Vacuum – help a retard out…
Thread Starter
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
Vacuum – help a retard out…
5 inches…….......of vacuum smart a**’s
Separated all forms of vacuum use (tranny, brake booster, “T” at back of manifold, ALL of them)
Timing OK
Changed carb and intake gaskets (no visual cracks while apart – didn’t remove heat shield)
Changed power valve (1.5 year old holley)
Compression good
Sprayed carb cleaner all over while running
Have it down to hole somewhere in my relatively new Holley, hole/crack in intake, or something I am absolutely missing…
Can I have a cracked head on the intake side and lose vacuum?
Discuss………….
Separated all forms of vacuum use (tranny, brake booster, “T” at back of manifold, ALL of them)
Timing OK
Changed carb and intake gaskets (no visual cracks while apart – didn’t remove heat shield)
Changed power valve (1.5 year old holley)
Compression good
Sprayed carb cleaner all over while running
Have it down to hole somewhere in my relatively new Holley, hole/crack in intake, or something I am absolutely missing…
Can I have a cracked head on the intake side and lose vacuum?
Discuss………….
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
you say timing ok, other than cam, timing at idle is the next largest thing to affect vacuum....
initital should be around ten or so maybe more....
I preferr running vacuum advance straight off manifold vacuum...gives much better idle charactoristics that way....
and good for about another ten+ points on vacuum readings...
GENE
initital should be around ten or so maybe more....
I preferr running vacuum advance straight off manifold vacuum...gives much better idle charactoristics that way....
and good for about another ten+ points on vacuum readings...
GENE
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
How much timing are you really running? If you have a little bit of a cam, you need about 15-18 degrees initial, 36 total, and 16 more degrees of vacuum advance so that you're actually idling with about 30-34 degrees of timing. This will usually get the vacuum up to where it belongs.
Burning Brakes
Joined: Jul 2000
Posts: 1,162
Likes: 2
From: Beaverton, OR
lars - You've just given me one more reason to sort out my timing/knocking issue . . . That way I can feel safe in plugging my vacuum advance back in. . . Not to mention that I just spend $40 for gas this morning . . . That got me to work only 4 or 5 times (14 miles round trip) 
Come to think of it, that is *REALLY* bad. At best that is 4.6 mpg. Crap, something else must be wrong too! Damn carb seems right on too.
Come to think of it, that is *REALLY* bad. At best that is 4.6 mpg. Crap, something else must be wrong too! Damn carb seems right on too.
Thread Starter
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
Thanks all of you. I just got in from swapping carbs. My brother has a 650, same as mine,...no change so that kinda rules the carb out..
the car has always run pretty good then started to run a little rough, so I went through the usual tune up stuff, plugs, wires, cap, and rotor...same roughness...
ran compression, all good (for an old worn out engine), checked vacuum, 5 inches - has run 11 or 12 in the past at idle so I am assuming that the roughness and the lack of vacuum are connected somehow.....I'm beginning to think that my intake could be cracked, even though I've never seen one do so...just can't think of much else that would lose that much vacuum since I've disconnected everything, changed intake gaskets, and changed carbs - even pulled the T out of the back of the intake just to eliminate one more thing..???
mild, nearly stock...not sure what size..
initital should be around ten or so maybe more....
I've always had about 11 or 12 at idle
I haven't done your correct timing set up, but as a base line I am timed at 11 degrees advance at idle, like it has almost always been... usually runs well, starts well - hot or cold so I haven't changed it..
the car has always run pretty good then started to run a little rough, so I went through the usual tune up stuff, plugs, wires, cap, and rotor...same roughness...
ran compression, all good (for an old worn out engine), checked vacuum, 5 inches - has run 11 or 12 in the past at idle so I am assuming that the roughness and the lack of vacuum are connected somehow.....I'm beginning to think that my intake could be cracked, even though I've never seen one do so...just can't think of much else that would lose that much vacuum since I've disconnected everything, changed intake gaskets, and changed carbs - even pulled the T out of the back of the intake just to eliminate one more thing..???
Originally Posted by MILO
Stock cam? Did you measure vacuum at the base of the carb or at one of the intake runners? TTT
Originally Posted by mrvette
initital should be around ten or so maybe more....
Originally Posted by lars
How much timing are you really running? If you have a little bit of a cam, you need about 15-18 degrees initial, 36 total, and 16 more degrees of vacuum advance so that you're actually idling with about 30-34 degrees of timing. This will usually get the vacuum up to where it belongs.
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
Take a look at the EGR valve...it's possible that it is sticking, or has a blown gasket. You can get a nice chrome block-off plate at most auto shops...to test to see if that's the problem.
Thread Starter
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
Originally Posted by Rockn-Roll
Take a look at the EGR valve...it's possible that it is sticking, or has a blown gasket. You can get a nice chrome block-off plate at most auto shops...to test to see if that's the problem.
with the cats, A/C, smog pump...etc..that stuff is for girls..
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Melting Slicks
Joined: Sep 2000
Posts: 2,406
Likes: 418
From: Chester Virginia
I had low vac issue with my 73 when I first got it. Tried everything with no improvement. A co-worker told me to remove the dipstick and measure for vac at the dipstick tube. I had 7" at the tube just like at the carb, it should be 0". Removed the intake and resealed it - vac went to 14", were it is today.
Roger
Roger
Thread Starter
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
Originally Posted by RMS73
I had low vac issue with my 73 when I first got it. Tried everything with no improvement. A co-worker told me to remove the dipstick and measure for vac at the dipstick tube. I had 7" at the tube just like at the carb, it should be 0". Removed the intake and resealed it - vac went to 14", were it is today.
Roger
Roger
have mine off now - second time this week. put a straight edge on the head side of the intake and it's slightly warped so I have a new one gonna be here friday...we'll see..