I've run my new/used holley 4175 long enough to get a good enough baseline to begin retuning her alittle.It seems to be real responsive and generally runs great,but at highway speeds,i can feel the car lean surge slightly.The plugs confirm the lean condition.I'm going to move up from the 59-2 jets that are in it now,to a pair of 63's.The power valve in it now is the 2 stage variety that opens at 12.5 hg/5.5 hg,I'm going to replace that with a 6.5 hg single stage one.I might have to go higher than this for a mild 350,but i thought this would be a good starting point.The secondary spring is right on,no bog and good response(purple spring).I'm really learning to love these Holley carbs.This $31 used Holley works better than than the $250 edelbrock carb i used to have.Just so you Know,I'm pulling about 20 inches of vac.at idle,and timing is set at 10 deg. initial.Any thoughts?

 

With a 6.5 PV you'll have to jet it much richer, too rich really, just to cover up the big loooonnnngggg flat spot you'll get off idle or with small throttle movements at light throttle when cruising, like stop & go traffic, from the PV never opening until you really stand on it kind of alot.

59 to 63 is a BIG step. Your description calls for just a small step.

Put 61 jets and a 8.5 or 9.5 PV in it.

 

Thanks.That makes perfect sense to me.Somehow,i gathered that a 1 or 2 step up in jets wouldn't make much difference.Most people seem to run jets of 65 or above for engines not much different than mine,so i thought i'd still be on the lean side with 63's.I guess i failed to take into account the power valve's job in the fuel curve.The Holley site said a 6.5 was good for all-around driving in near stock applications,so thats what i got.I'm glad i posted,i'd like to get it pretty close the first time.

 

When you check your vaccum when determining power valve selection have a friend put the car in gear, this will also make the vaccum a little different and might be enough to warrant a different power valve besides the 6.5, even though like you said that is the most common, when adjusting your jets, what will be the cars main use daily or alot of higher rpm spinning, you;ll have to tune the jets according to driving style, if it's a daily driver, drive it for 20-30 minutes on the hwy, come home and check a plug adjust from there, if it's gonna see alot of higher rev's go out and drive it like that then adjust the plugs otherwise you'll burn a piston if you set it for daily but spin the **** out of it, only draw back with setting Holley's, can't have the perfect mix for both worlds, I have mine set-up for 1/4mi. passes and they get pretty black driving in town, have'nt fouled them yet as I try to clean it out as much as possible, but after the end of a 1/4 mile run they look like a California beach babe color.

 

Thanks for the input.I baby my car most of the time,mostly around town daily driving.The only time i really get my hackels up is when i've got a 5.0 or a ***** givin me attitude.I don't think my carb is far off for the driving i do.I little bit richer to get rid of the surging at cruise and darken the plugs a shade and i'll be real happy.I'm gonna try the 61 jet-9.5 pv and see how it goes.I also got summit to send me a set of those re-useable gaskets.I'm assuming you don't get away with useing the gaskets more than once,even though i rebuilt this carb only a month ago.

 

Holley says to pick the power valve for mild street engines based on cruise vacuum, not idle vacuum. You'd be doing yourself a favor by hooking up a vacuum gage, check under flat steady cruise at 45 and 55, for instance, and pick one 1" below the lower reading (if 10" @ 45, 9" @ 55, go with a 7.5; if 11" is the lowest, 9.5, etc.). I'm using an 8.5, which is 3 steps up from what you would use based on idle vacuum. Throttle response is instantaneous. With a 4.5 (which is what was recommended to me based on idle vacuum), the flat spot was scary when driving on the street.

 

MTCamero, another wise investment besides the blue gaskets are the nylon bowl washers, the rubber ones fade away quickly with multi bowl removals.

 

All i've got is a hand- held vacuum gauge at the moment,but i plan to get one installed with a single 'A' pillar pod which will make it easier to do this right.I'm hopeing the 9.5 won't be too far off,seeing as how my original 2 stage pv begins opening at 12.5 (I'm assuming i'll get away with it opening alittle later since i'm jetting up some).While i'm on the topic,what is detrimental about the 2 stage pv's?I would think it would be alot more forgiving for selecting the right one,but visually,it looks like it wouldn't flow as much fuel.Am i on track?

 

Update: Put the 61 jets in with the 9.5 power valve this weekend,problem solved.The car pulls cleanly with no surging at all anymore.I hooked my hand held vacuum gauge up through the firewall with a 4-foot piece of hose and it turns out the 9.5 power valve was a great choice.Thanks for all the help guys(especially RB83L69!)

 

Glad to be of help!

I guess you can tell I've had a few of those carbs before....