Thread: vacuum advance woes

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where do I need to hook up my vacuum advance on my edelbrock carb? The two small vacuum ports on the front of the carb both pull manifold vacuum. car runs ok at idle, but the second that I open up the throttle while under load it starts pinging like crazy.

it is indeed the 85 5.0. my cam is a comp magnum 292h
Operating Range: 2500-6500 RPM
Duration Advertised: 292° Intake / 292° Exhaust
Duration @ .050'' Lift: 244° Intake / 244° Exhaust
Valve Lift w/1.6 Rockers: .518'' Intake / .518'' Exhaust
Lobe Separation Angle: 110°

i am running an edelbrock 650 cfm carb with electric choke. running wal-mart 91 octane fuel. (10.5:1 compression, D0oe cast 351 heads with adjustable rockers,exhaust thermactor bumps removed, gasket matched) edelbrock RPM-Airgap manifold, stock distributor with gear changed to match the flat tappet cam. I set my idle at 800 rpm with the vacuum advance unlugged and set my timing at 10 deg. adv. As soon as i plug the vacuum back in the idle jumps to about 1600 and the timing goes to 20+ degrees. everyone else that I have talked to recommended a holley 650 dp for me. I am buying one because this car will be raced, but I want/need to drive this thing now and I don't get paid till friday. Any help would be appreciated. No the dizzy is not locked out.

ok that's cool......I will just wait until I get the holley b/c there aren't any vacuum ports above the throttle plates on this edelbrock. I just got this damn thing running the other day and it has been hell since then. mostly just flywheel balance/ boxing errors. have you ever seen a combo like this and how did it run with all the bugs worked out?? car weighs about 3200 with me in it. 5- speed with 4.30 gears. 26x10.5 tires out back. just wondering.

that is where I have it connected.....any ideas as to why it could be pulling manifold?? I hooked my vacuum gauge up to the full time, and the ported and it showed the same amount of vacuum either way.

if ur gonna race this why worry about vacuum advance.

Going out on a limb here but, isnt the idle circuit supposed to be 600 to 750 rpm?? If you are idleing at 800+, the throttle could be open resulting in vacuum at ported connnection???

If you are at 800 rpm your mechanical advance maybe coming into play. Its normal for the rpm's to go up when you hook the vaccum line back up. Just close the throttle plates up to the slow it down and than adjust the idle screws. You need to get the idle and throttle plates right before adjusting any thing else.The edelbrock requires alot of piddling if it doesn't run right out of the box. Also the vaccum can may need to be replace with an adjustable one. To much vaccum at cruise and it won't drop off fast enough causing it to ping. A spring change on the metering rods may help also. Running the manifold vaccum for the street will help the engine to run cooler, esp. at idle.

Originally Posted by DennyW

I don't know. You haven't mentioned idle rpm's, and what your actual vacuum is.

If your not on the idle circuit, then you would have vacuum at that port.

Yep, right there it is. How did I miss that? Didnt mean to step on toes.

ok....since I last posted i have installed a brand new holley 650 dp, msd 6al, blaster 2 coil, and a mechanical advance pro-billet distributor. now it is a little better but now my car backfires when I try to put it under load, and at high rpm just revving. The dizzy came with the "heavy" springs installed so I left those in there. total timing checks in at about 2600 w/ 27 degrees advance. now it will idle at 600 but my vacuum still jumps a little between 6 and 3. Any more ideas?what is the best way to check for vacuum leaks that you guys have used. I appreciate it alot.

IMO thats not enough timing. if it had more timing the vacuum may go up

You had a point style distributor before. Did you take the resistor out so the have 12 volts to the coil?

it wasn't a points dizzy.....electronic from the factory on the 5.0 H.O.'s. my idle and BASIC driveability problems are pretty well fixed now. I have spent about 3 days just making small carb/timing adjustments and then doing a pre-determined road test to see what works. Probably about 40 runs so far. The only real problem that I have now is under full throttle. Base timing is about 13 degrees (+/- 1) and all in at 2600 rpm w/ 37 degrees.I put the timing where it runs better for daily driving so that I can finally drive it to work. But I really am burning to haul ass and there is a good test and tune tomorrow night. Vacuum at idle now is around 11 inches.that means that I should use approx. 5 in. power valve right?

[(idle vacuum/2)+1=power valve size]

i appreciate all of the pointers.