Recently installed a new Holley carb (750cfm) on my 1968 bird w/a 1972 455 in it. I set the ignition timing and idle w/the distributor vacuum advance plugged...after connecting the vacuum advance hose the engine races even at idle. Should I be getting vacuum off the carb at idle? I didn't think so, but I'm pretty new at this game.

make sure the vacuum hose is connected above the throttle plate.you prob'ly have it below, which would give it constant manifold vacuum.

i think thats right. i hook these things up wrong all the time. my worst is the vacuum modulator to the auto trans. i think it thru, and then invariably hook it up wrong, and it wont shift worth a crap. anyway, however that's hooked up, i hook it up the other way, and then it works.

i've just been informed by my resident holley user, that the vacuum advance often goes to the metering block between the bowl and carb body.

The owner at the machine shop that rebuilt my 350 told me to hook it up below throttle plate, but I have a rochester quadrajet. That might make a difference. He uses Holley on his race cars but recomended to me to go with a Q-jet. Pete

i always get these things mixed up, like i said. when i think it thru, it seems to me it should be above, so when you open up the throttle, vacuum is applied that was not there before, causing the diaphram to pull the ign. plate in the dist. in the direction of advance. but then again, when you floor it, your manifold vacuum drops, so i thought it thru, and it's prob'ly the opposite of however i would hook it up the first time. as far as i know, you only use advance at cruise.
i like q-jets cause they have little bitty primaries for low end torque, and big honkin' secondaries for high end torque, but my brother (ford man) swears by holleys.

if you hook it up below, doesnt that give you too much advance at idle?

There's three ports on the carb...the big power brake in the back, the manifold vacuum on the bottom and the "timed spark ignition" on the top. Either of the last two causes the racing at idle.

I was thinking maybe the advance itself on the distributor is not holding...but the carb is still pulling at idle in any event. Any other ideas?

Vacuum leak is another possibility. Use the ported vacuum at the metering block for proper vacuum at idle and at RPM's.

Steve

so, if you plug those two ports, it runs ok, but if you hook either up, it races? sounds like maybe you're hooking up a leak.
if they're not hooked up, and not plugged off, it should be racing due to a huge vacuum leak. plug one off, start engine with the other leaking, and then hook and unhook the hose and listen for any change. do the same for the other. if there is no change. ether the hose is not hooked up on the other end, or what tis hooked to is leaking. i hope this helps. its difficult when i'm not there looking at it. i'm assuming you have auto trans. if not, just keep that bottom port plugged.
it could be you have a bad diaphragm on the dist. advance.
actually, now that i think about it, that timed spark ign. port should not be drawing much vacuum with your throttle plate closed. double check your idle screws and make sure choke is open when you do all this.
i guess i'm just telling you stuff you already know, but keep us informed, with details.

OK, I do have one stupid question...what's a 'ported vacuum at the metering block'? Is it the same as a ported vacuum switch? Right now, I have the hose connecting the distributor vacuum advance directly to the carb timed ignition vacuum port. Should there be something in between? I don't think I have any other vacuum source...Sincerely, Cletus the slack-jawed yokel

OHC Scott...just saw your comments, thanks! What should I be checking the idle screws for? Do you mean the two mixture screws or the fast/curb idle?

I don't know if this will help or not but I have a 428 that the previous owner by-passed the PVS and I haven't fixed it yet. The q-jet, on the front drivers side of the fuel inlet has a hose outlet that works my vac advance. at idle hooked or unhooked the idle and timing is where it should be and hooked it advances with accelleration.I found a three way vac fitting,large one for power booster,1 sm one for heater control valve and 1 small one for the PVS switch.Hope this is correct for my GP,I have a 2 way for my Firebird.

