High Idle at start up: Depending on your carburetor you may have either a
temperature controlled choke, a vacuum controlled choke, or an electrical
choke. regardless of which type, if the idle speed drops after the engine is
running and you step on the gas peddle lightly a few times then all is
working as it should be. The actual warm idle speed can be adjusted up or
down with the stop screw located below the choke assembly. This actually
should be done with a tachometer and set to 800 RPM on a warm 305.
Note: a poorly timed distributor will affect your idle speed, also.
Depending on the production period in '85 you have (I'm betting) a
computer controlled ignition system. This is a very simplistic "computer"
that adjusts the timing advance based on a vibration or "knock" sensor
mounted to the engine block. After 30 years of use it's not surprising that
this circuitry has become "flaky" A quick test is to block your wheels, apply
full hard parking break, start the motor and put it in drive; attach a timing
light to the #1 spark plug and find the timing mark on the front flywheel. If
the mark seems to jump around randomly or move several degrees back
and forth (wiggles a lot) - the Electronic Spark Control (ESC) or the sensor
or both are bad. When the engine is under load and you give it more gas
to accelerate the timing should delay or "retard" just slightly until the
engine spins up. A bad ESC sometime ignores this condition and can
cause heavy pinging, backfiring, engine "lugging", and knocking.
Many mechanics of that period to today will tell you to replace the
electronic control in an '82 thru '88 chevy with the "old fashioned" vacuum
and mechanical advance system. If these are set up properly and you do
not change your truck's load often, you will not have problems. I just
performed this on an '85 305 suffering from your exact symptom. It runs
great now at all speeds - even if it's loaded to max and gets much better
gas mileage (went from 9 mpg to 16 cty/22 hwy mpg).
Bluish smoke means oil is burning with the gas in the cylinder. This could
come from worn rings, worn seals around the valve stems, a bad head
gasket, or worst case, a cracked head. If the smoke appears white at any
time it will be most likely one (or both) of the last two. Other signs of a
cracked head MAY be water in your oil and/or exhaust gasses in your
cooling system. Water contaminated oil will look like chocolate milk when
it's up to temperature. There are chemical fixes to this, but I have never
seen one last more than a few months. The real way to fix it is to pull the
heads and replace what ever is bad, valve seals, gaskets or the heads
themselves - and pray it is NOT a cracked block.
Good luck, kid. Don't be afraid, you will learn much. This engine is not that
complex. Just buy a good mechanic's manual for the year and make and
follow it's procedures to the letter. Chilton's is a good example.
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