not efficient at low power)
worn valve guides (makes fuel mixture too lean at low power)
vacuum leak from bad vacuum hoses or intake system gaskets (causes lean mixture)
dirty fuel injectors
bad fuel pressure regulator
bad MAP sensor for fuel injection
leaking intake duct between air flow sensor and throttle body
too much alcohol in gasoline with old car (requires carb. modification for correct mixture)
rusted or missing exhaust heat stove or pipe for heated air intake
loss of compression pressure due to worn rings or leaking valves
sticking EGR valve (causes rough idling and rich mixture on some engines)
exhaust backpressure caused by clogged catalytic converter, muffler or crushed pipe
stuck exhaust manifold valve for some V type engines. Intended for fast warm up.