later,
Ray

i am not there so this is difficult. plug off all vacuum coming out of carb and intake, or at least make sure none of it is causing a leak.
turn the mixture screws all the way in, then back out 1 an a quarter turns. start the car. adjust idle speed to specs (or best you can by ear) with choke open and engine warm. for automatics, adjust with car in drive.
dont worry about the choke or the fast idle yet.
turn mixture screws clockwise one at a time until rpm drops, then back out to position before rpm drop. readjust idle. repeat, turning mixture screws whichever way gives you the best idle (when in doubt go lean) if you have a vacuum guage, use it with the mixture screws to obtain highest vacuum. you should have 18-20psi of steady vacuum. verify ign timing.
repeat procedure until best vacuum is achieved.
once you know the carb is set right, your vacuum problems may be easier to find.
i've heard holleys have float adustment problems. i know nothing about them, they should be set correctly from the builder.
an easy way to find vacuum leaks is to spray carb cleaner around base of carb, hoses, and intake, if there is a leak, idle will change.
i dont think adjustment is your problem, but it's a good starting point.
i think the "ported metering block" is the vacuum block coming out of the manifold with hoses for different functions.
if this thing runs fine until you hook up your advance, there must be a leak in the advance line or diaphragm. but it really shouldnt be drawing vacuum there until you accelerate, as ray just pointed out (q-jet or holley)which is why i suggested adjustment.

This might help: I was helping a friend change intake & carb on his '84 TA. Went to a Performer intake & 670 cfm Steet Avenger carb. Engine idled at around 2,000 rpm and no matter what I did it would not idle down. Called the Holley tech line & was told it most likely a vacuum leak. The problem lies in the fact that the intake is designed for a spreadbore carb, the carb I was using is a square bore. I bought an adapter plate from Summit Racing Equip. that seals off the area that leaks. Problem solved. FYI: if you use this plate (approx. 1/16" thick) use a 3/8" drill bit to punch out the holes for the mounting studs to clear better. Seemed a little too tight.

Steve :rolleyes:

good tip steve. i forget about that 'cause that's the kind of mistake you (or someone you know) only makes once. thats enough.
if you use an adapter plate, always check your hood clearence before you slam the hood. (once)

so mr. bucket, is your "timed spark ign." port still drawing vacuum at idle? and if so, when you plug it off, does the idle settle? there should be enough info here now. tell us what went wrong?

Thanks, guys...I'm going to try out some of your suggestions this weekend. That adaptor may be the problem...when the port is plugged the timing/idle is OK. Once I connect, it races and idles too high. I'll update on Monday.

Bucket, did you get it figured out?

FYI: the ported vacuum is on the passenger side above the idle mixture screw. Did you try replacing your vacuum hoses?

Steve

thanks, guys...still working on it. I have replaced the hoses, but still at idle the vacuum is being drawn in. I've checked the timing while plugged, but still haven't done the mix screws. I don't believe there's a vacuum leak anywhere either at the carb or manifold. In the meantime, the radiator was really plugged so I'm putting in a new one. I'll update after this weekend.

The probelm appears to also be a catch-22. I'm fighting this exact problem on my 455 GTO.

Let me see if you have what I have:

With the vacuum line to the distributor diconnected, and the port on the carb plugged, the idle is low (like 600 RPM) and timing is set to 9 degrees BTDC. Plug in the vacuum advance, and it idles just like you set it. Blip the throttle, it won't come back to idle anywhere below 1200 RPM?

Is this what's happening to yours? If so, then there might be too much vacuum advance from the vacuum canister.

I have not tried to yet (lack of time and lack of sleep causing clouded judgement), but I was advised to limit the amount of vacuum advace and increase the amount of centrifugal advance. I am running a Mallory Unilite distributor, where both of these are easy to adjust. If you are running a stock distributor, I may not be all that much help in telling you HOW to do it, just that this might be the way to go.

I'll let you know if I solve my problem with this approach after the weekend.

Q

Thx, Q. I've got a stock HEI distributor, with a new Accel cap and rotor. I'm going to put in a new advance this weekend just to make sure that wasn't causing the problem. It sounds like we've got similar issues, just that I've been setting the idle and timing when the advance is plugged.