[DML] Re: Manifold Vacuum Question

["content22207" <brobertson@xxxxxxxxxxxx>]

I've got all the vacuum diagrams, but no scanner. Think my tortured
use of the English language will be enough?

Hopefully the moderator will let this through. It does have DeLo
application -- there's no reason a DeLo can't have alternative
plumbing installed (or even an entirely different flavor PRV for that
matter).

According to my literature, the dual barb spark advance diaphragm was
discontinued after 1979, but the delay valve was discontinued after
1978. Series 004 CPR was standard in 1979. Assuming it's original to
the engine, does your coolant distribution pipe have a thermactor
valve? That's a 1980 addition. I think I've seen you mention Lambda,
which is a 1980 addition as well (do you have CIS, added 1981?) In
short, there may be no telling what is original to your engine and
what has been added later (do you have a fuel/air mixture unit with
isolated throttle plates, or the single piece casting used through 1980?).

No matter, you can easily plumb it for what's onboard now.

The dual barb spark advance is an emissions concession. As fuel
mixtures got leaner & leaner, the risk of premature detonation
increased, especially with a conventional spark advance unit (don't
forget, base ignition times were creeping up too). To combat this,
manufacturers added an inner barb on the wrong side of the diaphragm,
attached to full manifold vacuum. This prevented the outer barb from
advancing ignition until the throttle plates were pretty fully open
(the only time fuel mixture approached anything close to pre-EPA specs
-- remember, this is the era of the spread bore carburetor too). But
you need spark advance on a cold engine, so the inner barb was usually
run through a thermactor valve in the cooling system. See where this
is going... (that's why late 1970's engines look like spaghetti).

The good news is: anything added to an engine design that predates EPA
regulations can be removed, returning the engine to its original
parameters, if that is your intention.

I'd recommend plumbing your spark advance just like a mid 70's model
-- simply run a single vacuum line from the ported barb under the
throttle plates to the outer barb on the spark advance diaphragm.
Don't worry about the inner barb -- with no vacuum attached your dual
barb unit will behave just like a single barb one. You can stick a
vacuum cap on it if the empty barb looks bad (an empty barb could also
confuse a 3rd party mechanic).

Both of your CPR's need full manifold vacuum if you want to use the
full throttle feature. They have 1/4" barbs on them, but your engine
may only have 3/16" and 3/8" barbs on the manifold. Where is your PCV?
If it's through the U pipes you can simply tap in there (same
diameter). I wouldn't recommend using a barb on the cold start tube
because they are calibrated much smaller. If you can't find a 1/4"
barb and decide to add one, McMaster Carr sells them with 1/8" BSPT
threads (the PRV does not use 1/8" NPT).

All CPR's except Series 066 work very simply. Full manifold vacuum
pulls them lean. As vacuum drops, they open richer. That's it.

If you're running Lambda, all this could be for naught. Unless you
figure out how to force the frequency valve inactive (Pin 11 on the
ECU?), it will fight every step you make to enrich the mixture.

Bill Robertson
#5939

>--- In dmcnews@xxxxxxxxxxxxxxx, "Rick" <rdevaux@xxxx> wrote:
> Bill, Thanks for the reply.  What I have is a 1980 B28 that has been 
> put into a 1978 262C.  It currently has the 004 CPR and the vacuum 
> delay/retard with two ports on the distributor vacuum diaphragm.  
> I've got a 038 and a 018 CPR ($10.00 new in the box,thank you UK 
> EBay).  Since I have the 004 and no other full throttle enrichment, 
> I'm considering putting on the 018 and wanted to hook it up to the 
> correct vacuum source.  With the 2 barb distributor vacuum diaphragm, 
> which barb should I hook up to manifold vacuum if I want to bypass 
> the vacuum delay solenoid?  What is the purpose of the delay solenoid 
> (emmisions?)and what will bypassing it affect?  I'm also looking into 
> adding the full throttle enrichment switch onto the throttle pulley 
> bracket and run the grounding circuit to terminal 11 of the ECU.  Do 
> you think the 018 CPR and the throttle enrichment switch are over-
> kill when combined?  What is the definition of ported vacuum?  Can I 
> hook up the CPR to one of the vacuum ports on the intake runner?  
> Thanks again for your help, Rick
> 
> 








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