Holley carb jetting advice

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Mopar to ya

Mopar to ya

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I know this has been beaten to death, but my carb started dumping fuel this morning. I know just enough about carbs to be dangerous. I have often wondered what the jets sizes were, since this is the carb it was dyno'd with and I'm sure it was set up for max power, not street driving / power. I took it apart and it looks like the numbers are 86 on the primary and 75 on the secondary side. I see no numbers on the power valve. Since I have it apart, I'd like to play with it a bit. Any advice on jet sizing? There are different power valves, too, aren't there? Also, when setting the floats, I assume you adjust them until fuel just runs out the side drain plug? Here is what I run. Sorry it's long, but it's simpler to copy and paste. Any help would be MUCH appreciated!

Here is what I have:
1992 magnum block
scat 4" stroke crank
Scat forged i-beam rods
Diamond flat top pistons (11.8:1 compression)
total seal rings
Engine Pro main, rod, Cam bearings
Fluid damper
Cloyes double roller timing set
Cometic head gaskets
Edelbrock performer rpm heads (gasket matched w/ light porting)
Smith Bros pushrods
Comp 1.5 ratio shaft mount billet roller rockers
comp hydraulic roller lifters
Comp Cam
242/248 Dur@ .050, .3660/.3630 lift @ .050, 112° lobe sep. 549/544 gross valve lift
Comp valve springs, retainers, keepers, locators
ARP head studs
ARP bolts used throughout
melling hv oil pump
Edelbrock RPM Air Gap intake
MSD Pro Billet distributor
bronze dist gear
Holley 750dp mech secondary manual choke
Mopar MSD 6 Ignition box
Blaster2 Coil
 
power valve in primary side? yes or no?

power valve in secondary side? yes or no?

86 in primary side is way to rich..

holley 750 dp...should be able 72 in the primary ..and about eight jets richer in the rear if no power valve is being used.....which would be an 80
 
the number on the power valve indicates at what vacuum it will supply additional fuel to the main circuit.

the is a power valve restriction channel that is behind the power valve dictates how much fuel is added to the main circuit.


sure you dont have the numbers backwards from the primary and secondary metering blocks?
 
I hope you have the jetting reversed.......that or the dyno guy was having a few adult beverages during testing....lol.
 
Floats, adjust so the sight plug threads just get wet.

If it's dumping fuel out the boosters, you may have a stuck/fouled needle and seat, sunk float, too much fuel pressure. If it's doing it from both sides, more likely fuel pressure or a real dirty fuel system.
 
If it ran ok before its not the jets. clean it up good and run it.
 
Here's how to set the float levels:

GENERAL GUIDELINES FOR
ADJUSTING BRASS AND
NITROPHYL FLOATS​

Two methods of float adjustment are provided for with Holley
performance carburetors depending on the style of float bowl
and needle and seat assembly employed. They are the internal
(dry) setting and the external (wet) setting. The internal
float adjustment is accomplished with the fuel bowl off the
carburetor. With “internally adjustable” needle and seats, the
fuel bowl is inverted and the float tang, or tab, is adjusted to
the point where the float surface is parallel to the fuel bowl
surface, just underneath. An initial dry setting can also be
accomplished with “externally adjustable” needle and seats.
To achieve this, invert the fuel bowl and turn the adjusting nut
until the float surface lies parallel to the fuel bowl casting
surface underneath.

Another, more accurate adjustment can be made with the
side hung style float if measuring gauges, such as drill bits,
are available. Here, with the fuel bowl inverted, the primary
float can be adjusted to the point where there is a 7/64" gap
between the “toe” of the float and the bottom of the fuel bowl
surface underneath. The float “toe” is the part of the float furthest
from where the arm is attached. The secondary float
can be adjusted to the point where there is a 13/64" gap
between the “heel” of the float and the bottom of the fuel
bowl surface underneath. The float “heel” is the part of the
float closest to the point where the arm is attached.

A “wet” level float adjustment can be performed on either the
side or center hung floats, if the fuel bowls have provision for
the externally adjustable needle and seats. This adjustment is
made as follows. Start the vehicle up and move it out of the
garage and into an open area where plenty of fresh ventilation
is available. Allow the idle to stabilize. Turn the engine
off and remove the sight plug from the primary fuel bowl to
inspect the fuel level. If it’s been determined that adjustment is
required use a large screw driver to crack loose the lock
screw. With a 5/8" open-end wrench turn the adjusting nut
clockwise to lower the float level.

