Holley 715 CFM #3259 float level adjustment

[Helmantel]

Hi,
I have a question about some adjustments of the 715 cfm Holley carburator. My car is just a regular Mustang but I hope you allow me to ask this question here anyway. My main concern is the float level height.

Quick background:
Engine: 289 with ported 351W heads, Shelby Hiriser intake, 10:1 CR, short duration high lift hydraulic roller cam, headers
I used a 600 cfm Holley 1850 previously, which ran OK but developed a lean stumble that I probably could have fixed if I had spent some time on it but I decided to try my Holley 3259 (reissue) instead. It started right up but didn't run right. After some reading I set the float levels and it ran decent, but very rich (sooty exhaust pipes, fuel smell), still had some stumble and required the idle screws to be closed much more than the typical 1.5 turns out. Fuel consumption was high.

I decided to recheck the float levels and check if the power valve doesn't leak. When trying to find the correct float setting, I found the following:
1.   1/2" front, 3/8" rear
2.   3/8" front, ½" rear (to the top of the bowl), confirmed by the same person who confirmed setting #1 was correct in another forum post 
3.   13/32"
4.   Float arms parallel to the bowl
5.   Set floats 1/8" lower than stock to compensate for modern fuels

Needless to say, I was a bit confused. I decided to go for #2, since it was recommended twice by people who seemed to have clue and because somebody with a Cobra 289 said that the correct levels were sensitive to the angle of the engine in the car. Since the engine leans backwards in a Mustang, I figured the front float had to sit a little higher to compensate.

The float levels turned out to be much too low. I probably measured from the bottom instead of the top, last time I set them. Half an hour of tinkering later, I considered them close enough to 3/8" and ½" and put the carb back on the car. It started right up, idled OK and ran good with considerably less stumble (a little remains) and the fuel smell seemed to be less. Back home and the engine warm, I adjusted the idle screws and had to turn them in more (not sure how much). Vacuum ended up at 18" but the RPM is too high at 950. I guess I need to close the secondaries a little, because the primary blades are right where they need to be, relative to the transfer slots.

Before I go on and fine tune this carburetor further, can anybody confirm that these float levels are correct? I'd like to avoid to tune and adjust it based on the wrong float level. Also, do you think that the float type makes a difference? Mine are the Nitrophyle type and I figured their setting may differ from a brass type? I don't know what was original though and if the 3/8" and ½" float settings apply to brass or nitrophyle floats.

It would sure be easier to have some side plugs (and an internal adjuster!) to check the level.... The bowls have flat areas where a side plug would normally be mounted . I'm considering to drill and tap them for side plugs.

Well, a long story but I figured that visiting this forum means you like reading about Mustangs, so you probably wouldn't mind 

[shelbydoug]

I don't think that you are having an issue with the fuel levels.

The carb that you are working with was originally sourced by Shelby from the 427 Ford. It was selected for racing Homoligation reasons and really is too big for a 289 street engine. Period.

It was just carried over on the following model years as it had become a selling feature of the GT350. It was the era of bigger is better.


Normally the plugs are going to read rich with it. You need to use a cam with timing of at least that of a 289hp cam.

It likes lots os static compression. Around 10.5 to 11 to 1. Lots of ignition timing and as a result high octane fuel.

Today's ethanol fuels are causing lots of issues never seen before in the '60s simply because they didn't exist even in concept form.


Even 10% ethanol requires you to adjust fuel levels down and can cause isolated hot restart issues. Carry a can of starting fluid with you.


Even the 289hp cam, by today's standards, is a pathetic display of overselling by just calling it a high performance cam. Back then, relatively speaking to other factory offerings, it was.

To really make that carb work, you need more cubic inches, real 4 into 1 headers and a camshaft with lots of lift and duration that will compliment a scavenging effect to both the intake and the exhaust.

The Holley CFM v. CI chart shows something like 450 to 500 cfm for a 289. Not a 715.


I ran that carb on a 351 Cleveland with little positive to say about it. When it comes down to it, it's just a nostalgia thing as are the 289hp engines in general.

People are building 331's and 347's (me) out of the original basic concept, using better heads (AFR, Edelbrock) and real headers like the JBA 6610's.


There if you are going to turn the engine into the 8,000rpm range, that is where that carb comes into it's own. In around street driving it really is just disappointing and makes a mess.

It does look impressive though.

[67350#1242]

The rear fuel level is set lower than the front to prevent spillover into secondaries when braking hard so the 1/2" measure top of float to top of bowl was the original spec. and  3/8" for the front.
I suggest adding at least 1/16" to these measurements to lower fuel levels a little more for today's fuels.
Set the primary transition slot to "square".  This should be pretty close to 1 full turn of the primary idle screw after it contacts the tang on the throttle rod.   Set the secondary butterflies to completely closed and just bump the secondary idle screw so the plates won't bind in the bores - you want secondary closed as possible.  This way the primary is in control of the idle.

When setting idle speed try not to move the primary idle screw more than about 1/8 turn.  If idle too low try bumping the timing to increase it. Goal is to keep the transition slot very close to square.
These are the steps I have taken to dial in my 3259.  Shelbydoug is right the carb is too big but these steps should help to make it easier to live with.
Kurt.

