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Old 09-17-2010, 10:12 PM
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Float level, how does it improve performance?

Supposing on a holley. If your carb float level is set to the bottom of the hole/ or sight glass. The needle is seating properly, and fuel is not dripping from the ports. How does rasing or lowering the float help performance? Knowing vacuum pulls over the ports leading to the metering plate. Also that more fuel is above , on top of the jet extension where the fuel gets pulled in.

Yet there is always the float adjustment recommendation in tuning advice.
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Old 09-17-2010, 11:09 PM
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The fuel level in the emulsion well in the metering blocks is the same as the level in the bowl. Raise the float level and it puts the fuel in the well closer to the top of the well where it is pulled into the booster, speeding up its reaction to the airflow draw that pulls it over the top of the well into the booster leg.
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Old 09-18-2010, 04:37 AM
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it's sort of like a toilet. to high a level in the bowl and the water will constantly run down the drain. Too little water in the bowl, and it won't flush
worth a darn!
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Old 09-18-2010, 07:57 AM
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Usually the base float level (just running out the sight hole) is a perfect setup. Sometimes, if the carb runs too lean or too rich you can adjust the level slightly to compensate. But, after you adjust the float level you also should readjust idle mixture. Floats are your supply of fuel basically. I wouldnt say you improve performance by adjusting them, more that you "correct" performance if it wasnt tuned properly

A car with 400hp and another with 600hp might have the bowls at the same level using the same carb.

Same goes for fuel pressure, more isnt always better. Holleys are good at 6psi or so, too high and they flood out, too low and the bowls run dry
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Old 09-18-2010, 06:59 PM
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Yes, the fuel pressure can over come the needle, that is understood.

More or less, in the past my floats were set to accomodate the angle of the carb pad. The pad is never level, and the floats need to be adjusted back to the recommended hieght.

Sometimes the secondary isn't feeding correctly, and needs more vacuum to draw fuel to the secondaries. Almost like the WOT secondary hit is weak. My fix for this, a heavier spring is used in the vacuum pod. It gives the engine something to keep pulling against to maintain flow. Maybe?
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Old 09-18-2010, 08:17 PM
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If your going to make 400hp or more, you need to stay away from the chrome dual feed fuel lines as in pic #1 for a dual inlet holley's as the primary fuel bowl under WOT will rob some pressure and fuel flow from the rear secondary bowl.
For 400hp on up you need to go with a Y block inlet with equal length lines going to each bowl from the Y block such as the 1969 chevy 396/375hp engine as in pic #2. The Y block will keep equal pressure to the front and rear bowls from the fuel pump line. When you only run 6lbs of pressure to your carb, this can make a difference in the higher rpm fuel supply.
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Old 09-18-2010, 09:52 PM
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Quote:
Originally Posted by sqzbox
If your going to make 400hp or more, you need to stay away from the chrome dual feed fuel lines as in pic #1 for a dual inlet holley's as the primary fuel bowl under WOT will rob some pressure and fuel flow from the rear secondary bowl.
For 400hp on up you need to go with a Y block inlet with equal length lines going to each bowl from the Y block such as the 1969 chevy 396/375hp engine as in pic #2. The Y block will keep equal pressure to the front and rear bowls from the fuel pump line. When you only run 6lbs of pressure to your carb, this can make a difference in the higher rpm fuel supply.
Hogwash, unless the fuel pump can barely keep up with the flow needed to maintain pressure at one needle and seat, which would be a pump problem, not a line problem.

That factory "Y" fitting is one of the most restrictive pieces around, you won't find them brass factory or hardware store fittings around anything where the builder knows what they are doing, the size of the passages in them and the step/ledge drilled at intersections internally are real flow killers. Factory did that because it was CHEAP, not because it flowed high volumes.
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Old 09-18-2010, 11:19 PM
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You don't have to use THAT Y block. I only used it as an example because I couldn't find a pre-fabed item picture. The equal length between the main fuel line and the F/R bowls trick still works. It was published in hot rod magazine, "tips and tricks" not to long after you got out of your diapers and before you learned to read.

Here's an example of what I was talking about on dual single feed carb's.
The trick works on dual feed single carbs also.

