chas0218

So when the car is idling it pulls 5-7 inches of vacuum at 1000 rpm. So that would mean I need a 3.5 powervalve or should i go with a 4.5? When I give it gas the vacuum drops to zero then starts to rise up to about 2 or 3.

Am I doing this correct?

Right now it has the 6.5 installed and it is burning my eyes with the idle mixture screws almost all the way in. Also the car is kind of sluggish when the accelerator is snapped.

Thanks,
Chas

pancherj

Some specs on the cam and ignition timing? Sounds like you have the timing at idle retarded and are compensating by opening the butterflies too far on the carb (with the idle screw). This is exposing the transfer slot, dumping in a lot of fuel and causing your vacuum to be really low. I have a pretty big cam in my 350 and it pulls 10" of vacuum at 850RPM idle. I think you can get more vacuum and lower your idle. When you do this, the 6.5PV may end up just right.

84 z28

Get your vaccum reading then cut it in half and thats what power valve you need. But from the sounds of it first you need to time your motor and tune your carb.

Damon

You must have a very big cam in that motor to only pull 5-7" of vacuum at idle (in the category of "lumpy bastard cam"). What cam do you have and what is your ignition timing curve?

And are you checking vacuum from a full manifold vacuum port (not the timed vacuum port)?

pancherj

My personal experience has been that the "half of vacuum reading" for sizing the PV has NEVER worked. The power circuit doesn't kick in fast enough when you step on it. To cover the hole, you put in bigger shooters and a more agressive pump cam. That works, but then you are burning through gas at an obscene rate when cruising around because the pump cam is actuating every time you touch the throttle.

84 z28

Well guess you had a bad " personal experience". Thats the way you get your power valve number. Maybe holley dosent know what there talking about either.

http://www.holley.com/data/TechServi...wer_valves.pdf

pancherj

Well, maybe Holley is gearing their literature (and basic tuning) to cover the widest variety of combinations? I am not the only person to run into this issue. In fact, it was info that was passed onto me from someone else. I complained and he said "this should fix it". This is why no out of the box carb is perfect...but they will all run. Holley (or whoever) builds them to suit many applications.

onebad82z

iTrader: (7)

That is good when the carb is a mismatch for the motor it is on. Otherwise follow sofakingdoms tuning advice on choosing PV size. Can't go wrong.

onebad82z

iTrader: (7)

Bingo. Like often done, too big of a carb on too small of a combo.

onebad82z

iTrader: (7)

What vacuum does the motor produce at a steady cruise?

84 z28

Its has nothing to do with the carb being mis matched to the motor.

chas0218

The timing is set to 8* advanced and 36* full out. The cam is 240-246 at .050" its lopey but drives nice on the road. I'm checking manifold vacuum on the actual manifold not thru the timing vacuum port. Idle is set for 1000-900 rpm, It wont go lower without shutting off. I believe it won't go lower because its running so rich. This is a auto car with a 700 dp (I don't want to hear "You should run a VS with auto.") It has a large 35 secondary squiter but I will be ordering a new one when I order the powervalve.

Thanks,
Chas

pancherj

That isn't nearly enough initial for that size cam. You need to be more like 16-20* initial and limit the distributor to only 20* advance (36* total is good). I have a similar cam and I run vacuum advance so at idle I am at 44* and under part throttle I am at 52*. I have 20* initial and 36* total. Seems to work good. Don't bump your initial up to 16* without limiting the mechanical advance! I can get away with 92 octane no problem (aluminum heads and 10.2:1 compression).

These timing changes should get you some more vacuum and also vastly improve your throttle response. After you make these changes, you'll have to mess with the carb some more (idle screws, idle enrich, maybe the PV, etc.).

onebad82z

iTrader: (7)

X2.

chas0218

Sweet thanks a ton. My buddy's dad said that I only needed 8-10 initial. I'll try bumping it up and see how things work out. This might help my hard starting problem.

right now i have 11.2:1 compression on iron heads without any issues but its mostly because the cam bleeds a lot of it off.

pancherj

That is a good bit of compression for iron heads and pump gas. Go slow with the timing changes (don't jump right to 18*. Start at 14*)! Keep an eye on the spark plugs. Get a good magnifying light so you can see down inside the plug (at the base of the porcelin). I'd hate to see you hurt the motor. But you are right...at low speeds that cam is going to bleed off a lot of cylinder pressure.

chas0218

Ok so i bumped my initial timing up to 16* without any problems but my vacuum went down to 3 inches. When I burp the throttle and release it drops to 2000 and sits there for about 3 seconds then drops back down to 900ish (idle.) My eyes are still burning at idle with the timing advanced. I can put my idle mixture screws in and it will still burn my eyes.

I don't understand why I'm not pulling much vacuum at idle. My transition slots are perfect but not sure what else could effect my idle.

Thanks,
Chas

kd5icr

iTrader: (2)

I wonder if you may be running to much carb for the engine. What size is it?

84 z28

Sounds like your butterflies are open too much and your transition slots are off.

chas0218

Is a 383 bored .060 over....I doubt its too much carb, Holley recommends a 720 cfm carb but im only running a 700 soon to be 750 without the choke horn and new main body. My guess is that the reason for the high idle then drop is because of the power valve being open and allowing too much fuel thru.

I will pull the carb and check the transition slots to make sure they are correct.

pancherj

Also, check around for a vacuum leak (maybe the intake gaskets). I don't think the carb is too big. It is probably something little and after you find it you will want to curse at the car!

chas0218

It ALWAYS seems to be something small or something you over look because "nah that doesn't make a difference."

But i will check for vacuum leaks I don't think it would be my intake gasket seeing how it my vacuum just dropped after advancing my timing. I'm thinking I might have loosened one of the hoses when adjusting the timing...I'll be sure to let everyone know as soon as I get a chance to look at it.

chas0218

Ok so i got everything all buttoned up and I still pulls 5-7 inches of vacuum with 20* of initial timing.

sprayed carb cleaner all around my vacuum connections and no leaks. Im thinking that its still just way to rich and doesn't pull vacuum because of it.

Ill pull the carb and check the transition slots

Should I drill a small hole in the butterflies. --> Good idea?

onebad82z

iTrader: (7)

Make sure you exhaust all other possibilities before making a change that cannot be reversed.

Set it IM screws before you decide on anything....