Re: Carb specs...

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

With the crack of the whip, Caesar Naples <caesar@domain.elided> moaned with
pleasure and gasped:

>I posted this note to the IHC digest the other day, and I was wondering
>about your thoughts on the whole thing.

I saw it but thought someone else had answered.

Are you having specific problems or just wondering about the details of the
carb?

>First the main jets.  My books says "#53", but I found a "531" and
>a "542" (but the "2" was upside down?) in the new carb.  The "542" 
>was on the accelerator pump side.  I assume the "531" is close enough,
>but what about the "542"?  Can I use that?

If I were you, I'd stick with the jets that are in the carburetor.  They will
be perfect for 99% of the applications the carb was designed for.  If after
running the vehicle with the new carb you run into problems, then maybe we can
consider other options.  But remember... this is an emissions type 2 barrel
carb and it's designed to be slightly on the lean side.  Performance is
acceptable, but nothing to write home about.  It's just your good basic carb!

The basics... jet numbers like 51, 52, 53, 54, etc. would be standard Holley
jets available at any good auto parts store.  But each jet has an allowable
flow tolerance, so you could buy a number 52 jet that flows on the high side
of the tolerance limit and a number 53 jet that flows on the low side of the
tolerance limit so that there is very little difference between the two.  This
can be frustrating when you're trying to change the jetting, only to find
swapping the jets does little to effect the air / fuel mixture.

Because of this, Holley came up with "close tolerance" jets.  They added an
additional digit to the end of the normal jet number to indicate where in the
tolerance limit the jet falls.  A number 531 would mean the jet is on the lean
side of a normal number 53.  Number 532 means it's in the center and 533 would
be on the rich side.  Get it?

Even such small changes in flow from 531 to 532 to 533 can make a big
difference when trying to properly tune a carburetor to meet Federal emissions
requirements.  Vehicle manufacturers can't risk throwing standard "wide
tolerance" jets into the carburetor... some vehicles with jets on the rich
side of the tolerance limit might exceed emissions limits.

This is why you'll typically find jet sizes in these 2210C Holley carbs like
531 on one barrel and 542 on the other.  And having two different sized jets
installed in the carburetor isn't a mistake either.  The construction of the
different height runners in the intake manifold and the heat riser on the
exhaust manifold lead to this requirement.

>Next, the power valve.  I can't find the spec for that in my manual.
>The new carb has a power valve with a "40" on it.  The old carb has a
>power valve with an "11" stamped on the top (I don't have the right 
>tool to remove it).  Should I stick with the "40" that came w/ the
>new carb?  What's "stock" for the 304?

Yes... stick with what's in there.  This power valve in this model of
carburetor is not subject to "blowouts" (from backfiring) as another Digest
poster incorrectly stated.  This is not a diaphragm type power valve that
suffers from the rupture problem.  It's more like a check valve with a plunger
and spring.  On the other hand, these power valves are sort of rare in this
day and age (semi-obsolete) and you're not going to find them for sale
anywhere other than at Holley direct.  You'd have to call them to get the
specs.

Again, try the carb like it is and if you have a problem, we can brainstorm.
The power valve gives you that extra rich air / fuel mixture when you need it
at low vacuum conditions (heavy acceleration or load).  It activates at a
certain vacuum and enrichens the mixture a certain amount in several stages.
Again, its particular specifications aren't documented in any readily
available book anywhere that I'm aware of.  But Holley technical service could
look it up if your *really* needed to know.

Hopefully you don't drive around much in that condition (low vacuum / heavy
load) because your gas mileage will suffer when it kicks in.  As another
Digest member mentioned, you can change out the power valve to delay its
activation and change the amount of enrichening, but believe me, you can rest
assured that the stock valve has been chosen to reasonably balance fuel
economy, emissions and performance.  So personally I'd try it like it is.

>And finally, the accelerator pump adjustment.  There's no value listed 
>for my year, but the '73 is listed as 17.78mm (.700").  Should I use 
>this setting, or does someone have the correct one for the '78 304?

This setting is also correct for your year.

>And about using this number... the manual says "Close throttle valves
>tightly. Measure distance between top of air horn and end of plunger
>shaft."  I read this as I should measure from the end of the plunger 
>UP to the top of the rim of the airhorn, but there's almost no way you
>could adjust it anywhere near 17.78mm if you measure that way.  Is the
>right way to measure this from the end of the plunger DOWN to the slot 
>in the air horn that the plunger comes through?

It sounds like you're simply having trouble understanding what they want you
to measure.  After you follow the previous paragraph in the manual to the one
you just mentioned (back off the idle screw and make sure the choke fast idle
cam is out of the way), you hold the throttle closed.  You then stick a small
machinists rule beside the accelerator pump shaft (that rectangular bar) and
rest it on the surface of the carburetor top (air horn as they call it in the
manual).  Be sure the ruler is not resting on the slotted washer that's around
the accelerator pump shaft and resting on the air horn.  The correct
measurement (.700") is from the bottom of the ruler to the end (top) of the
pump shaft.  NOT the top of the pump shaft to the edge of the air filter
housing mounting lip as I suspect you might have thought it said.  Hope this
clears that up?

If the measurement is still not close to .700", then you may have the
accelerator pump connector rod in the wrong slot of the accelerator pump
rocker arm.  The 2210C from my 1976 Traveler with a 345 was in the top slot to
achieve this distance measurement.

>I'm kinda new to carbs (couldn't tell, huh?) so if anyone wants to offer
>any other advice, feel free (stuff like "oh, don't ever do this when
>replacing your carb" is greatly appreacited).

Put the carb on as it is and properly adjust it per the manual and see how it
performs!  If new, I'll bet most of the settings are right where you need
them.

Good luck,

John

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