Massive Vacuum Leak
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Well 9, 10 or 11, it should be higher than that. You should be seeing 17 to 21 inches of vacuum.
I would suggest you get a compression tester. You are not really looking for how much(after all, it is a 30 year old engine), but comparing the difference between cylinders. A large difference would make me look towards bad intake gaskets or a cracked manifold.
But before you do that, unhook your exhaust and see what she does. I know you said it is leaking, but go ahead and open her all the way. Since it has been sitting for a good while the exhaust could be clogged up. I don't know where you have had it stored, but I do know that rats, birds and other critters like to make nests in weird places...
You never know.
I would suggest you get a compression tester. You are not really looking for how much(after all, it is a 30 year old engine), but comparing the difference between cylinders. A large difference would make me look towards bad intake gaskets or a cracked manifold.
But before you do that, unhook your exhaust and see what she does. I know you said it is leaking, but go ahead and open her all the way. Since it has been sitting for a good while the exhaust could be clogged up. I don't know where you have had it stored, but I do know that rats, birds and other critters like to make nests in weird places...
You never know.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Originally Posted by nitronick
Well 9, 10 or 11, it should be higher than that. You should be seeing 17 to 21 inches of vacuum.
I would suggest you get a compression tester. You are not really looking for how much(after all, it is a 30 year old engine), but comparing the difference between cylinders. A large difference would make me look towards bad intake gaskets or a cracked manifold.
But before you do that, unhook your exhaust and see what she does. I know you said it is leaking, but go ahead and open her all the way. Since it has been sitting for a good while the exhaust could be clogged up. I don't know where you have had it stored, but I do know that rats, birds and other critters like to make nests in weird places...
You never know.
I would suggest you get a compression tester. You are not really looking for how much(after all, it is a 30 year old engine), but comparing the difference between cylinders. A large difference would make me look towards bad intake gaskets or a cracked manifold.
But before you do that, unhook your exhaust and see what she does. I know you said it is leaking, but go ahead and open her all the way. Since it has been sitting for a good while the exhaust could be clogged up. I don't know where you have had it stored, but I do know that rats, birds and other critters like to make nests in weird places...
You never know.
You should be seeing greater vacuum, unless you have a big cam. Either have a leak or internal engine problems. A leak down test would be better than a compression test.
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Originally Posted by glen242
How would a bad intake gasket or cracked (intake) manifold cause a bad compression reading? Compression measures how well a cylinder (closed) holds pressure. No direct link to the intake when the valves are closed.
You should be seeing greater vacuum, unless you have a big cam. Either have a leak or internal engine problems. A leak down test would be better than a compression test.
You should be seeing greater vacuum, unless you have a big cam. Either have a leak or internal engine problems. A leak down test would be better than a compression test.
I meant a leakdown test...
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
nitronick
I would suggest you get a compression tester
but comparing the difference between cylinders
Are you refering to doing a compression test on each cylinder?
unhook your exhaust and see what she does
I would imagine you're referring to unhooking it at the collector nick?
I don't know where you have had it stored
In my garage, but I know that rats and such can live almost anywhere.
glen242
A leak down test would be better than a compression test.
Sorry glen but I'm confused, what is a leak down test?
Just to recap, I replaced;
Cap, rotor, wires, pcv,rebuilt carb (rochester quadrajet), fuel filter, fresh gas.
And today I replaced:
several vacuum lines, new ac R43ts plugs with no change.
Thanks for sticking with me guys I really appreciate it.
Frank
I would suggest you get a compression tester
but comparing the difference between cylinders
Are you refering to doing a compression test on each cylinder?
unhook your exhaust and see what she does
I would imagine you're referring to unhooking it at the collector nick?
I don't know where you have had it stored
In my garage, but I know that rats and such can live almost anywhere.
glen242
A leak down test would be better than a compression test.
Sorry glen but I'm confused, what is a leak down test?