I forgot I had this pic. or I would have used it in the other post.
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Last edited by sqzbox; 09-18-2010 at 11:33 PM.
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Old 09-18-2010, 11:55 PM
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Y fitting

Heres the one I was looking for. Russell, 3/8" no restriction.
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Old 09-19-2010, 12:56 PM
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The fuel pump is oem style and has never had a problem providing fuel, and no trouble keeping the bowls full of fuel. In the past my fuel pumps have never dropped off psi on a gauge. The oem pump self regulates around 6 psi and 80gph. Low pressure high volume type system with a return line. The usual case was that the pump was replaced with a performance type, giving high volume/psi , and needed a regulator.


the sub 12 second race cars needs to fill a gallon in 20 seconds. A 350 road car with a performer cam is completely different in thrist requirement.

For holley vacuum secondary users , do you oversize your carb a little, to compensate for the fact that the blades are not open all the way?

My engines perform greatest when the stiffer spring is used. Problem is the secondary blades only open 3/4 of the way. Does anyone else buy the 750 for this reason when they require a 650 cfm airflow?
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Old 09-19-2010, 07:05 PM
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Quote:
Originally Posted by spinn
The fuel pump is oem style and has never had a problem providing fuel, and no trouble keeping the bowls full of fuel. In the past my fuel pumps have never dropped off psi on a gauge. The oem pump self regulates around 6 psi and 80gph. Low pressure high volume type system with a return line. The usual case was that the pump was replaced with a performance type, giving high volume/psi , and needed a regulator.


the sub 12 second race cars needs to fill a gallon in 20 seconds. A 350 road car with a performer cam is completely different in thrist requirement.

For holley vacuum secondary users , do you oversize your carb a little, to compensate for the fact that the blades are not open all the way?

My engines perform greatest when the stiffer spring is used. Problem is the secondary blades only open 3/4 of the way. Does anyone else buy the 750 for this reason when they require a 650 cfm airflow?
750 is the most common choice but I think its based on requirements, 650 might be too low. You might need to change to a different secondary spring, which is why they offer so many options.

But if you're only opening 3/4 is it possible your engine doesnt require any more air?
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Old 09-19-2010, 07:30 PM
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Does any spring open them up to full cfm at 5, ooo rpm? The yellow short is open wide around 5, 6oo. Problem with that is, it opens too soon, sometimes under 1, 5oo rpm. Even a in a 650 carb that will ruin throttle response, interfere with the pump shot, and power valve hit. The secondaries for my taste like to come in around 2, ooo rpm. There is no 2ndary accelerator pump so if you mat it , and the secondary blades flop open, you do not accelerate with proper authority.

Nice to here the ideas and points of veiw from others' experience. Thanks.
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Old 09-20-2010, 07:00 AM
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The springs do not do the same thing on every vehicle due to the differences between engines. Your car might perform better with a spring that would bog my particular car. It's trial an error with vacuum secondaries, I have a similar carb I think (mine is a 3310-c vac secondary)

I'd set the secondary floats at factory (just below the hole) and leave it that way forever. If you keep changing that you throw all other adjustments off. Then, try different springs and see how it feels. If you feel the secondaries kick in (like you feel the car lunge forward like it got a push) then the spring it too heavy so go one spring less.

There's also a metering plate inside the secondary bowl that can be changed but it's rarely required

if you post your engine specs someone might be able to see a problem if you do actually have one. But as far as I can see things are working as they should. If the motor doesn't require more fuel immediately then the spring changes might be wrong.
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Old 09-20-2010, 07:10 AM
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Perhapps a higher stall TC might be in order. If your running a stick, a DP.

I'm curious as to how you've determend your secondaries are only open 3/4 of the way at whatever rpm you claim. Maybe your acc. linkage is out of adj. and not pulling the carb. linkage all the way open.
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Old 09-20-2010, 07:20 AM
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I forgot to ask: do you have the quick change secondary kit installed or are you removing the secondary diaphragm housing each time to install springs? If you're removing the housing maybe you killed that little cork gasket inside the housing that compresses against the carb?
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