Ported Vacuum vs FullTime Vacuum

[knish71]

Hey guys, I am having problems with detonation and higher than normal engine temps after my '71 LT-1 was rebuilt. I'm pretty sure the shop that did the work may messed up the way the vacuum hoses are routed.

I've been told that my vacuum can is attached via a ported method off the carb and it would be more beneficial to change the canister to a VC1810 and attach it to a fulltime vacuum source.

By no means am I a mechanic, but after skimming thru the internet, ported vacuum is obtained by a port high on the primary side of the carb near the choke plate, where full time is obtained from a lower port on the carb.

Am I correct? If so, my 4801/4150 carb does not have a port vacuum access, so is it safe to assume that I'm already using full time vacuum?

Any help will be appreciated

Robert

 

[vette-dude]

Use the search function and you can probably find the answer. This may help.

http://corvetteactioncenter.com/forums/showthread.php?t=60830&highlight=ported+vacuum

Randy

 

[knish71]

I've spend all morning reading great posts by Johnz. I know that it's best to use full vacuum. My real question pertains to my list-4801/4150 holley carb. I didn't think there is a ported vacuum port on it. Am I wrong? I've seen a picture of a ported holley carb, and my list-4801 doesn't have a port such as this did. The only vacuum ports are located on the base. Where on this carb should the vacuum advance can on my distributer connect to?

Thanks

 

[motorman]

I've spend all morning reading great posts by Johnz. I know that it's best to use full vacuum. My real question pertains to my list-4801/4150 holley carb. I didn't think there is a ported vacuum port on it. Am I wrong? I've seen a picture of a ported holley carb, and my list-4801 doesn't have a port such as this did. The only vacuum ports are located on the base. Where on this carb should the vacuum advance can on my distributer connect to?

Thanks

just put a vacuum gauge on the vacuum ports and see if you have full vacuum reading at idle speed. i have converted ported vacuum carbs the full vacuum ones by modifiy the passages in the throttle base plate

 

[knish71]

Ok, that's a pretty good idea. I will have to buy a vacuum gauge. I would still like to know if there indeed any ported vacuum ports on a list4801/4150 carb, or are they all full vacuum.

Robert

just put a vacuum gauge on the vacuum ports and see if you have full vacuum reading at idle speed. i have converted ported vacuum carbs the full vacuum ones by modifiy the passages in the throttle base plate

 

[redvett]

Hey guys, I am having problems with detonation and higher than normal engine temps after my '71 LT-1 was rebuilt. I'm pretty sure the shop that did the work may messed up the way the vacuum hoses are routed.

I've been told that my vacuum can is attached via a ported method off the carb and it would be more beneficial to change the canister to a VC1810 and attach it to a fulltime vacuum source.

By no means am I a mechanic, but after skimming thru the internet, ported vacuum is obtained by a port high on the primary side of the carb near the choke plate, where full time is obtained from a lower port on the carb.

Am I correct? If so, my 4801/4150 carb does not have a port vacuum access, so is it safe to assume that I'm already using full time vacuum?

Any help will be appreciated

Robert

If you are using the port on the side its manifold and you must use the solenoid for spark control. It only allows vacuum in 4 th gear other wise you get too much spark advance. You can take the carb. to a carb. specialist and have him make a port for ported vacuum. I did on my 72 LT-1 Air Coupe.

 

[knish71]

I don't want to use ported vacuum. So, from what you have said, the carb doesn't have a ported vacuum port stock.


Thanks

If you are using the port on the side its manifold and you must use the solenoid for spark control. It only allows vacuum in 4 th gear other wise you get too much spark advance. You can take the carb. to a carb. specialist and have him make a port for ported vacuum. I did on my 72 LT-1 Air Coupe.

 

[redvett]

I don't want to use ported vacuum. So, from what you have said, the carb doesn't have a ported vacuum port stock.


Thanks

The manifold vacuum is going to add too much timing and that is what is causing spark knock and overheating. You cannot run that much timing at idle and low RPM's. The only other way is to use a special vacuum advance that Accel used to make. Dont know if they still do but it was adjustable. Used an allen wrench in the vacuum port to adjust it.
Hope this helps.

 

[motorman]

The manifold vacuum is going to add too much timing and that is what is causing spark knock and overheating. You cannot run that much timing at idle and low RPM's. The only other way is to use a special vacuum advance that Accel used to make. Dont know if they still do but it was adjustable. Used an allen wrench in the vacuum port to adjust it.
Hope this helps.

you need full spark advance at idle to prevent overheating because once GM went to smog controls they started to use ported vacuum and this is when corvette started to run hot at idle and low speed

 

[redvett]

you need full spark advance at idle to prevent overheating because once GM went to smog controls they started to use ported vacuum and this is when corvette started to run hot at idle and low speed

Thats funny but its 95 deg here today and been driving mine today with the air on and no overheating. No mods all stock.