Hot Rod Forum banner

1 - 15 of 15 Posts

·
Smile if you know Jesus
Joined
·
515 Posts
Discussion Starter #1
I don't understand why or how this is happening.

I've been running full vacuum to the distributor. When I was tinkering under the hood I decided to switch to ported vacuum to see if there was much of a difference.

I went to start it up and it wouldn't even turn over. It would start to turn over then stop. Switched the line back and it fired up just fine. I haven't attempted it again.

This is on my SBC w/ Holley Truck Avenger. I normally run vacuum from base plate. I plumbed the line to the vacuum port on the primary metering block and plugged the base plate connection.

Any ideas or insight?
 

·
Hates: Liver. Loves: Diesel
Joined
·
5,659 Posts
Ignition systems designed to operate on manifold vacuum use a modest mechanical advance and then use big idle vacuum to get the proper initial vacuum setting. systems designed for ported vacuum don't have the vacuum at idle, so they are set with lots of initial advance.

If you want to try the change to ported advance, you'll have to change vac canisters, recurve the mechanical, and advance the timing to compensate. Chances are you'll not see much difference except maybe a 1/2 mpg at cruise by switching to ported.
 

·
aka Duke of URL
Joined
·
4,815 Posts
:confused:

Are older GM systems designed for manifold vacuum? I know FORD uses ported and mixes both on some later smog designs.

I had a FIREBIRD years ago (working on-not owned) that drove me absolutely crazy regarding hard starting and pre-ignition. Was I tricked? :rolleyes:
 

·
Glad To Be Here
Joined
·
2,240 Posts
70bird said:
I went to start it up and it wouldn't even turn over. It would start to turn over then stop. Switched the line back and it fired up just fine.
Any ideas or insight?
Look for something else to be the cause of the engine not turning over.

Until the engine starts; you will not have any vacuum (whether ported or manifold); so switching the connection would not cause the engine to not turn over.
 

·
Smile if you know Jesus
Joined
·
515 Posts
Discussion Starter #5
I have my initital set a ~14-15. I have an adjustible vacuum canister that I have set to pull no advance at idle.


Frisco said:
Until the engine starts; you will not have any vacuum (whether ported or manifold); so switching the connection would not cause the engine to not turn over.
That's what I thought also. Manifold pulls 16 in.hg at idle.
 

·
Smile if you know Jesus
Joined
·
515 Posts
Discussion Starter #6
1/2 MPG doesn't sound bad when I am getting about 9 right now.

I think I'll hook my vacuum guage do the porded and see how it reacts when driving.

I'm beginning to think that the the only time my vacuum advance is working is when decelerating and vacuum goes to about 24 in hg.
 

·
Registered
Joined
·
14 Posts
RE: Vacuum

Do your self a favor and read the ignition system information on the
Century Performance web site. This will clear up all the miss information.
 

·
Hates: Liver. Loves: Diesel
Joined
·
5,659 Posts
KULTULZ said:
:confused:

Are older GM systems designed for manifold vacuum? I know FORD uses ported and mixes both on some later smog designs.
Yes. In the 60s things started switching over to ported. HD trucks and vans remained manifold vacuum up until they went EFI in the late 80s. If you pull a Qjet off a 70s or 80s truck or van they usually don't even have a ported vacuum source.
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
Frisco said:
Until the engine starts; you will not have any vacuum (whether ported or manifold);

.
Excuse me. :welcome:

NON ported or rather "manifold" vacuum happens the instant any cylinder starts sucking. If it didn't then no idle fuel would be drawn through the idle circuits...

Take a hose off a line that goes directly into the manifold or plenum. Hold your finger over it when somebody cranks the engine... It DOES suck your finger, instantly, before the engine starts.
 

·
Administrator
Joined
·
18,400 Posts
xntrik said:
Excuse me. :welcome:

NON ported or rather "manifold" vacuum happens the instant any cylinder starts sucking. If it didn't then no idle fuel would be drawn through the idle circuits...

Take a hose off a line that goes directly into the manifold or plenum. Hold your finger over it when somebody cranks the engine... It DOES suck your finger, instantly, before the engine starts.
Yeah...but still dont think that was the problem...he said that the engine wouldn't even turn over............I think he has starter or electrical problems. Just coincidence that it worked when he put the hose back.
 

·
Smile if you know Jesus
Joined
·
515 Posts
Discussion Starter #12
I attemped this about 6-7 months ago.

Now that I am thinking about it may have been at the same time I tried to bypass the vacuum canister.
Would messing with the line that goes from the base plate to the vacuum canister to fuel tank cause a no start condition?
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
poncho62 said:
Yeah...but still dont think that was the problem...he said that the engine wouldn't even turn over............I think he has starter or electrical problems. Just coincidence that it worked when he put the hose back.
:welcome: You could be right. I don't know what to say about his problem. :welcome:

I was just addressing Frisco's statement so everyone concerned can be better mechanics. :thumbup:
 

·
Glad To Be Here
Joined
·
2,240 Posts
xntrik said:
Excuse me. :welcome:

NON ported or rather "manifold" vacuum happens the instant any cylinder starts sucking. If it didn't then no idle fuel would be drawn through the idle circuits...

Take a hose off a line that goes directly into the manifold or plenum. Hold your finger over it when somebody cranks the engine... It DOES suck your finger, instantly, before the engine starts.
You are correct! :D

Thank you for noticing. :thumbup:

As a matter of fact IF the butterfly's of the carb are opened enough there will even be some vacuum at the ported connection. ;)

My statement was technically incorrect. :( :nono: My bad!!!

My point was that switching the vacuum advance hose connections would not cause the engine to fail to turn over. This is what he was asking. The amount of vacuum when the engine is being turned over by the starter motor is very low due to low RPM's but increases dramatically once the engine starts.
 

·
Save a horse, Ride a Cowboy.
Joined
·
5,122 Posts
Frisco said:
You are correct! :D

Thank you for noticing. :thumbup:

As a matter of fact IF the butterfly's of the carb are opened enough there will even be some vacuum at the ported connection. ;)

My statement was technically incorrect. :( :nono: My bad!!!

My point was that switching the vacuum advance hose connections would not cause the engine to fail to turn over. This is what he was asking. The amount of vacuum when the engine is being turned over by the starter motor is very low due to low RPM's but increases dramatically once the engine starts.
Correct :thumbup: Cranking vacuum is so low that a vacuum advance will not move during cranking. Most take at least 8" to begin movement. :thumbup:

ditto for the venturi ported vacuum. None til the air rushes through the venturi.
 
1 - 15 of 15 Posts
Top