Re: Vac advance?

[jlandry@domain.elided (John A. Landry)]

In a brilliant stroke of genius, Tom Mandera <tsm1@domain.elided> blurted out:

>Is this a possible culprit for my bog/stumble when off-idle?  I still
>haven't tracked the problem down and have succumbed to bumping the idle
>up in the meantime.. 

I don't think so Tom...

>Anyhow.. it'll idle OK.  If I accelerate *slowly* it accelerates just
>fine.  If I go from idle to WOT, it has a slight stumble then roars like
>a lion.  If I punch it from idle to half throttle with any speed, it
>typically just lean-starves and dies.  The Acc pump squirters have long
>since shot their fuel, and the main jets never start flushing gas down
>the throat.

Still sounds to me like the classic inadequate accelerator pump shot.  Did
you try different accelerator pump cam profiles yet??? 

>Kinda thinking the vacuum leak was causing the vacuum signal to be so
>bad that it's not getting the to carb and screaming "GIVE ME GAS" unless
>I nail it to WOT.. but plugging the brake booster didn't eliminate this
>stumble (so maybe there are more leaks to plug).

In my experience, lack of ignition advance... whether it be vacuum or
mechanical... usually makes the engine power feel gutless across the power
band.  An improperly calibrated advance curve can lead to gutless
performance more restricted to certain engine rpms.  Lack of advance also
leads to excessive fuel consumption.

In your case, your performance seems to be fine once you get beyond the
(what seems to be) lean stumble.  This points directly at the accelerator
pump.

>If it were set to 16BTDC static, would that indicate a shot vac. advance
>or mech. advance?  Or rather, would it be used to *mask* it?

Yes... excessive advance at idle is generally used to cover up and gain
back a lack of advance at higher rpms.  If the vacuum advance and / or
mechanical advance aren't working (or only partially so), you have to dial
in more advance up front.  This doesn't completely solve the problem, but
is (as you suspect) a coverup at best.

I personally contend (and others will freely disagree if they wish) that
any IH engine running on regular pump gas set to something like 16 degrees
BTDC is covering up for a problem!  Either they're distributor advance
curve is way out of spec, the vacuum advance is shot, or the mechanical
advance is frozen... or maybe even all three.  It's been my observation
that very likely your advance curve will be out of spec if you have a chain
auto parts store rebuilt distributor... like the Cardone brand.  I've had
my hands on three of these and every single one was not right... usually a
very delayed and limited mechanical advance curve.

>Frustrating problems... grrr...

Play with the accelerator pump cam... go for a *longer* duration.  But it
certainly can't hurt to check your distributor too.  You do know the basic
procedures... right?

Good luck,

John L.
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