Just to recap, I replaced;
Cap, rotor, wires, pcv,rebuilt carb (rochester quadrajet), fuel filter, fresh gas.
And today I replaced:
several vacuum lines, new ac R43ts plugs with no change.
Thanks for sticking with me guys I really appreciate it.
Frank
Last edited by Maymyvetteliveforevr; Aug 6, 2005 at 10:15 PM.
Racer
Joined: Feb 2001
Posts: 405
Likes: 15
From: Minneapolis
I haven't read ALL of the replies, but it appears that you still have the problem. For whatever it's worth, when I rebuilt by motor I had a similar problem. Wouldn't idle below1500 rpm. After disconnected all possible vacuum sources, I took a can of starting fluid and sprayed it around the base of the carb, as well as the joint between the heads and manifold. Engine smoothed right out when I got to the rear of the manifold. After I tightend it down real tight, the problem was solved.
If you haven't tried that one, do it now. The propane trick didn't work in my case. Don't know why, but the ether in the starting fluid did the trick.
I feel for you - don't give up. There IS a cause behind every problem. You just have to be patient and methodical.
If you haven't tried that one, do it now. The propane trick didn't work in my case. Don't know why, but the ether in the starting fluid did the trick.
I feel for you - don't give up. There IS a cause behind every problem. You just have to be patient and methodical.
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
if you cap all the vacuum sources from the manifold, and you are sure it is not the base gasket, it has to be the intake manifold or gasket leaking, try a new intake gasket it is only about $15. process of elimination time
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Go here to read up on the leakdown test.
If it was me I would do both compression and leakdown. And yes, on each cylinder. If it is a crack or some other problem, you will know which cylinder it is after performing the tests.
And yeah, unhook your exhaust at the collector. A clogged exhaust will keep the engine from breathing and could cause a dip in vacuum readings.
If it was me I would do both compression and leakdown. And yes, on each cylinder. If it is a crack or some other problem, you will know which cylinder it is after performing the tests.
And yeah, unhook your exhaust at the collector. A clogged exhaust will keep the engine from breathing and could cause a dip in vacuum readings.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
7T9SharkI took a can of starting fluid and sprayed it around the base of the carb, as well as the joint between the heads and manifold
I'll consider doing that as soon as I can get out and get me some.
gator79
if you cap all the vacuum sources from the manifold,
I just did that as you can see from the photos below and I got the idle down to about 950 rpm with the mixture screws set at around 2 1/2 turns out each. I changed the timing by feel, which helped a bit but now it touches the intake. The problem I have now is that the engine surges from about 950 to 1400 rpm. it constantly runs back and forth between the 2 ranges. If i got it to stop surging I may be able to get it down alittle more.
nitronick
Go here to read up on the leakdown test
[IMG]
I'm about to read that right after I finish this point.
Guys I'm getting there.
I know I know that pictures are to small..........crap[IMG]http://pic4.picturetrail.com/VOL733/3792904/inbox/t-
http://pic4.picturetrail.com/VOL733/3792904/inbox/t-57135.jpg[/IMG]
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Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Maymyvetteliveforevr
I changed the timing by feel, which helped a bit but now it touches the intake
I just checked the timing, I don't have one of those adjustable timming light so I figured it out to be around 24 degrees BTDC at approximately 950 rpm just before the surge, with the vacuum line disconnected from the vaccum advance on the disrtibutor.
I changed the timing by feel, which helped a bit but now it touches the intake
I just checked the timing, I don't have one of those adjustable timming light so I figured it out to be around 24 degrees BTDC at approximately 950 rpm just before the surge, with the vacuum line disconnected from the vaccum advance on the disrtibutor.
