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Originally posted by 73mach351c:
holley carb, (dunno think its a 650 dp)
dave

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There could be something right there. If in fact it's a mechanical secondary carb, is it bogging when abruptly sinking the pedal to the floor at low rpms?

It may infact be a carb tune issue as well. Also need to know what pump shooter(s) it's got in it, power valve size, jetting size.
The list number on the airhorn crossed with the numbers here http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSC/FMSC.html will tell you what you've got and how it was originally, not necessarily how it's tuned now as there's no telling who's done what to it.

On the ignition issue, Clevelands like plenty of timng; like 40+ overall. Of course the rest of the system must be up to the task of igniting all that air/fuel mixture those gargantuan heads flow

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1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9"

Welcome to M&M

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1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9"

Mike

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1968 Mustang Coupe 302

thanx for the help!!!

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Hello
Thanks for contacting Holley Performance Products Inc. If you are getting
backfires through the carb you are probably lean, and if it is backfiring
through the exhoust you are probably rich. You may want to check your
timing. Be sure that the vacuum advance is running off a timed vacuum
source. That is a vacuum that comes in as rpm rises. It is usually the
vacuum port on the side of the metering block.
You need to start off by doing the basic settings first. First pull the
fuel bowl sight plugs out and set the float level so that the fuel in the
bowl is at the bottom of the sight hole and it should only run out if you
rock the vehicle. Then you will need to set the idle mixture screws. You
will need to run the mixture screws in and lightly bottom and then back
out about one and a half turns to get a starting point. Start up the
engine and let it warm up to operating temperature, then hook up a vacuum
gauge to manifold vacuum. If you have a automatic put it in gear and hold
the brake, if it is a standard shift let it idle in nuetral and adjust the
idle mixture screws together in equal amounts until you get the highest
possible manifold vacuum reading on the gauge. Check vacuum of engine to
select correct power valve next.The power enrichment system basically
supllys extra fuel to the main well under heavy load conditions. The
power valve works off of vacuum. When the predetermined set vacuum
reading is reached the power valve will open and give the engine a little
extra boost of fuel under a load to help prevent a stumble caused from a
lean condition when a engine is put into a heavy load condition. As the
rpm of a engine rises you will notice that your engine has less vacuum,
but under a full throttle or full load position you will find that an
engine wiil have little to no vacuum. If you have a high performanc
engine with a performance camshaft you will find that your engine will
make less vacuum than a stock engine which is normal. The procedure you
go through to check to see what size power valve you need will vary from
an automatic and manual shift vehicle. In order to check on a automatic
transmission vehicle you will need to let the engine run and get up to
operating temperature. Then hook up a vacuum gauge to a manifold vacuum
port. Put the transmission in gear at idle and take a vacuum reading.
You will need to take the vacuum reading that you got and divide it by two
and the answer that you get will be the size power valve you need. If
your vehicle is a standard shift you will need to let the engine warm up
to operating temperature. You will hook a vacuum gauge up to a manifold
vacuum source, and take a reading with your engine at idle and
transmission in neutral. Then take the reading and devide it by two and
get your power valve. Example: If you have a vacuum reading of 9"hg you
would devide that by two which would come out to be 4.5"hg. So, it would
take a 4.5" power valve.
In order to check the jetting you will have to do plug readings.When
you are checking the jetting of your carburetor you will have to read your
sparkplugs. When you want to check the jetting on the primary side, you
will first need to let your engine warm up to normal operating
temperature. Then take your vehicle out and run it at a steady cruising
speed without kicking in the four barrel. Then kill the engine and coast
to a stop, and pull out a couple of spark plugs and look at the porcelain
on the end of the plug and see what color is on it.
If you want to check the secondary side you will let your engine run up
to operating temp like before. This time when you run your vehicle you
will need to run it like you are racing at wide open throttle with the
four barrel kicked in.Run it wide open for a short distance maybe a
1/4mile or something like that. As soon as you let off the throttle kill
the engine and coast to a stop. Get out and pull a couple of sparkplugs
and see what color they are as before.
Here is the plug color and what that tells you.
NORMAL: The porcelian on the plug has a light tan or gray or very light
brown or grey color on it with no specks or deposits.
OVERHEATED OR LEAN CONDITION: The porcelian is a chalky white and the
electrode could be burn or eroded. If you see this you will need to jet up
for more fuel.
FOULED OR RICH CONDITION: The porcelian and electrode is coated with black
sutty carbon. If you see this on your plug you will need to jet down for
less fuel flow.
Keep in mind that if the engine is not immediately shut down after any
of the two run test that the plug reading could be unacurate, and by
reading the plugs you should be able tell ant problems with the fuel
mixture, spark advance, fuel grade or plug heatrange.
Any time you jet up or down you will need to go two jet sizes at a time.
Keep in mind also that a hotter plug is not always the right thing for a
engine. If you have a high performance engine that produces alot of
cylinder temperature you really need a colder plug. If you have a engine
that doesnt get drove much,burns a little oil, or the fuel mixture is a
little rich you may want to use a hotter plug.
High energy ignition systems can handle a much wider plug gap than a
standard system. You might even gain a little performance by widening the
plug gap when using a performance ignition coil, because it increases the
chances of encountering a combustible mix of air and fuel. If you try
widening the plug gap on a stock ignition sytem you will probably find you
will cause misfires.

If you open the throttle and your vehicle accelerates and then hesitates
you will need to extend the fuel shot by installing a smaller pump
discharge nozzle(shooter). You may even find that it might help you to
step up a size on the accelerator pump cam.
If your vehicle initially bogs or hesitates but then picks up and moves
on well you may want to install a larger discharge nozzle(shooter) to get
your engine more initial fuel.
Thanks
Matt