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Topics - Helmantel

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1965 GT350/R-Model / Holley 715 CFM #3259 float level adjustment
« on: August 26, 2020, 06:13:56 AM »
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, ˝” 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 ˝” 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 ˝” 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  :)

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