I have a mild 355sbc. and I am trying to set-up the carb.It is a
600cfm holley with the choke removed and the lightest v.s. spring
installed.When I dynoed the car last fall there was a small dip in
the power curve and the line was not smooth.The guy running the dyno said that it needed more fuel.On an earlier post on this
board someone mentioned that I could get a lean bog from running that v.s. spring.I already plan on rejetting the carb(67on
the primaries and 76 on the secondaries).Do you think an 8.5
power valve would help?Please let me know if I am going in the right direction or not.

p.s. motor has about 350hp.@ the crank and is shifted at 5750rpm

 

The light spring will produce a bog from lean midrange, every time. It allows too much air in at too low RPM. That would expain "dips" in the power curve.

Put the brown spring in, and leave the jets and PVs alone until you see what that does.

Optimum PV depends on idle vacuum. If it has 14" or more with a stick or with an auto in gear, 8.5 is OK; if it has less, then you have to use a lower value PV. Low idle vacuum + high # PV = very bad gas mileage.

600 CFM is really too small for anything beyond a bone-stock smog 350. I doubt you are getting 330 HP out of it; HP = flow, and 600 CFM may not be enough air to burn 330 HP worth of gasoline.

 

I'll beg to differ with you slightly, RB, about the power valve (yes, I'm waiting for lighting to strike). According to Holley, the idle vacuum is only to be used for engines with agressive, high overlap cams. For the more mild street engine, they recommend determining the lowest typical cruise vacuum (i.e., level road, steady speed, checked at medium and highway speeds - like 45 & 60 MPH). Then, get a PV that is rated 2" below that vacuum.

 

Does a double pumper create the same lean condition as the
lighter spring?

 

Does anyone think this will work?