Tuning Idle Speed Circuit. (was: Re: idle problem)

["therealdmcvegas" <DMCVegas@xxxx>]

--- In dmcnews@xxxxxxxxxxxxxxx, "checksix3" <jetjock11@xxxx> wrote:
> ...I have yet to see the proper way to set up the idle speed system 
> (the Bosch recommended method) in any post or archive.
<SNIP>

I know I've seen the procedure before, but I can't remember if it was
here, or off list in a private e-mail. but I believe the procedure was
to open the first/main screw by giving it 4 equal turns, then give
each throttle tube 2 screws. Then just fiddle with the main screw to
set the idle.

>As long as you 
> have no air leaks and your CO is set closely this will solve all idle 
> hunting...period. Some airflow trough the throttle body and metering 
> plate is required for the system to work properly. This is obvious if 
> the conductance of the system is measured. 
<SNIP>

Well, um. This is true, but only dependng upon your point of view. The
Idle Speed Motor does a fine job of setting the idle speed, and of
maintaing it. But as I found on my car, only when you drive gently.

If the engine is at a high rev (usually about 4K or more RPMs), then
when you push the clutch in to drop (or let off the gas on an auto
from my experience), the RPMs will fall VERY quickly. In my case, I
had two senarios: I'd would either kill the motor occasionally, or
mostly I would just strain the hell out of my electrical system. Dim
lights, dropping volts, you name it. This was compound at night when I
had the clearance and headlights on. And if the A/C was running, then
forget it. I could drop my RPMs low enough that battery light would
flicker, of not kill the motor again.

Now I had to compensate on my car to bend the bracket for the Idle
Speed Microswitch, and extending the stop bolt on the throttle
assembly. This now causes my Idle Speed Circuit to trip early, and
activate the ICM so that it will open early enough so the engine
doesn't dip too low, and I keep my electrical system stable.

However, there has of course been a trade off. I now have no gradual
powerband. Gently pressing my acellerator pedal, The ISM then tries to
compensate for a "fast idle" by choking my engine, rather than
allowing me to add the extra airflow to my acelleration. Once the
Microswitch is deactivated, my RPMs then jump to 1,500! Not really
fun, since it makes my clutch a bit "grabby" IMO, but nothing really
damaging. Just annoying.

The other problem with the Idle Speed Circuit is that it doesn't like
to work well with a motor that's not properly tuned. If you do have a
vaccum leak, or you're running rich, then the idle seeking/hunting
will of course occur (that's the LAMBDA system cycling). However, the
Idle Speed Circuit seems to compound this problem. Instead of suttely
compensating the ISM to gently stabilize the idle, it likes to panic.
Anyone who's had a vaccum/mixture problem, and has tried to start
their car on a cold morning know exactly what I mean. The ISM goes
"WOT" to increase the speed. And when it gets too high, it goes
"Choke". And when the RPMs drop too low, it goes "WOT" again. This
cycle occurs until the engine finally warms up and/or the fuel mixture
is balanced.


Now with this said, don't think that I'm putting the stock Idle Speed
Circuit down. On the contrary, I think that to remove it is perhaps
one of the worst mistakes that you could make. However, I do feel that
as great of a system that it is, that it needs to be supplemented. And
that is where setting the Engine Idle Speed manually comes into play.

By setting the initial Engine Idle Speed manually with the 3 brass
screws, we create a "pillow" of sorts for the engine speed. The idle
should stay far more stable. When the RPMs drop hard, the engine isn't
going to have to wait for the ISM to activate, and open up to feed the
engine air. When cold starting, the idle seeking should be kept to a
minimum by way of a solid supply of air keeping the RPMs from dipping
too low. And of course, strains on the cars electrical system could
greatly be quelled. Extending the life of your Alternator and Voltage
Regulator, and keeping the motor from stalling out in some cases. And
of course, you can keep the lights bright. Another thing is that this
should hopefully reduce engine noise a bit. When my motor is cold, the
sound of extra air rushing thru the ISM, and the brass tube is quite
noticeble.


Now, if there is any downside to utilizing the Manual portion of the
Idle Speed Circuit, then please, let me know. I myself really don't
see any harm in using it. Only benefit. And everything that I've read
against it has been mainly along the lines of, "You shouldn't need
to.", and "The manual doesn't say it's nessisary." I understand both
these points, but in using the Brass Screws, I really only see how it
can improve upon the existing system.

-Robert
vin 6585 "X"