Float setting and power piston spring

[bcarlson78248]

My carb is a 17080213 from an 80-86 GM truck, but I will be using on my '75 GMC.   The info that came with the rebuild kit has a variety of settings for the float, but I don't know which one is the best match to my engine. 

My engine is nearly identical to the original '75 350 that came in my truck, but with a slightly longer duration cam than original (mine is 194/204 at .050) and an Edelbrock Performer intake.  It is not a performance engine.  Should I use a 3/8" float setting, or something bigger?

I have also used the power piston spring that is the closest match to the original one I pulled from the carb.  Why would I want to switch to a stronger or lighter spring? 

I have also tapped a hole in the carb so I can adjust the APT screw after I have it installed.

Thanks,

Bruce

[Cliff Ruggles]

The float settings on all Quadrajets were application specific, and the carbs entire calibration was based on that setting.  They also used quite a few different fuel pumps, some having pretty low pressure, others really high pressure.

When using a later model Q-jet on a non original application, we recomend setting the float to 9/32 to 1/4" as a starting point.  After building many thousands of these carburetors for a wide variety of applications, this has proven to be the best place to be.

As an experiment I tried a really low float setting of 15/32 on a later model carb a few weeks back.  At 6psi from an electric pump, there wasn't enough fuel in the bowl to effectively fill the accellerator pump well.  The level of fuel was exactly in line with the lowest fill point, it really needed to be at least 1/8" higher.

A low float setting and low fuel level not only effect the carbs calibration, but they don't leave much fuel in the bowl for evaporation.....Cliff

[bcarlson78248]

Cliff,

Thanks for the information on float setting.  I have it at 3/8" now, but will bring it up to about 1/4".

Now which power piston spring should I use?  I have about 20" of vacuum at idle.

Or does the externally adjustable APT screw make it easier to dial in without swapping springs?

Bruce

[Cliff Ruggles]

The orange and light blue (plain color in some kits) are the lightest and for big cams and low vacuum.  I'd use the green or dark blue spring for what you are doing.  We make the green spring extra long so it can be clipped for custom tuning.......Cliff