Conversely, turn the adjusting nut counter-clockwise to raise
the float level. Tighten the lock screw. Restart the vehicle and
let the engine idle stabilize. Shut the engine off. Remove the
sight plug to reinspect the fuel level. The fuel level should stabilize
at just below the level of the fuel bowl sight plug hole.

This same adjustment procedure is performed on the secondary
bowl.​


GENERAL GUIDELINES FOR
ADJUSTING DURACON (PLASTIC)
FLOATS​

The Duracon float rides higher on the fuel than either the
brass or nitrophyl float and, therefore, a higher setting is in
order. A Duracon float, set at the same level as either a brass
or nitrophyl float, would make the carburetor run leaner,
everything else being equal. This is because there would be
less fuel available in the fuel bowl. The Duracon float setting
must be higher to compensate for this condition.

Dry Setting for Duracon Center Hung Float:

The primary side setting is .3125" (5/16"), measured with
the fuel bowl inverted, at the middle of the float. The secondary
side setting is .3750" (3/8"), measured with the fuel
bowl inverted, at the middle of the float (back side).

Dry Setting for Duracon Side Hung Float:

The primary side setting is .2188" (7/32"), measured with
the fuel bowl inverted, at the toe of the float. The secondary
side setting is .3125" (5/16"), measured with the fuel bowl
inverted, at the toe of the float.

Wet Setting for Duracon Float:

Refer to “Wet Float Setting”, discussed previously.​

WARNING:​
​
​
​
​
Caution should be exercised when doing the wet
level float adjustment. Fuel at the needle and seat is under
pressure from the fuel pump. Some may leak out when the
adjustment is made and shop rags should be available to
immediately wipe up any fuel spillage. Gasoline is flammable
and proper precaution should be taken.​


CAUTION:​
​
​
​
​
Once again, remember that these are general
guidelines for adjusting floats. Your particular application
may require additional fine tuning over and above these
listed procedures.​


NOTE:​
​
​
​
​
The float adjustment feature on Holley carburetors cannot cure
a poor running engine, a bad ignition system, a clogged fuel filter, an
improperly operating fuel pump or fuel pressure that is too high or
low. This adjustment is provided solely to ensure that the fuel in the
bowl can be adjusted to the correct level for the carburetor to perform
its function. There is no need to “wrench” excessively on the adjustment
nut. A quarter of a turn one way or the other should be enough​

to bring you into spec.


http://www.holley.com/TechService/Library.asp

 
Power valve info:



POWER ENRICHMENT SYSTEM​

The power enrichment system supplies additional fuel to the
main system during heavy load or full power situations. Holley
carburetors utilize a vacuum operated power enrichment system
and a selection of power valves is available to “time” this system’s
operation to your specific needs. Each Holley power
valve is stamped with a number to indicate the vacuum opening
point. For example, the number “65” indicates that the power
valve will open when the engine vacuum drops to 6.5" or
below. An accurate vacuum gauge, such as Holley P/N
26-501, should be used when determining the correct power
valve to use. A competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmission,
take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAMPLE:
8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.


Most of the popular Holley “Street Legal” and “Street
Performance” carburetors incorporate a power valve blow-out
protection system. A special check valve is located in the throttle
body expressly for this purpose. This check valve is
designed to be normally open but will quickly seat to close off
the internal vacuum passage when a backfire occurs. Once
closed, the check valve interrupts the pressure wave caused by
the backfire, thus protecting the power valve.

If you have a carburetor older than 1992 (or you have experienced
an extreme backfire) and expect a blown power valve,
use this simple test. TEST: At idle turn your idle mixture screws
(found on the side of the metering block) all the way in. If your
engine dies the power valve is not blown.​


THE TRUTH ABOUT POWER
VALVES USED WITH HOLLEY
CARBURETORS​

There still seems to be a lot of misconception about
Holley carburetors blowing power valves. Nothing
could be further from the truth. Holley performance
carburetors built since 1992 have utilized a power
valve check system that effectively eliminated this
infrequent problem. Consisting of a spring, brass
seat and check ball, the check ball system is 100%
effective protecting the power valve diaphragm
from damage due to engine backfire.

