Re: Proper jetting help

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

The aliens commander decided Russel DeArman <russde_93@domain.elided> would
make a perfect specimen for dissection, and he yelled...

>  I have two different size jets in there; a '54' on
>the choke side, and a '56' on the accelerator pump
>side.
>  I called Holley tech service today and gave them the
>carb number. The tech told me that this carb should
>have two '60' jets in it. What gives? Why would
>someone put in two different size jets?

Russ,

I hate to say this, but as usual with many tech service centers, the Holley
people are full of crap regarding your carb.  Well... to be fair... they
might be right about your carb if it's not originally from an IH vehicle.

Having two different sized jets is actually quite normal in these carbs.
Because the intake manifold is a crossover "dual plane" design, the intake
air has to travel different distances and likely at different velocities...
depending on which side of the intake manifold it's on.  The different size
jetting compensates for differences in the air / fuel mixture which results
from the intake manifold design.  Most aftermarket carbs would never have
mixed jets because it's nearly impossible to analyse the exhaust gases in
such a way as to figure out what you'd need.  Because of the intake cross
over design, you'd have to sample the exhaust gas of each and every
cylinder to determine the best jetting for the carb.  If you sample one
"collective" side of the engine's exhaust, you're sample the burned air /
fuel mixture which originated from both sides of the carb!

Going from memory (I don't have the information with me, but will look it
up tonight), the richer (larger) jet should be on the accelerator pump side
as in your carb.

>  I am going to tear the carb apart tonight to try and
>find/eliminate the blockage I apparantly have on the
>choke side barrel. While I'm at it I would like to put
>the proper size jets back in...but how do I know that
>the jets are correct? Is there a procedure for
>'tuning' the jets sizing? I called Hi-Lo and they have
>a package of two '58' jets. Would I damage anything by
>putting these in place of the current mismatched ones?

Tell us again what model / engine / year truck this is and I can look
everything up tonight.  Please also supply any and all code numbers from
the carburetor so I can attempt to very what it is.

The way to tell if you have the correct jet size is by using one or more of
the following: an exhaust gas analyzer, an oxygen sensor in the exhaust, by
reading the spark plugs, and by "feel."  The last two are the hardest to do
accurately, while the first is the most accurate.  A small portable exhaust
gas analyser is sold by JC Whitney, but it's probably cheaper to stop by a
repair shop and pay them to tell you what your air / fuel mixture is at any
given engine rpm.  A dyno test result would be the best.  But most guys
just go by feel because that's the cheapest of all.

Good luck in finding the carb blockage.

John L.
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