Timed Vacuum pulls vacuum at idle...
Thread Starter
7th Gear
Joined: Dec 2017
Posts: 7
Likes: 0
Timed Vacuum pulls vacuum at idle...
1974 350ci, top end build. Edelbrock carb and intake, bigger cam, hei distributor, heavy valve springs...the usual. It's been running great for months and today out of nowhere it started wanting to die at stop lights and at idle. Checked it out and noticed that my timed vacuum port on my Edelbrock 1406 600cfm is pulling vacuum at idle. It runs rough with the vacuum advance connected, but basically can't run when I disconnect it. The idle drops and the pipes start popping a bit. It did this last summer, I took the carb off, sprayed it with cleaner and put it back on, and it still ran rough. Then after a few days of sitting it was perfectly fine. I need to get this fixed ASAP....please help.
Thanks
Thanks
Melting Slicks
Joined: Oct 2006
Posts: 2,417
Likes: 336
From: Texas
I would check the vacuum advance on the hei. Make sure it's working. May be stuck or have a hole in the diaphragm or hose. Just a thought. Is this a corvette or a truck engine. Just curious.
Last edited by speedreed8; Apr 14, 2018 at 09:01 AM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
A good investment for you right now is one of those vacuum pump tools. That would verify the Vac Can on the dizzy is truely moving like Speedreed said and can be used later for brake bleeding, checking for vac leaks, etc.
For now, move the dizzy vac line to Full Manifold Vac port, see how it runs (better).
For now, move the dizzy vac line to Full Manifold Vac port, see how it runs (better).
Last edited by HeadsU.P.; Apr 14, 2018 at 05:20 PM.
Thread Starter
7th Gear
Joined: Dec 2017
Posts: 7
Likes: 0
Distributor is working fine and it still tight. Believe the problem is in the carb. It shouldn't be pulling any vacuum through the timed vacuum port, but it is pulling quite a bit at an idle. Not sure why it would suddenly do this.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you are getting significant vacuum at idle from a 'timed' / 'ported' outlet, then your carb's primary throttle plates are open too much at idle. When nearly closed, the timed vacuum port feed orifice is above the throttle plate so there can be no vacuum present. With the throttle plates open to where those orifices see vacuum, the idle and mixture settings are way off.
But, I would never hook a distributor vacuum advance can to a timed/ported outlet, anyway. That was all for EPA and emissions requirements. For best idle, cooler engine running and least fuel use at idle, it should be connected to a manifold vacuum port.
But, I would never hook a distributor vacuum advance can to a timed/ported outlet, anyway. That was all for EPA and emissions requirements. For best idle, cooler engine running and least fuel use at idle, it should be connected to a manifold vacuum port.
Thread Starter
7th Gear
Joined: Dec 2017
Posts: 7
Likes: 0
If you are getting significant vacuum at idle from a 'timed' / 'ported' outlet, then your carb's primary throttle plates are open too much at idle. When nearly closed, the timed vacuum port feed orifice is above the throttle plate so there can be no vacuum present. With the throttle plates open to where those orifices see vacuum, the idle and mixture settings are way off.
But, I would never hook a distributor vacuum advance can to a timed/ported outlet, anyway. That was all for EPA and emissions requirements. For best idle, cooler engine running and least fuel use at idle, it should be connected to a manifold vacuum port.
But, I would never hook a distributor vacuum advance can to a timed/ported outlet, anyway. That was all for EPA and emissions requirements. For best idle, cooler engine running and least fuel use at idle, it should be connected to a manifold vacuum port.
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regarless of how tight the distributor is, recheck the timing with the vacuum advance disconnected.
check for vacuum leaks at the carburetor base, hold down screws/bolts could be loose. Check for cracked or disconnected rubber vacuum lines.
Your idle speed should be around 750/800 rpm, depending on camshaft used, what is it currently?
check for vacuum leaks at the carburetor base, hold down screws/bolts could be loose. Check for cracked or disconnected rubber vacuum lines.
Your idle speed should be around 750/800 rpm, depending on camshaft used, what is it currently?
Racer
Joined: Jul 2015
Posts: 396
Likes: 101
From: Lehigh Valley Pennsylvania
Please post what you find. I had the same issue and changed carbs.
Here is what mine did and maybe you can check if yours does the same
I had the edelbrock on my 400 cu in Cordoba and if you let it idle long enough (30 to 45 seconds) it would stall. It would idle with 16"Hg vacuum which was way too low for the cam I have. The vacuum would deteriorate over time and the car would stall. The car ran great if I could keep it from stalling at idle. I had no adjustment on the idle screws which the Edelbrock rep claimed was "normal".
I called edelbrock and the guy really wasn't interested in helping and didn't seem to have a clue. I put the Qjet from my 73 BB vet on the Cordoba and it runs great despite the fact that Qjet sat on the shelf for 10 years.
I am pretty sure I had a vacuum leak and I am pretty sure it was internal to the carb. Changing the carb resolved my problem.
Here is what mine did and maybe you can check if yours does the same
I had the edelbrock on my 400 cu in Cordoba and if you let it idle long enough (30 to 45 seconds) it would stall. It would idle with 16"Hg vacuum which was way too low for the cam I have. The vacuum would deteriorate over time and the car would stall. The car ran great if I could keep it from stalling at idle. I had no adjustment on the idle screws which the Edelbrock rep claimed was "normal".
I called edelbrock and the guy really wasn't interested in helping and didn't seem to have a clue. I put the Qjet from my 73 BB vet on the Cordoba and it runs great despite the fact that Qjet sat on the shelf for 10 years.
I am pretty sure I had a vacuum leak and I am pretty sure it was internal to the carb. Changing the carb resolved my problem.