Low Vac?
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
Low Vac?
My vette been running a low vac since I got her, no leaks in vac system, just changed my intake manifold and carb, still runs the same vac, about 11/12 hg.
what are the reasons for the low vac??(cam or worn engine?)
If rev the engine it goes upto 20/21 hg, is that normal??
I have also noticed that my timing keeps on changing and also my idle is abit stumbly, and sometimes wants to cut out and also the engine seems to be running hotter at idle, I'm using full manifold vac from carb for vac can on dizzy.
Ive read Lars, papers this matter, it does suggest I need a different vac can, which one would suit me best??(I just dont know what cam the car has)
specs
78 5.7L
rpm airgap intake
holley 750 sec vac carb(new)
heads and cams??(I did notice on the heads, when I changed my intake, looks like someone had done some porting
cheers
dave
what are the reasons for the low vac??(cam or worn engine?)
If rev the engine it goes upto 20/21 hg, is that normal??
I have also noticed that my timing keeps on changing and also my idle is abit stumbly, and sometimes wants to cut out and also the engine seems to be running hotter at idle, I'm using full manifold vac from carb for vac can on dizzy.
Ive read Lars, papers this matter, it does suggest I need a different vac can, which one would suit me best??(I just dont know what cam the car has)
specs
78 5.7L
rpm airgap intake
holley 750 sec vac carb(new)
heads and cams??(I did notice on the heads, when I changed my intake, looks like someone had done some porting
cheers
dave
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
different cams can produce different vacuum levels. Is the cam stock or aftermarket? If you are POSITIVE there is no vacuum leaks in the system than you probably have an aftermarket cam with a more radical design than the stock cam and that would help explain the lower vacuum level.
revving the motor doesn't matter and the vacuum levels with the motor revved up while sitting still in the garage will produce a higher vacuum reading but those figures are useless - when driving and at those higher RPM's the motor will be under load and therefore reduce the vacuum level anyway.
It's very possible that if the cam was changed out that you may have an incorrect vacuum can on the distributor. you want to make sure you use a vacuum can that brings the vacuum advance in 2" HG below the level of vacuum that your motor makes. Since you say you have 11" HG of vacuum at idle than you want to make sure that the can you use has a spec of bringing in the vaacuum by 8 or 9" HG.
you can get a listing of the various vacuum cans and their specs in one of Lar's papers I have posted up on my website's tech articles page here:
http://lbfun.com/Corvette/Tech/vettetech.html
scroll down to the section called: ****Distributor & Timing***
and look for the file called:
Vac Adv Spec.pdf
by Lars Grimsrud
you didn't list the year or type of motor or type of distributor but Lars paper will list the vacuum cans for both points distributors and HEI distributors so be sure to look in the correct section for what you have.
EDIT:
ok, I just noticed in your other thread about your distributor being in wrong that you still need to tune and adjust your carb. Before changing out the can on the distributor first properly set your idle muxture on your carb as this will also affect your idle vacuum.
To set the correct idle mixture, connect your vacuum gauge up to a full manifold vacuum source and adjust the idle fuel mixture screw(s) until you get the highest STEADY vacuum reading. only make small changes at a time, no more than a 1/4 turn at most on any single adjustment than let it stabilize for 15-20 seconds before taking a vacuum reading off the gauge, than make another small adjustment, let it stabilize and read the vacuum level, and continue this way until you get the highest vacuum level.
as you make adjustments you may need to adjust the idle speed also to keep it at the correct idle level as you are setting the mixture. don't let the idle increase above the normal amount as you set the mixture than simply adjust the idle back down afterwards as this will than also affect the vacuum level reading. Also make sure the motor is warmed up enough that the carb is completely off the choke as you make the adjustments.
If your carb has two idle mixture screws than make sure each one is turned equally and when you are done they should be adjusted the same amount on both.
Forget any vacuum level readings such as your previously stated 11 or 12" HG until you have your carb adjusted properly (and the distributor has the timing correctly set) as they will be off. reply back after the carb is set up and the distributor is set up with your new vacuum reading.
revving the motor doesn't matter and the vacuum levels with the motor revved up while sitting still in the garage will produce a higher vacuum reading but those figures are useless - when driving and at those higher RPM's the motor will be under load and therefore reduce the vacuum level anyway.
It's very possible that if the cam was changed out that you may have an incorrect vacuum can on the distributor. you want to make sure you use a vacuum can that brings the vacuum advance in 2" HG below the level of vacuum that your motor makes. Since you say you have 11" HG of vacuum at idle than you want to make sure that the can you use has a spec of bringing in the vaacuum by 8 or 9" HG.
you can get a listing of the various vacuum cans and their specs in one of Lar's papers I have posted up on my website's tech articles page here:
http://lbfun.com/Corvette/Tech/vettetech.html
scroll down to the section called: ****Distributor & Timing***
and look for the file called:
Vac Adv Spec.pdf
by Lars Grimsrud
you didn't list the year or type of motor or type of distributor but Lars paper will list the vacuum cans for both points distributors and HEI distributors so be sure to look in the correct section for what you have.
EDIT:
ok, I just noticed in your other thread about your distributor being in wrong that you still need to tune and adjust your carb. Before changing out the can on the distributor first properly set your idle muxture on your carb as this will also affect your idle vacuum.
To set the correct idle mixture, connect your vacuum gauge up to a full manifold vacuum source and adjust the idle fuel mixture screw(s) until you get the highest STEADY vacuum reading. only make small changes at a time, no more than a 1/4 turn at most on any single adjustment than let it stabilize for 15-20 seconds before taking a vacuum reading off the gauge, than make another small adjustment, let it stabilize and read the vacuum level, and continue this way until you get the highest vacuum level.
as you make adjustments you may need to adjust the idle speed also to keep it at the correct idle level as you are setting the mixture. don't let the idle increase above the normal amount as you set the mixture than simply adjust the idle back down afterwards as this will than also affect the vacuum level reading. Also make sure the motor is warmed up enough that the carb is completely off the choke as you make the adjustments.
If your carb has two idle mixture screws than make sure each one is turned equally and when you are done they should be adjusted the same amount on both.
Forget any vacuum level readings such as your previously stated 11 or 12" HG until you have your carb adjusted properly (and the distributor has the timing correctly set) as they will be off. reply back after the carb is set up and the distributor is set up with your new vacuum reading.
Last edited by BarryK; Oct 9, 2007 at 05:51 PM.
