Re: Vac advance?

[Tom Mandera <tsm1@domain.elided>]

jmbrodsky@domain.elided wrote:
> 
> The vacuum advance on my Scout has never worked.  In 8 years I had it tuned
> thinking that everything was great until I remove the vac advance line at idle,
> and nothing happened.  rev the motor, then open the line, nothing.  I've checked
> they arc at night.  The vacuum advance really only compensates for the instant
> 'bog' when you open the throttle plates.  It's the only opportunity for the
> vacuum advance can to change.  Smooth operations in throttle use no vac advance
> change at all.

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 still have a vacuum leak on the booster that isn't helping.. I
installed a new check valve and gasket last week with plenty of clear
RTV (yup, no colors!) and I still get a whistling/hissing from the check
valve area.. put my hand over it, no sound, move my hand, sound.  Plug
the vacuum port to the booster, idle vac. goes up a few inches.

I think it's around 15 or so w/ the booster plugged.  Timing is at
10BTDC (did I mention the light stays solid when the motor isn't
stumbling from the vacuum leak?).

I do not believe my engine has an EGR.. it's a '72 304A.  Looking at my
parts book, the EGR should look something like a a coffee cup diameter
pancake, located between the carb and the bellhousing, on the top,
right?  Only thing there is a "T" which is also listed in the parts
book.  Should my '76/77 196 have an EGR?  Or a '77 345?  I may have to
look at the other two motors and see...

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.

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).

Interestingly, the motor *was* timed at 16BTDC by a professional carb
shop when I had the carb installed 3 years ago.. ran fine with a slight
hesitation from idle to throttle.. but, when I got it back from it's 2
years of captivity, it was exhibiting these symptoms.

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?

If I accelerate slowly to medium throttle, then stab it, it still has a
hesitation.. doesn't stall, but I can DEFINITELY feel the shock to the
drivetrain when the main jets kick in harder.

Seems to me these are all low-vacuum conditions when the vacuum advance
should be working?

Frustrating problems... grrr...

And it all combines to 5mpg in town, too... ;-)

-Tom