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 Post subject: Holley N&S info
 Post Posted: Mon Dec 06, 2010 5:19 pm 
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Joined: Mon Oct 29, 2007 7:07 pm
Posts: 1422
Location: Florida
Info is from Tom Vaught, former Holley employee.

1) Why the Holley Hex Nut above the the Float Bowl has 6 sides (vs say 8 sides):

The thread used on the needle and seat that screws into the top of the float bowl is a 3/8-32
thread. There are 32 threads per inch cut on the needle and seat where it goes into the bowl.
One full turn of the Hex Nut will move the needle and seat 1/32” or .03125”
or for easy math about .030” upward or downward.

2) What the actual float moves vs the tip of the needle (away from the seat):

The float has a 6 to 1 multiplication ratio so you turn the needle and seat hex nut one full turn downward, the float movement will be .03126 X 6 or .1875” lower in the float bowl and the average fuel level in the bowl will be for easy math .180” lower. Two “Flats on the Hex nut works out to about a 1/16” of fuel level change in the float bowl. If you keep accurate records of your float level position you can track the air/fuel ratio change (with an accurate air/fuel meter and get the fuel bowl fuel level vs the mail well fuel level dialed right in for your intake manifold/ carburetor/ engine installation angle dialed right in for best fuel control.

3) Why the fuel level rises in the bowl, (above the Holley Calibration Level), when a higher fuel pressure (vs the Holley fuel pressure spec of 6 psi) is used:

When the needle and seat (.110” size) is used with a factory float bowl and at the Holley fuel pressure calibration point (6 psi), the average float level height will be close to the “design fuel level” in the bowl and and the carburetor will will meter fuel properly. In some of the old Holley Books they used to show an external gage mounted on the fuel bowl to check fuel level before the “Sight Plug” type bowls were designed. If you add extra fuel pressure to the fuel line going to the bowl the needle and seat will have a higher pressure acting on the tip of the needle and seat. A small change here in force acting on the tip of the needle vs a fixed float buoyancyforce X a 6 to 1 float ratio mentioned above means that the FUEL LEVEL IN THE BOWL WILL RISE TO A NEW AVERAGE LEVEL WITH INCREASED FUEL PRESSURE.

4) Why the fuel level rises in the bowl, (above the Holley Calibration Level), when a larger needle and seat is used (vs the Holley .110 needle and seat).

In the previous paragraph 3), the fuel level went up because the pressure on the float was higher due to the fuel pressure. The other way the level can go up is by using a larger area needle and seat which also imparts more force against the float, (raising the average fuel level.
A Holley 6-504 part Number is Holley's standard .110” Viton Needle and seat assembly
A 6-505 needle and seat will flow about 170-180 lbs of fuel per hour at 7 psi (depending on the
specific gravity of the fuel used). 175 x 2 = 350 lbs of fuel per hour. About 630 horsepower if
assuming a .55 lbs/horsepower/hr. Using the larger needles and seats naturally will move more
fuel BUT flue level changes in the bowl will have to be monitored for each combination vs
expected air/fuel ratio and drivability.

5) Why “Float Drop” is important:
If you have the wrong float drop, the float can “hang” against the needle as it rides on the “hump” on the float level. There are two needle and seat lower needle dimensions. One needle has a 2 cm lower diameter and the other one has a 4 cm diameter. Naturally the 4 cm diameter needle and seat will not “hang” as easily as the “footprint” (diameter) is twice as large. If the float “hangs” you will have open flow into the bowl with no fuel control and flooding, fuel coming out of the vents, and potential “hydra-locking” of the engine. As second issue with the wrong float settings is that the fuel level in the bowl could be too low and uncover the power valve on acceleration. Special power Valves can help eliminate this issue.

