Engine vacuum
Thread Starter
3rd Gear
Joined: May 2009
Posts: 3
Likes: 0
Engine vacuum
Greetings, I need some ideas about low vacuum at all engine speeds and rough running on a 73 small block. Pulling about 8 in on a 350 that is not stock..normal vacuum before the problem was about 18 in. This is not a stock vette 350. Lines all check with no leaks, Compression in engine is good/normal. Carb gasket looks good. I suspect intake gasket but leak check on exterior of it shows nothing. Soapy water/oil test. Vacuum increases with engine speed and stays steady but low. Rough running at all speeds. Just need any ideas you can offer before i pull the intake. Thanks!
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
From the BTDT school of personal experiences, I'm afraid my 1st suspicion is that an intake manifold vacuum leak from the cam valley is lurking under there, which probably doesn't offer anything new you're not already thinking.
Instructor
Joined: Jul 2008
Posts: 100
Likes: 0
From: Long Jetty NSW
Things I would check, and perhaps you have already checked yourself, are ...........
Power Booster hose/diaphram
Rocker settings, have you changed them, is one riding?
Ignition timing.
Another way to check inlet manifold is to pressurise it and listen for leaks. To do this .......
Release all rockers so that all valves are shut.
Remove carb.
Remove/block off all vacuum ports on manifold.
Make up a plate similar to picture below.
Apply 5 to 10 pound air pressure from a compressor.
Listen for leaks.
Hope this is of some help.
Power Booster hose/diaphram
Rocker settings, have you changed them, is one riding?
Ignition timing.
Another way to check inlet manifold is to pressurise it and listen for leaks. To do this .......
Release all rockers so that all valves are shut.
Remove carb.
Remove/block off all vacuum ports on manifold.
Make up a plate similar to picture below.
Apply 5 to 10 pound air pressure from a compressor.
Listen for leaks.
Hope this is of some help.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
That's a good idea. But I think he only has to release the intake valve rockers, that's eight instead of sixteen. 
Assuming the rockers are correctly set, he could also use an indelible pen and mark each of these rocker nuts, just to be sure to set them back in the same position.
If engine is at TDC ( firing ) on cylinder #1, only 3 rockers should be enough : Intake #2, Intake #1, Intake #8.
Some leak detector spray can be useful on the intake manifold near the gaskets and around the fittings. It makes nice bubbles with VERY SMALL leaks and does not stain or corrode.
Last edited by 73StreetRace; May 17, 2009 at 07:07 AM.
Thread Starter
3rd Gear
Joined: May 2009
Posts: 3
Likes: 0
Thanks for the ideas. Timing is a-ok. All rockers are operating normaly as observed and none have been recently changed. All vacuum lines to the manifold are in good shape and not leaking. I may try the leak test if I can put a plate together. Thanks again.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Low vacuum is the result of low cylinder pressure. Normal causes are poor fuel metering, either a vacuum leak or maladjusted or bad carburertor, or poor ignition performance. The easiest way to diagnose a vacuum leak is to gradually choke the carb inlet. If vacuum improves and the engine smooths out you know your engine is seeing unmetered air.
On the ignition side, don't assume the timing is correct based upon what you see on the balancer. The inertia ring is known to slip. But if it was fine before and you didn't mess with it, it's not likely to have gone out of time on its own. If the spark is weak, the only way you'll know that is to graph the ignition primary and secondary.
I'd approach this as a fuel system problem. You already tested the compression and your results show good readings so going invasive is probably a waste of your time.
On the ignition side, don't assume the timing is correct based upon what you see on the balancer. The inertia ring is known to slip. But if it was fine before and you didn't mess with it, it's not likely to have gone out of time on its own. If the spark is weak, the only way you'll know that is to graph the ignition primary and secondary.
I'd approach this as a fuel system problem. You already tested the compression and your results show good readings so going invasive is probably a waste of your time.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Greetings, I need some ideas about low vacuum at all engine speeds and rough running on a 73 small block. Pulling about 8 in on a 350 that is not stock..normal vacuum before the problem was about 18 in. This is not a stock vette 350. Lines all check with no leaks, Compression in engine is good/normal. Carb gasket looks good. I suspect intake gasket but leak check on exterior of it shows nothing. Soapy water/oil test. Vacuum increases with engine speed and stays steady but low. Rough running at all speeds. Just need any ideas you can offer before i pull the intake. Thanks!
Timing chain jumped ?
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Advanced
Joined: Dec 2006
Posts: 64
Likes: 3
From: Florida
I found this site to be very usefull. From your post, I understand that you already have a good working vacuum guage. go to this site and click on all 15 scenarios. let me know if it helps.
e.p.
http://www.secondchancegarage.com/public/186.cfm
e.p.
http://www.secondchancegarage.com/public/186.cfm
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
#1 If you haven't done so already, remove ALL vacuum lines from the manifold and carb and cap/seal-off all of the open fittings. Don't forget to remove and cap the line for the brake booster (if you have P/B), any emissions vacuum lines, and the vacuum advance can on the distributor. Now, connect a vacuum gauge [yes, you need one if you have a C3] to the manifold fitting so that the gauge is the ONLY item connected to engine vacuum. Now, start the car and let it idle and read the vacuum level. Is it still at around 8 in. Hg....or is it higher? If it is still about the same as you had before, you either have a very wicked cam, badly worn or damaged valves/valve seats, or some serious intake manifold gasket leaks. Let us know what you find when you check the 'base' vacuum level at idle. If you have 15 in. Hg or higher with the 'base' vacuum check, you can add back in one system at a time to find out which is the 'culprit' that is causing the leakage.
Thread Starter
3rd Gear
Joined: May 2009
Posts: 3
Likes: 0
Thanks for all the suggestions. I revisited the ignition system and found fouled plugs. This was caused I think by my having an hei conversion system that was connected to the resistor system. I now have a full 12 volt feed to the system as well as new plugs. Started easy and runs great now. Vacuum is about 16 I beleive because on my 500 lift cam. This seems to be the normal vacuum since I built the engine. Thanks again! Also the vacuum diagnostic website is great.