Race Director
Joined: Oct 2002
Posts: 12,065
Likes: 2
From: Smalltownville Michigan
Cruise-In 5, 6 & 10 Veteran
St. Jude Donor '09
i know that i may come off as a dumazz by asking this but, what about that open port on the front of the carb? have you tried plugging that to see if it makes a difference? i'm no carb genius by any stretch of the imagination so i have no clue if that is vacuum related or not.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Aflac
what about that open port on the front of the carb
I found there to be no vacuum on that port therefore I never plugged it. When I plugged it with my finger there was no difference.
gator79
it looks like the brake master cylinder line is still hooked up, it may just be the picture angle. good luck
I ended up plugging it just now with no difference.
New progress:
I just came back with a couple of cans of ether and after spraying around the intake I noticed a conciderable increase in rpm around the rear of the carb just below the fitting where the line comes out for the brake booster. It's not the fitting since I just plugged it up. I think it's either the new base plate gasket or the section of the carb between the lower part of the quadrajet and the middle part.
How do you know if you have the proper base plate gasket for your quadrajet?
Thanks everyone.
what about that open port on the front of the carb
I found there to be no vacuum on that port therefore I never plugged it. When I plugged it with my finger there was no difference.
gator79
it looks like the brake master cylinder line is still hooked up, it may just be the picture angle. good luck
I ended up plugging it just now with no difference.
New progress:
I just came back with a couple of cans of ether and after spraying around the intake I noticed a conciderable increase in rpm around the rear of the carb just below the fitting where the line comes out for the brake booster. It's not the fitting since I just plugged it up. I think it's either the new base plate gasket or the section of the carb between the lower part of the quadrajet and the middle part.
How do you know if you have the proper base plate gasket for your quadrajet?
Thanks everyone.
Le Mans Master
Joined: Jan 2003
Posts: 6,575
Likes: 1
From: Tuttle OK
Cruise-In VIII Veteran
I didn't see it mentioned, so I will...Your quad looks like an older one and they can be a bit notorious for leaking at the base gasket due to the base of the carb being warped from overtightening...might be something to check, take the carb off and put a straight edge along the base...it's one of those quick and easy elimination things...
hey aflac...
hey aflac...
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
I do notice that your carburetor is a lean burn type, which would make it around a 78 or 79 model. I don't think those came out on Corvettes!
It would be nice if you could give me the carb number. I would also like to see a pic of the carb off of the manifold, a pic of the carb mounting area of the manifold and maybe even a pic of the mounting gasket(carb side).
Who built this carb? Was it actually remanufactured or did someone just put a kit in it. Big difference there. Also, it might be of help to know the kit number used, if that is the case.
Don't worry bud, we'll get you going!
It would be nice if you could give me the carb number. I would also like to see a pic of the carb off of the manifold, a pic of the carb mounting area of the manifold and maybe even a pic of the mounting gasket(carb side).
Who built this carb? Was it actually remanufactured or did someone just put a kit in it. Big difference there. Also, it might be of help to know the kit number used, if that is the case.
Don't worry bud, we'll get you going!
Last edited by nitronick; Aug 9, 2005 at 12:18 AM.
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Originally Posted by Maymyvetteliveforevr
what about that open port on the front of the carb
I found there to be no vacuum on that port therefore I never plugged it. When I plugged it with my finger there was no difference.
I found there to be no vacuum on that port therefore I never plugged it. When I plugged it with my finger there was no difference.
New progress:
I just came back with a couple of cans of ether and after spraying around the intake I noticed a conciderable increase in rpm around the rear of the carb just below the fitting where the line comes out for the brake booster. It's not the fitting since I just plugged it up. I think it's either the new base plate gasket or the section of the carb between the lower part of the quadrajet and the middle part.
How do you know if you have the proper base plate gasket for your quadrajet?
I just came back with a couple of cans of ether and after spraying around the intake I noticed a conciderable increase in rpm around the rear of the carb just below the fitting where the line comes out for the brake booster. It's not the fitting since I just plugged it up. I think it's either the new base plate gasket or the section of the carb between the lower part of the quadrajet and the middle part.
How do you know if you have the proper base plate gasket for your quadrajet?
Last edited by nitronick; Aug 9, 2005 at 12:18 AM.