Bryan:
Just went out and took a good look at exactly where I drilled the hole. It
took me two tries to get it right. The spot is at 11:00 on the front
throat, and goes through the bore only a few thousanths in front of the
butterfly. Too far away from the butterfly and there won't be enough vacuum
to opperate the advance. The choke (enrichment hole) is a few thousanths
after the butterfly and would be subject to vacuum at all times. If I
understand vacuum advance correctly, it's purpose is to improve fuel economy
under light load only, like at highway cruising speeds. At Idle, an engine
has to have minimal advance to prevent backfiring and hard starting.
Centrifugal advance corrects the timing necessary as engine RPM increases,
and Vacuum advance allows the timing to advance or retard based on engine
load (how am I doing so far?) On a race engine, you would be under load
most of the time, so vacuum advance wouldn't be necessary (hence Cooper
Distributors with no vacuum advance provision). Manifold vacuum will not
work correctly because manifold vacuum is too high at idle and under
acelleration, resulting in timing that would be too far advanced in both
those situations.
That is how I understand it, anyway. My fuel economy improved about 4 mpg
highway when I hooked up the vacuum advance.
David Riker
74 Midget
63 Falcon
----- Original Message -----
From: Bryan Vandiver <Bryan.Vandiver@eng.sun.com>
To: <Bryan.Vandiver@eng.sun.com>; <dwramsey@worldnet.att.net>;
<davidr@sunset.net>
Cc: <spridgets@autox.team.net>
Sent: Tuesday, August 03, 1999 3:00 PM
Subject: Re: Timing the 1275 ? (vac. advance with a sidedraft)
> David,
>
> Thanks! This is specifically the information I was looking for.
Unfortunately
> your picture didn't come out (I'm on a Sun workstation), but I have a
pretty
> good idea of where I should tap a vacuum hole. Since the dellorto, and
weber
> DCOE are virtually identical, your instructions should apply to my setup.
I just
> want to make sure that I know exactly where the hole should be placed...
> With the plate fully closed, should it be right up against the plate (on
the
> filter side), or out from it a few millimeters, or is it that critical??
For
> this to work properly, I am under the impression that you want to have
'minimum
> 'vacuum when the throttle plate is fully closed, and have the vacuum
increase
> the moment the throttle starts to open. There is a hole where the 'choke'
lets
> in fuel, and I'm wondering if I can tap into that( since the choke never
gets
> used anyway),but I need to double check to see if that is in front or
behind the
> throttle plate. Does that sound like a possibility??, or do you think I'm
better
> off just drilling a new hole in the side of the throttle body?
>
> Regards,
> Bryan
> >
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