[shelbydoug]

The rear fuel level is set lower than the front to prevent spillover into secondaries when braking hard so the 1/2" measure top of float to top of bowl was the original spec. and  3/8" for the front.
I suggest adding at least 1/16" to these measurements to lower fuel levels a little more for today's fuels.
Set the primary transition slot to "square".  This should be pretty close to 1 full turn of the primary idle screw after it contacts the tang on the throttle rod.   Set the secondary butterflies to completely closed and just bump the secondary idle screw so the plates won't bind in the bores - you want secondary closed as possible.  This way the primary is in control of the idle.

When setting idle speed try not to move the primary idle screw more than about 1/8 turn.  If idle too low try bumping the timing to increase it. Goal is to keep the transition slot very close to square.
These are the steps I have taken to dial in my 3259.  Shelbydoug is right the carb is too big but these steps should help to make it easier to live with.
Kurt.

+1 

[Helmantel]

Thanks for your answers. I agree that 715 cfm is more than necessary for a street driven 289 and if I were to buy a generic Holley for it, I would have picked a 600. But I had this one and figured that since Shelby used it for several years it can't be all bad and that I at least could try and make it work as good as on a 65-67 Shelby.

I have now driven it a little more with the readjusted float settings. ~50 miles to and from work including city, freeway and stop-and-go driving plus some spirited driving on country roads. It starts easy, idles smoothly, smells much less, has much less hesitation and when I took out a spark plug, the tip and porcelain were light tan/beige (while the metal was still covered with thick black soot, from earlier I guess). Don't know about the fuel consumption yet.
I may try lowering the float settings a little later on, but I think I first will hook up my wide band AFR sensor and see how things look.

[Drew Pojedinec]

Lot of Really bad info here.

There is zero reason you cannot do some basic tuning and have a 1 5/16, 1 3/8 venturi, 1 11/16 bore carb work perfectly on a 289.

Ford had many sizes available both larger and smaller. There is a valid reason to use this size.

[Helmantel]

I finally got a wide band AFR sensor hooked up which showed that it's still running (much) too rich: ~10-11 at idle and ~12,5 at cruising speed. My first thought was a leaking Power valve but vacuum testing it showed that it was OK. I then noticed that front float was bottoming out against the bowl. I had set it at 3/8" (see first picture) but that is apparently is right where the float hits the bowl. It is hitting at the edge just below the upper bolt holes (see second picture)

The probably means that the float valve wasn't closing properly and the actual fuel level in the bowl could have been anything. The engine actually ran surprisingly well despite this (although rich at idle).

I guess that means that I have to set the float a little lower (as was already recommended for modern fuels).

[shelbydoug]

Go to Air Fuel Spark on Facebook and ask Drew. He's done a zillion of these carbs and will help you get it right.

[Helmantel]

The same Drew as above? I don't have Facebook so I hope he'll answer here

[shelbydoug]

The same Drew as above? I don't have Facebook so I hope he'll answer here

Yes.

[Drew Pojedinec]

Yup the float can hit the bowl. Just set it lower.
When manually working the float you can feel if it hits the bowl or the needle and seat
(One feels spongy, one clunks)

[Helmantel]

Yes, I noticed the difference (spongy vs clunk). Any tips on how much lower?

Odd that the official setting is 3/8" then, or is that specification for a different/older float design?

[Drew Pojedinec]

I dunno. I just eyeball them and adjust as needed.

Appears new vs old floats are same size and construction despite being a different color.

[gt350hr]

  Let me add to this a bit.
     "ALL" engines will ONLY flow a specific amount of air through a carburetor REGARDLESS of CFM capability of the carburetor. Now it would be stupid to take my statement and put a ONE barrel , 6 cylinder carb on a 427 or vice versa. Those who subscribe to the 289=600cfm rule as an absolute are WRONG. Last time I checked there are about 60 DIFFERENT 600cfm Holleys and only about 5 or 6 that really work well on a 289. WHY? Because they are "generic" in design. In late '64 , Holley engineers spent allot of time calibrating an existing Holley four barrel to work on a 289 High performance engine with a "high rise" intake and tubular steel headers. The result is the 3259 , "GT350" carburetor. It was revised ( better front metering block) and given the 3259-1 list number and the S2MS9510-A ID number added . The adjustments made allowed this former 427 FE carburetor to work on the smaller 289. While the maximum air flow potential is 715CFM , (again) the 289 will only flow a lesser number as produced. The extra air capacity does NOT  "flood" the engine!!!!!!! THAT is a physical impossibility! Pure and simple. Carburetors are not designed to POUR fuel into an engine , they are designed to add fuel to air SUCKED in by the engine underneath it. The vacuum "signal" that the "boosters" "see" , pulls fuel out of the float bowls. YES a smaller CFM carburetor "can" be easier to calibrate because the smaller venturi helps the "pull" on the fuel. The amount of air flowing through the carburetor is UNCHANGED from the 715. Now change the camshaft , port the heads etc . and the AIRFLOW the engine creates will change and a larger carburetor "might" be needed. Random picking of a carburetor by size alone "may" not provide perfect calibration and require adjustments ( much like Holley engineers did on the 3259). There are at least 30 different 750 cfm carbs and "most" of them would require adjustments to fine tune them to a specific engine.
     This is what keeps carburetor specialists like Drew busy. Drew has the experience needed to match a carburetor to an application OR tell us it won't work. Most of us can change jets , power valves , gaskets .FEW of us can "optimally" tune a carburetor with air bleeds, power valve restrictions etc. like an expert does.
    Randy

[S7MS427]

+1, Randy.  Good explanation in terms most everyone can understand.