6) Why Floats collapse after a Carburetor Fire:

If you have a carburetor backfire, and the carburetor is burning inside the venturis, the main-body gets hot quickly. This heat transfers to the air in the carburetor heating it and the also to the parts around the air. The brass floats have air inside of them when they were made. The air in the float gets hot, expands, and trying to get out of the float. Many times there are very small places where the air can escape but fuel cannot get into the floats. As soon as the engine is restarted, the cold fuel enters the bowl, the float is still hot. The float cools rapidly vs the lost air inside the float. Because the air pressure outside is now higher vs the inside of the float the float collapses (just like it would under boost). Now you have a big problem. The black Nitrophyl floats do not suffer this problem therefore as we know are a better component to use in a boosted carburetor. At least now you can explain to your buddy why his float looks like Chit when he was not running any boost through the carb, LOL!

7) What the “Standard” Brass Float should weight vs what the Black Nitrophyl Float weighs:

A Brass float weighs around 15 grams. (An average paper clip weighs 3 grams). The black nitrophyl float weighs about 11 grams. When you cut notches in the black float to allow for jet extensions, (or if you buy a black float with the notches already there), you should check the weight of the float on a gram scale to see if it is close to the right factory weight. You never know how much epoxy the guy used sealing the float, how deep he made the cuts, etc. Some floats for the road race guys and NASCAR guys are heavily modified. If the float is too light, (less than 11 grams stock, or if you want to make the float the same weight as the brass float you can add extra lead shot to get up to the brass float weight of 15 grams.

Cool Adding weight to a black float:
You drill a hole in the black float, add the weight, do a trial measurement on the gram scale, and when you are close you add the light coating of epoxy to seal the whole deal up. I am not a big fan of the making the black float weight the same as the brass float as the black float dimensions are different vs the brass float. The brass float has a longer arm therefore more force on the needle and seat vs the float weight.

9) Lean Acceleration caused by an improper float level setting:

As was mentioned in 5), if the float level drops too low, the “head” that the fuel has to overcome in the main-well gets higher and the engine calibration goes lean. If the fuel level is too high, the “head” that the fuel has to overcome in the main-well, will be much less and the engine runs rich. There is a “sweet spot” that was designed into the carburetor fuel calibration over many years of testing vs the parts used and random “experimenting” will typically cause issues over time. High pressure boosting is adding another level of knowledge to this carburetor database of proper settings.

10) Why Bowl Vent Clearance is important:

The vent whistle at the top of the metering block was added to prevent fuel slosh out of the carburetor bowl vents. A Rivet typically holes it in place. There may be a slight leakage around the vent but typically it is there for fuel control in the bowl. Trimming the vent is commonly done. You do not want the fuel to slam against the back wall of the carburetor under acceleration, climb the back wall, hit the roof of the bowl, move forward, and then enter the vent whistle. A whistle about 1.25' long typically works well. I trim the opening with an exacto knife to allow more area through the vent whistle too.

11) “Sight Plugs” Why they are different Front Bowl to Rear Bowl and why some people have carburetor problems when swapping out Holley Bowls for after-market fuel bowls:

Few Holley bowls today do not have a “Sight Plug” but the location of the sight plug can be in several different locations. Primary bowls will always have a higher sight plug location vs the secondary bowl location. The number varies all over the map. The average bowl difference will be around .220” difference with some going over .300” difference. Why is this important? Because the sight plug location sets where the “bottom of the threads” location is that everyone sets their fuel level to. As said earlier, you do not want to screw up the Holley Fuel Level Calibration Setting Point if you can help it.

12) “How to use a Primary Float Bowl on the Secondary side of a Holley carburetor:

IF you put a Primary bowl on the secondary side of a carburetor you can typically set the fuel level to the bottom of the threads deal and the LOWER the fuel level 4 FLATS.

