Idle vacuum low

[Renxus]

According to Andy Kruse in this video () the idle vacuum reading should be close to 20 on idle. Then you adjust for the best vacuum for the best mix of air and fuel.
Well, no matter how I start and turn the mixture screws I cannot get much over 11.
I have the settings the same as Andy, same carb, timing 10ish degrees in advance, and the new (real) Edelbrock repair kit was put in lately. All should be the same. But not. By the way, the engine runs smoothly by ear. Where is my vacuum?
(Never mind the ported is blind and not connected to the distributor - another story).

 

[giantpune]

My first thought is what sort of cam are you running? The bigger the cam, the worse your vacuum.

If thats a stock or mild cam, then I'd say your gauge is reading low. And assuming your gauge is accurate, I would look for vacuum leaks. It is possible that you have a vacuum leak and then have tuned the idle mix screws overly rich which causes the AFR to be close to correct while the carb's idle circuit is in use.

I would disconnect anything that is connected to the manifold vacuum and plug it and see if it still idles the same. Then start spraying starting fluid around the intake, at any place where there's a gasket or a port. Your power brake hose and PCV setup look "custom". One of them could be the leak.

It may help to count how many turns your idle mix screws are out. Usually they are 1 - 1.5 turns from being all the way in (but not cranked down tight).

 

[Steve McMahan]

I’m wondering if you should try another vacuum port?

 

[Vicus]

I would try to set the ignition at 14 degrees advance.
My engine is a stock 351-2V with an Edelbrock Aluminum intake and a Holley 4160 and I read 15.5 on my vacuum gauge. I'm located at 500 m altitude.

 

[Kilgon]

+1 with giantpune. A large cam will cause low vacuum. Also, as he mentioned, try plugging all the vacuum ports. If your vacuum goes up, then you know one of the things you disconnected it your problem. Start reconnecting them one at a time and check your vacuum after each time. When your vacuums drops then you know which one is the culprit. Also, if you do have a larger cam, they like a more advance timing usually in the 16 to 20 degree area.

 

[Renxus]

Thanks for your thoughts, will try all this out. Not much history came with the car but I suspect it may have Edelbrock's Performer camshaft; it does have Edelbrock's Intake Manifold. Perhaps the full Performer kit is installed. The Edelbrock says: Increase initial setting to 10-14° BTDC.Total advance not to exceed 34-36°.With initial and centrifugal weights combined timing should be at full advance by 3000-3500 rpm. Will play with it soon and let you know.

Another mystery is that the distributor (Proform HEI 66980) does not like a (ported) vacuum at all. As soon as it is connected and I push the throttle it starts sputtering badly. Seems it is advancing the ignition way too much. But this is another challenge to solve.

 

[351c1971]

Here are my thoughts;
1. Verify maximum mechanical advance before setting the initial advance.
2. Connect the vacuum gauge to the vacuum tree on the intake.

My “H” has the Performer 2V intake, RV cam and low compression. It pulls 17 “ of vacuum at idle.

Regards, Jim

 

[logicbomb]

Mine was running around 12in after having it tuned at the Ford shop. I didn't like how it was running, so I tuned it with the idle mixture screws and got it up to 15in. Mine is a 351C 4V, and a Holley 750.

 

[Renxus]

Busy times with real life but now got some time to play with the carb again. Not done the vacuum plug test yet but this is what I got last night (BTW I am at 100 feet from sea level and taking the vacuum reading from manifold):
1. Estimated 12 deg mech advance gives vacuum about 13 in. Cannot really be sure on degs cause missing the proper pointer. Although, have made a new mark on the engine of TDC where the 1st cylinder is at the top. Should be close enough. Now, turning the distributor earlier gives a few additional inches - max to 15 in but the engine does not sound happy anymore. And the reading shows close to 30 BTC on idle. Turing later, the vacuum drops and the unhappy engine again. Tuned it back to 13 in.
2. Now I tested it on 3000 RPM - nice green up to 20 in. Acceleration test drops to 0 in. Looks good.
3. Attached the vacuum to dist from ported. No changes at idle. Rising RPMs - very unhappy engine.
So, I will have the vacuum leaks test to run but unlikely there are leaks - the fresh original carb repair kit was recently installed and tested.
I suspect the cam is aggressive (it might have some Edelbrock performer kit installed, at least the intake manifold says Edelbrock) so might be ok with the low vacuum at idle.
But the challenge now is to get the vacuum advance to work correctly. Will get soon the adjustable vacuum advance mechanism to my Proform dist - perhaps this would help to make this vacuum advance less agressive.
(Made few videos but cannot load them here - some error occurs with mp4 and avi)

 

[351bb]

I also had vacuum problem a few weeks ago, it ended up with a bad PCV valve!
Pluged all vacuum ports (PCV, Brake Booster, Auto. Trans., etc.) and the engine immediately ran much smoother
(Edelbrock Intake, 650cfm Holley, Comp Cam)
Good luck

 

[Steve McMahan]

Have you run the “everything disconnected from intake manifold” test
to confirm you have no vacuum leak?

 

[351bb]

Yes, I did this “everything disconnected from intake manifold”, and the engine immediately ran much smoother.
Then I connected one source after the other back on (Brake Booster, PCV, Auto. Trans., etc.) 'till the poor idle came back again. It occurred, when I pluged the PCV into the vacuum port. So that was my problem part. I replaced the PCV-Valve and the engine runs/idels how it should!

 

[midlife]

Nice troubleshooting there! Well done!

 

[Steve McMahan]

Well done.