Date: Mon, 17 Mar 2003 11:31:50 -0800
Reply-To: Tom Young <tomyoung1@ATTBI.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tom Young <tomyoung1@ATTBI.COM>
Subject: Re: How Does Timing Affect Idle Mixture?
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----- Original Message -----
From: "Jay L Snyder" <Jay.L.Snyder@USA.DUPONT.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, March 17, 2003 10:35 AM
Subject: How Does Timing Affect Idle Mixture?
> Back to my main question. I have my timing advanced to 5 BTDC, which is
> about 10 degrees advanced from stock. I have been running this way for
> quite a while and this was even suggested by Bob Donalds. With the O2
> sensor disconnected, I am having trouble getting the idle mixture
adjusted.
> It is running lean even with the idle mixture screw all the way in.
I'd guess changing the ignition timing might have an effect on the mixture
"seen" by the O2 sensor since I suppose the timing would have some affect -
what, I don't know - on the burn of the mixture. Since I *don't* know how
timing affects these things I don't mess with the timing, even if it's
suggested by someone, unless they have a lot of science to back it up. So
my advice is to stick with the timing suggested by the manufacturer. The
way these AFM's work is that you set the fuel/air ratio *at idle* and the
contact strip/return spring are calibrated to deliver the "right" mixture
from there on out. The O2 sensor is really there to "nudge" the ECU if
something goes wrong.
> (When the O2 sensor is re-connected, the mixture is corrected.)
Well, sure... the O2 sensor is "fighting" the incorrect basic setting, and
winning.
>The problem
> is on cold starts, the O2 sensor doesn't do anything for about a minute or
> so until it heats up, so cold starts are a little funky. If I retard the
> timing back towards the stock setting of 5ATDC, will this have a richening
> effect o the idle mixture?
Don't know, but I don't think you can go wrong with a stock setting for
timing.
> It seems that tightening up the AFM spring has
> added to this idle mixture problem. I didn't see the lean idle mixture
> before this weekend. Is the AFM flap supposed to be moving around at
> idle? I am thinking it is now held shut by the higher spring tension.
Of course it has. Tightening up the spring affects the mixture over all
ranges. And yes, the AFM *does* move around at idle as your air pump
(engine) doesn't have a steady draw. You could tighten the spring up enough
to correct this, but the engine wouldn't run.
> My
> AFM is 18 notches tighter than when purchased new. The display still
goes
> to full rich (1.0VDC) when the pedal is touched and goes full lean when
the
> pedal is backed off. It is still getting plenty of gas, and I think
there
> may be a few more mpg left in it yet. I need to get the idle sorted out
> first. First, I will back the timing off to TDC (split the difference)
and
> see if that helps the mixture. If not, back off the AFM spring a few
> notches. Any other ideas?
Here's mine: Set the ignition timing to stock. Back off your spring until
it's just a few notches advanced from stock. Try getting the idle mixture
set correctly, with the mixture adjustment screw *not* at the extreme end of
its adjustment range. If that doesn't work "sneak up" on it by tightening a
couple of clicks more and working with the adjustment screw. I'll also tell
you that there can be air leaks into the system when you have the plastic
cover off so you should put the cover back in place, with a bit of sealer to
ensure against leaks, when doing your idle mixture screw adjusting.
> My guess is: Retarding the timing will force me to open up the idle
screw,
> which will in turn allow the mixture screw to have more of an effect.
Your guess is as good as mine.
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Tom Young '81 Vanagon
Lafayette, CA 94549 '82 Westfalia
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