13) Float Bowl Screws can be different:

A stock Holley Metering Block type bowl screw is 63mm long from the metal surface the screw gasket seals against to the end of the screw threads. About 2.48” long. The Metering Plate Holley screw is about 48mm long, about 1.89” long. You will notice that neither of the screws is the standard 2” long or 2.5” long hardware store bolts. At one time there were some screws out there that looked the same as the Holley typical parts but they were VERY CLOSE to the 2” and 2.5” length dimensions. You use one of these screws in your carb you are almost guaranteed that you will strip a thread in the main-body as the screw will bottom in the casting, have no where to go, and any additional tightening will pull threads.

14) Never use “Hot Rod” needles and seats parts vs the “Holley Design” needles and seats:

Above in line 13 we had a screw length issues that could cause problems. In this line we have a needle and seat thread issue that needs to be addressed. Holley needles and seats ARE as mentioned above a 3/8-32 thread. Some of the “hot rod” old style needles and seats were a bastard thread then therefore would strip out the threads in the float bowl after installation in the bowl. USE ONLY HOLLEY DESIGN TYPE PARTS.

15) Differences between Holley needles and seats:

Holley used a few needle and seat designs over the years. Initially Holley had steel needles and later offered Viton tipped needles for many applications. Holley offered several orifice sizes, with the .110” needle and seat being the most common. Holley also changed the feed strategy to the needle. The old units used drilled holes in the upper and lower chambers (typically 4 holes). The later generation parts used a “Picture Window” rectangular shaped entry hole for more flow to the carburetor bowls. Today we have the stock type needles and seats and also “Bottom Feed” type racing needles and seats.

16) Viton tipped needles and seats vs steel needles and seats:

Last but not least Viton Tipped needles and seats vs steel needles and seats. The Viton stuff seals very well unless you get metal chips stuck in the viton tip. A steel needle and seat will typically allow the trash if small enough to pass through the needle and seat to the bowl. Steel needles and seats are used on higher performance applications when using exotic fuel blends as the viton tip parts are designed for normal pump fuels and some racing fuels. Most alcohol carbs will use a steel needle and seat for two reasons: Increased orifice diameter required where the viton part is not offered and durability from the corrosive fuel. The increased fuel orifice diameter requirement can be corrected bu the dual needle and seat strategy but the corrosive fuel issue is still there.

 Post subject: Re: Holley N&S info
 Post Posted: Mon Dec 06, 2010 7:49 pm 
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Joined: Wed Oct 24, 2007 6:36 pm
Posts: 718
Location: Blanco, Texas
Good info Mark,



 Post subject: Re: Holley N&S info
 Post Posted: Mon Dec 06, 2010 9:28 pm 
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Joined: Thu May 18, 2006 9:35 pm
Posts: 541
Location: savannah mo.
WOW =D> =D>

ET 6.49 60FT 1.37. 107.54 mph

 Post subject: Re: Holley N&S info
 Post Posted: Mon Dec 06, 2010 10:01 pm 

Joined: Sat Nov 06, 2010 12:34 pm
Posts: 14
Good read!

 Post subject: Re: Holley N&S info
 Post Posted: Mon Dec 27, 2010 9:44 pm 
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Joined: Sat Oct 21, 2006 11:41 pm
Posts: 768
Location: Morrison, Co
Great info!

Mark Goulette
Owner/driver, Livin' the Dream Racing Dragster
Speed Kills, But It's Better Than Going Slow!!!

 Post subject: Re: Holley N&S info
 Post Posted: Tue Dec 27, 2011 10:55 pm 

Joined: Mon Nov 28, 2011 10:24 pm
Posts: 21
Excellent write-up Mark, thank you!


 Post subject: Re: Holley N&S info
 Post Posted: Wed Dec 26, 2012 11:48 pm 
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Joined: Fri Feb 17, 2012 8:09 pm
Posts: 166
Location: Maryland
Thank you for posting this Mark.

We used to set the float level to the bottom of the threads, put the plug back in and go 2 flats higher. Every racer I knew set them that way in the 70's. Now everybody seems to use a 3/8's bit under the float, with no regard to a "bottom of the thread" measurement. Is the level supposed to be at the bottom of the threads on a race engine?

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