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I just bought from Man-a-Fre
a Weber 38MM carb for my 1978 FJ 40. The increase in power has been great
but I'm not getting the proper gas mileage. What can I do to improve gas
mileage?
More than likely there
are several things you can do to increase mileage. The first is the most
critical, give the engine a proper
tuning. Adjust the valves, this is supposed to be done every 12,000 miles,
or 12 months whichever comes first. The key to this is doing the valve
adjustment properly. Engine must be at operating temp, valves must be
adjusted one cylinder at a time, in firing position, 008" intake, .014"
exhaust. If you have a distributor with points, replace them, and the
condensor if you have one. Set the points to 36-39 degrees of dwell. Install
new spark plugs properly gapped. Set ignition timing @ idle of no more than
650 RPM, to 7 DBTDC (small steel ball peened into flywheel), with all vacuum
hoses to distributor disconnected. Check to make sure when you rev the
engine that the timing advances, if it does not, time for a new distributor.
Make sure vacuum lines have proper type of vacuum for style of distributor,
i.e. vac advance units require "ported" vacuum signal. This means no vacuum
at idle, and gradual vacuum increase as throttle is rolled on. Highest
vacuum reading for ported vacuum will be at light throttle. Vacuum retard
distributors require switch controlled manifold vacuum. These switches are
located in the thermostat housing. Manifold vacuum has highest signal level
at idle, and decreases as throttle is rolled on. Check to make sure vacuum
dashpot
is working, apply vacuum to a vac advance dashpot @ idle, and engine speed
will raise. Apply vacuum to a retard type dashpot, and idle speed will drop.
Next comes carb adjustment. After completing other tuning work, warm up
engine. Set idle speed using idle speed screw to 600-650 rpm. The Weber
38DGAS has 2 idle/transition circuits, and 2 idle mixture screws at opposite
ends of the carb. You will need to adjust/balance both. Using a tach to
watch rpm, pick either of the idle screws to start. Turn the screw
out(counter clockwise) 1/2 turn, and wait to see if the idle speed raises.
When you turn the screw out
and the idle no longer raises, stop, wait a little bit, then turn in the
screw (clockwise) 1/4 turn, wait, keep doing this until the rpm
drops, once the idle drops, turn the screw back that last 1/4 turn. Reset
the idle to 600-650 rpm if it is no longer in spec. Then start the same
process with the other mixture screw. After you are finished with the second
mixture screw, and have reset the idle speed, recheck the first mixture
screw. Test drive the vehicle, after the test drive, recheck idle speed,
you may need to go through the entire process again, but this is part of
tuning an engine properly. After all this you will at least have the engine
tuned. At this stage you'll be able to judge if you need to rejet the carb.
This would be done with either an exhaust gas analyzer, or the old fashioned
method of spark plug readings.
Other things that affect mileage, tire size, condition, and pressure. Make
sure your tires are set to the recommended inflation
pressure. Big tires are harder to roll, and cost some mileage, this is the
trade off we all deal with. Wheel bearings, these are supposed to be
repacked and adjusted every 24,000 miles, or 24 months. Gearing/tire size,
if you are going 55 mph, and are turning 3,500 rpm, with your foot to the
floor, it's time to think about tire size, and gears. Exhaust system, if
your muffler is collapsed internally, or you have some other exhaust
restriction, you will loose mileage, and power. Check to make sure your EGR
(Exhaust gas recirculation) valve is not stuck open. A properly working EGR
system will cost you 1-2 mpg, one that is stuck open, will make the engine
run poorly, and will cost even more in mileage. EGR's allow exhaust gas to
be fed
back into the engine after it has been run through a cooler. The exhaust gas
has already had the oxygen used up, and therefore does not dilute the
mixture like a vacuum leak would. This exhaust gas takes up space in the
combustion chamber at certain rpm, with the purpose of making the engine
ineffecient, and lowering NOX levels. Change to all synthetic lubricants, if
you read the 80 series online chat, a common thread is the mileage increase
from changing to all synthetic oils/grease. These guys typically claim a
1/2-2 mpg increase related to the changeover, and I would believe this to be
accurate. The engine's overall condition, if your engine is worn, or your
valves don't seal too well, you'll never get good mileage out of it. Of
course if your right foot is REALLY BIG, and you've got it to the floor all
the time, expect to pay for it at the fuel pump.
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