The power valve check ball is designed to be
normally open but quickly seals to close off the
internal vacuum passage when a backfire occurs.
Once closed, the check valve interrupts the pressure
wave generated by the backfire, thus protecting the
power valve diaphragm. There is no way that the
power valve’s diaphragm can rupture due to an​
engine backfire!

http://www.holley.com/TechService/Library.asp
 
What cracked said....float adjustment, junk in needle assembly, or a sinking float.

EDIT: Since you have it apart, blow compressed air through ALL the passages, both ways if possible, to clear any obstructions and crapola out. Not always recommended, but I also run thin wire through the passages I can get to....for insurance sake.
 
I may have them backwards. As I said - just enough to be dangerous. It ran fine all the way home Friday night. I installed a new battery on Sunday and started it up. This morning it would hardly run due to fuel dumping down the throat of the carb. Billowing smoke out the pipes. I visually saw the fuel pouring down this evening. I figured I would pull it off, clean it up, and check the jets - change them if the consensus was something different. It has always run rich. The power valve is getting changed because I am there. There is only one power valve. It was on the side with the smaller jets. The floats move freely and are not full of fuel.
 
Pouring fuel has to be too much pressure or floats too high in my opinion. As far as the PV, looking at your combo, 6.5 will probably be low enough, and it may have that already?
 
It would have been best to have opened the sight plug while it was still on the engne. Then you would have narrowed down the culprit(s).

My guess is as was written above - needle and seat not holding due to dirt or too much pressure for condition of the needle. But it also could be any of several leaks, so sure change the gaskets and change the PV. Use a Holley gasket (blue or plain) You'l need a gasket for the PV too.

Power valve numbers are stamped light on the sheet metal or sometimes on the cast flat.

If the car ran good, I'd not change a thing unless conditions are greatly differernt than the dyno. If you really want to experiment, then for street use, change 1 or 2 jet sizes smaller on the primary side. Gently accelerate on the hwy. As the car gets near 60 or 70 mph it will either be happy or start to surge. Surging means the jetting on the dyno was as lean as you can take the primaries. If its happy you should get better milage. However you may need to jet up for WOT (1/4 mile).

My own experience has been that on a chassis dyno (inertia alone) jetting tends to be on the lean side compared to the real world with air resistance.

Get a copy of Urich & Fisher Holley Carburators & Manifolds if you want some good explanations and illustrations along with tuning tips. Urich was Holley's VP of Engineeering which seems to help the technical accuracy and clarity of the explanations.
 
ps. I agree with Tony that is a big spread in the jet sizes. But the determining factor is not what we typically see, but the size of the power valve restriction channel. You could measure this if you have tiny drills or gages. Otherwise, you can just experment with the seconday jetting when at the drag strip or on a chassis dyno.
 
Yes. if gas is coming out the top vent like a water fountain, check for a stuck needle/seat or a piece of dirt between the needle and seat.
 
I bought a gasket kit. I always change gaskets when i take a carb apart. The engine was dyno'd on a stand, not in the car, but I see your point. It has no drag at all as it would when driving. It has always run well, but quite rich. Black tailpipe and when I run race gas, the rear bumper gets coated in white crap. I am ordering a programmable MSD box to play with my timing as well, but I want to get the carb set up properly as well. The jets are easy to change, so I can play as much as needed. Great advice so far, thanks! I'll take all that you've got.
 
Mopar to ya said:
Oh and it is running straight down the throat, not jetting up. It's like a faucet is turned on.
Click to expand...

So you're saying that the gas is flowing up the vent tube and into the carb?

Then definately check the needle and seat for foreign debris (dirt, chips, etc). Or if you have a spare new one, replace the inlet needle and seat. I posted how to set the fuel levels straight from the Holley web site above.
 
Mopar to ya said:
Oh and it is running straight down the throat, not jetting up. It's like a faucet is turned on.
Click to expand...

Agree with the above, float was likely stuck, or dirt on seat, sometimes the o-ring on the needle/seat will leak,, but more usually after an adjustment..

your kit will have new needle/seats..
 
Yep,, is the way it should be,, primary on right..

Did fuel come out one of those angled tubes, vents ?
 
krazykuda said:
So you're saying that the gas is flowing up the vent tube and into the carb?

Then definately check the needle and seat for foreign debris (dirt, chips, etc). Or if you have a spare new one, replace the inlet needle and seat. I posted how to set the fuel levels straight from the Holley web site above.
Click to expand...

No. Not up and then down. Like a stream running into the carb. Straight down. I could actually get it running, but there was so much fuel it was choking out. Instead off a nice squirt, it was running into the carb.
 
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