Re: Vaccuum Advance on '71 Scout

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

The aliens commander decided MikeIIDC@domain.elided would make a perfect
specimen for dissection, and he yelled...

>I have not use a timing
>light on the thing, but the timing was set with the tune-up-done by a
>buddy-when I first got it, and I have checked it "by ear" and it seems
>to be pretty optimally set.

Mike Mike Mike... how hearing this breaks my heart!

Do we need to take up a collection to buy you a timing light and a tach /
dwell meter?  Please save yourself some serious headaches and actually
check and verify what the timing, dwell and rpm are set at.  You're just
making more work for yourself by failing to use the proper tools.

>I checked the points gap, and the tune-up is very recent.

But that doesn't mean the "tuneup" was done right.  Never trust a
mechanic... especially a "buddy"!  I don't expect you to trust me either!

>I went out this morning to try to check vacuum advance.  First I tried
>spraying "starter" fluid around the base of the carb and all hose
>fittings and junctions to listen for acceleration.  Nothing here.  I
>then hooked up the vaccuum pump to a "t" fitting that has multiple
>branches coming out of the distributor.  It has a branch to the
>"charcoal canister," one to the distributor, one to the carb, and one to
>a plug sticking out of the top of the rear of the intake manifold.  When

Something is definitely wrong here Mike.  Typically, the vacuum advance on
the distributor will be hooked directly to a straight vacuum or metered
port on the carb *or* (in it's simplest form) directly to the intake
manifold.  Sometimes this vacuum advance hose will be routed through a
thermal control valve, but never should it also be connected to the
charcoal canister, etc.  If in doubt, install a new hose directly to a
vacuum source on the intake manifold.

>I disconnect the carb hose from this conglomeration and try to build up
>some vacuum, nothing holds.  I can hear the hiss of the carb hose, but
>the pump builds up no pressure no matter how hard or quickly I pump.
>There is also no reaction in terms of RPMs.  If I reattach this and hook
>my vacuum pump directly into the spout from the distributor, it holds
>pressure up to about 20-25 when I quite pumping, but I get no reaction
>from the engine here either.  I was under the belief that this vaccuum
>advance would trigger an increase in RPM's.  Is this true?

A couple of things need mentioning here Mike... Applying vacuum to the
distributor's vacuum advance might cause a *slight* increase in rpm
depending on several factors such as the curb idle rpm and base ignition
timing.  But the correct test (to verify if your vacuum advance is working)
is to hook up a timing light and watch for movement of the timing mark on
the harmonic balancer.  The vacuum advance only gives you about 5 to 10
degrees of advance, which in the big picture isn't all that much, so you
may not be able to "hear" the change in rpm.  I applaud you for having a
hand held vacuum pump to test your vacuum advance.  Most shade tree
mechanics lack one.

>If so, does
>this automatically mean I need a new distributor?

Definitely not.  Do the test with the timing light.  If you do see movement
of the timing mark as you apply vacuum to the advance unit, then the next
test to perform is the mechanical advance.  With the vacuum advance hose
disconnected and plugged (and the timing properly set... usually to zero
degrees BTDC), increase the engine rpm while watching the timing mark with
the timing light.  As the engine rpm's increase, the timing should advance
smoothly and return to the initial setting when the engine returns to curb
idle.  Depending on your engine and year, you should see as much as 20+
degrees of mechanical advance... this is normal.

If for some reason the mechanical advance isn't working, then you either
need to disassemble, clean and lubricate your distributor or exchange it
for a rebuilt unit.  If the vacuum advance isn't working (and the
mechanical is) then you can buy just a replacement vacuum advance unit.
But the fact that your advance unit is holding good vacuum, it's likely not
in need of replacement.  Usually they don't hold vacuum when they fail (the
internal rubber diaphragm ruptures).

>what about the other
>lines not holding vaccum?  Should they?

Not necessarily.  The lines connected to the charcoal canister will usually
have a metering valve somewhere in the line to prevent a huge flow of
vacuum, but vacuum will always be flowing nonetheless.

>Should I change the hoses
>first, then see if it runs better, or does the lack of vaccuum effect on
>the distributor mean that I definitely need a new dist regardless?

No... see above.  Lack of a functioning vacuum advance (in your case
probably caused by the vacuum hoses connected all wrong) will lead to
generally gutless performance and poor fuel economy.  Couple that with the
timing set wrong (possibly to compensate for the screwed up vacuum hoses)
and you could easily have your engine running like crap.  Now I'd be really
suspect about the quality of your buddy's tune up skills!

>And
>does the lack of vacuum in the others mean I definitely need new hoses,
>or is there a possibility of a bad component?  Something in the canister
>leaking, etc?

Not necessarily.  Again... wide open vacuum leaks are generally bad.  But a
*metered* amount of flow through certain components is acceptable... like
in the vacuum canister.  The idea is that the engine is sucking stored fuel
vapors from the charcoal canister when running.  So vacuum has to pull
through the canister for this to work.

But sure... you might have bad hoses on top of everything.  You simply have
to carefully examine them to see if they are causing a problem.  Look for
loose or cracked hoses.

> If the dist is bad, how much interchange did IH use on
>these?  For testing purposes, could I pull the one out of my '68 345 and
>try it out?

Yes you could... but it's better and easier to preform the tests I
described above, because your alternate distributor may be screwed up too.
You won't know unless you test it with a timing light as described.

>Any other input on this?

Yes, but I'll save them until I see you at Tulare... you will be there
won't you?  We need to sit down and have a long talk Mike. <grin>

>Thanks in Advance, once again!

Good luck,

John L.
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com      | 
Conservative Libertarian        |  "The road to  tyranny, we must never
Life Member of the NRA          |   forget, begins with the destruction
WA Arms Collectors              |   of the truth."
Commercial Helicopter - Inst.   |                     William J. Clinton
http://www.halcyon.com/jlandry/ |    10-15-95, speech at the Univ. of CT