Lots of response to my "MG's idle rich" post. Recommendations about
changing the needles were quite common. There were also cautionary notes
about the detrimental effects of a rich mixture on the rings (washing the
cylinders) and the combustion chambers (buildup of crud).
These are good points and quite correct. I could change the needles on the
A. My point was more that the cars, as set up by the factory, seem to idle
rich- especially once they get a few miles on them. My A and B both had /
have stock needles, I believe. On the A, I could and probably will lead it
out a flat or two to see if I can "sneak up" on a better overall mixture
setting. However, when tuning the car this Spring by the standard methods
(lift the dashpot) I ended up with a car that idles on the rich side of
optimal. It runs great with no stumbles or misses except at idle. When
cool, it idles great but once it is totaly warm it has a slightly lumpy
idle and long term idling in hot weather requires that I "clear the carbs"
occasionally or it will slow below 500 rpm (it hasn't quit yet). It is
funny that all of the tech manuals that I have read recommend clearing the
carbs occasionally during tuning. The only reason to do this would be that
the carbs are rich or drift to rich on extended idle or low rpm operation.
Concerning the positives / negatives of running an engine rich or lean.
The worst thing that you can do to an engine is let it go lean at full
power. This will burn a hole in a piston fairly quickly. Other than that,
there is a theoretical concern about overly rich mixtures washing the
cylinders and causing wear or causing carbon buildup. I have seen cars
that wouldn't start due to flooding / use of starter fluid end up with
"washed cylinders" where you had to pull the plugs and squirt oil in the
cylinders to re-establish a seal. This is not common but can happen. Some
aircraft engines have upper end trouble on 100LL (low lead) fuel if they
aren't sufficiently leaned at cruise due to buildup on the valves.
However, this is due to the higher lead content in 100LL compared to the 80
that they were designed to burn and low combustion temps associated with
rich conditions.
Aircraft engines, in fact, spend a lot of time running rich. Listen to/
smell a small plane at idle and takeoff. You set the mixture to full rich
and there is plenty of fuel around. The reason for full rich at idle is
that the engine doesn't cool well on the ground and a rich mixture lowers
temperatures. A friend of mine flies a turbo Navajo and he says that the
engine is liquid cooled and that liquid is gasoline. Aircraft engines,
which spend a lot of time at or near the rich limit, have a recommended TBO
(time between overhauls) of 2000 hours which is about equal to 100k miles
on a car. The FAA isn't too worried about cylinder washdown by rich idle
so I don't worry about it either. I will say this, my car does not
overheat at idle. It has yet to reach 200 F on the gauge this year
including ten minutes in a McDonalds drive through at 80+ F. I did have to
clear the carbs once or twice, though :-)
Someone mentioned that their car doesn't "smell rich" at idle. I know the
my car idle rich but it doesn't smell rich either. It could be that my
nose isn't as good as it used to be but I suspect that the formulation of
new gasolines don't smell the same when partially burned as the fuel that
we grew up with. My car has more of an industrial cleanser smelling
exhaust at idle.
I agree that a 160F thermostat is too cool. I have been meaning to buy a
180F t-stat but I keep forgetting. Maybe once the finance department
approves my request for funds on my wheel bearings, I will sneak a t-stat
on the order. I could also use points and a condenser. I removed about
3/4 of the material on my points last time I filed them.
Regards,
Bill Eastman
61 MGA with another 200+ miles logged this week.
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