QuestionI have a '78 Midget, recently overhauled. Advertised gas mileage 40 mpg highway..actual 20 mpg @ 55 mph. Will different weight oils in the damper affect mileage? i.e.: lighter weight oil for leaner mixture/better mileage? Thanks...you are a godsend!
AnswerHi Stephen,
The damper oil weight only effects a quick throttle movement so if you are on and off the throttle a lot a thicker oil will give you a richer mixture. At a steady speed the damper is not in use and has no effect at all.
As far as mixture goes the best fuel mileage will not come from the leanest mixture.
All fuels have an ideal mixture called stoichiometric. As you lean down from that ideal mixture you loose horse power and fuel mileage. As you richen the mixture above that ideal mixture you gain horse power to a point then the power starts to drop off. Many factors determine what that point is. I have seen tests that show that point as 15% richer and others that show that point at 40% richer than ideal. As you richen above stoichiometric even though you gain horse power you loose fuel mileage. As you lean out lower than stoichiometric you loose fuel mileage. So leaning a mixture out even though it may seem to run smoother, it is loosing mileage.
Many factors determine fuel mileage. You have something wrong with your car to only get 20 MPG. You should get twice that amount. finding out what is wrong will require a systematic diagnosis.
Many times when I was working in dealerships I would get the same complaint. I found it necessary to set up a policy that I needed to stick to so I could correct the problem.
First you must do an accurate fuel mileage test. You don't need one of those test tanks to do it. Find a long straight road with a service station on it and fill the tank as full as you can. Log the odometer and drive normally straight down the road at least 10 miles and turn around and come back to that same station and same pump facing the same direction and fill it up again and read the odometer. Be sure to drive steady as possible and not over 55 MPH.
Now you can figure your mileage.
Go back and do a compression test (Throttle open and at least 4 or 5 revs)and set the valves and timing accurately. Be sure your air filter is clean and use engine oil in the top pot. Remove the air filter and very slowly start to restrict the air flow to the carb. If at any time the engine speeds up or smoothes out as you restrict the air intake, you have an air leak either at a vacuum hose or gasket. Replace the air filter and lift the small spring loaded pin under the lip of the top pot and note the reaction. If the engine tries to die the mixture is too lean and if it tries to speed up or smooth out it is too rich. When it is correct it will change a little and run steady. (Different with dual carbs)
After you have completed ALL of the above go do the fuel test the exact same way as before.
You should see from 35 to 40%+ MPG on that car.
Let me know,
Howard
