Massive Vacuum Leak
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Massive Vacuum Leak
I'm at my wit's end 
with this problem and I don't know where to turn. You may have read my previous posts regarding my '75 L48 but nothing has changed since. I need help!!!
PROBLEM: Car won't idle below 1200 rpm. Hand over the primary (carb) idles approx. 700 rpm more.
WHAT I'VE DONE: Installed rebuilt quadrajet carb, new plugs, wires, cap, rotor, fuel filter, PCV, pinched every possible vacuum line on intake, sprayed water and/or brake cleaner all over intake and vacuum lines to see if there was a change in RPM, plugged vacuum lines to headlights.
OVERALL RESULT---NO CHANGE. Present vacuum pressure is 9-10 lbs. Timing has been set. I'm losing hope and have all but given up. HELP!!!
with this problem and I don't know where to turn. You may have read my previous posts regarding my '75 L48 but nothing has changed since. I need help!!!PROBLEM: Car won't idle below 1200 rpm. Hand over the primary (carb) idles approx. 700 rpm more.
WHAT I'VE DONE: Installed rebuilt quadrajet carb, new plugs, wires, cap, rotor, fuel filter, PCV, pinched every possible vacuum line on intake, sprayed water and/or brake cleaner all over intake and vacuum lines to see if there was a change in RPM, plugged vacuum lines to headlights.
OVERALL RESULT---NO CHANGE. Present vacuum pressure is 9-10 lbs. Timing has been set. I'm losing hope and have all but given up. HELP!!!
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
If you back the idle screw off does the engine die or drop or what?
Not the mixture screws the one on the accelerator.
What is your timing at 1200 and what is it below 1200 if its still running.
Not the mixture screws the one on the accelerator.
What is your timing at 1200 and what is it below 1200 if its still running.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Believe me when I say that I really appreciate all your input since this car is driving me to drink. I'll answer all your questions one by one:
Yes, the engine stalls when the idle screw is set to low and the mixture screws have been set evenly from 1 1/2 turns out as far as 4 full turns out.
The choke is not stuck open or closed. This is the 2nd rebuilt carb on the car and both had new base plate gaskets which came with the rebuild kits.
Timming is set at 12 degrees BTDC with the vacuum advance plugged and no I can't set it below 1,200 rpm since it stalls.
The brake booster was disconnected and plugged up with no change in rpm's.
I'm not using the metal base plate gasket and I'm not sure what you meant by the heat riser slots on the front.
How can I tell if I have the correct base plate gasket?
Please keep the suggestions coming since I'm at my wit's end and am willing to try all.
Yes, the engine stalls when the idle screw is set to low and the mixture screws have been set evenly from 1 1/2 turns out as far as 4 full turns out.
The choke is not stuck open or closed. This is the 2nd rebuilt carb on the car and both had new base plate gaskets which came with the rebuild kits.
Timming is set at 12 degrees BTDC with the vacuum advance plugged and no I can't set it below 1,200 rpm since it stalls.
The brake booster was disconnected and plugged up with no change in rpm's.
I'm not using the metal base plate gasket and I'm not sure what you meant by the heat riser slots on the front.
How can I tell if I have the correct base plate gasket?
Please keep the suggestions coming since I'm at my wit's end and am willing to try all.
Last edited by Maymyvetteliveforevr; Aug 3, 2005 at 06:24 PM.
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Racer
Joined: Mar 2005
Posts: 306
Likes: 0
From: WAY SOUTH OF HEAVEN inebriated
Still here for a moment but when i pulled the carb of my 350 the gasket was shot had a nice passage for air to be sucked through clearly seen by the deteriorated gasket and the marks on the intake flange.try a new gasket that should solve the problem,if not sell it! just my two cents see ya on the flip side.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
This is the 3rd new base plate gasket on the car in the last several months 1980 454. As for selling it, it's part of my past, to many memories, especially since I'm the original owner.
Frank
Frank
Instructor
Joined: Nov 2002
Posts: 228
Likes: 0
If you have a stock manifold there is a slot with two holes one on each of the front primarys. I have seen gaskets that don't cover these holes . I lay the gasket on the manifold to ensure coverage. The matel gasket is to keep the hot exhaust from coming in contact with the fiber gasket .
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
A 75 model Chevrolet QuadraJet takes either one of the following mounting gaskets:
or
There are no difference between the two as far as sealing. Just different manufacturers. It may be marked top, but that won't matter as long as you don't have it reversed front to back(Hey, I have seen it happen!)
Have a look and make sure you don't have a wire or vacuum hose pinched under the carb. It is very easy for something to slip under unnoticed while installing one of these, and most definitly will cause a vacuum leak.
The stainless steel heat shield that was mentioned was used on earlier models (late 60s) and are not required on your intake, assuming everything is stock 1975.
or
There are no difference between the two as far as sealing. Just different manufacturers. It may be marked top, but that won't matter as long as you don't have it reversed front to back(Hey, I have seen it happen!
) Have a look and make sure you don't have a wire or vacuum hose pinched under the carb. It is very easy for something to slip under unnoticed while installing one of these, and most definitly will cause a vacuum leak.
The stainless steel heat shield that was mentioned was used on earlier models (late 60s) and are not required on your intake, assuming everything is stock 1975.
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Originally Posted by Perfomer
If you have a stock manifold there is a slot with two holes one on each of the front primarys. I have seen gaskets that don't cover these holes . I lay the gasket on the manifold to ensure coverage. The matel gasket is to keep the hot exhaust from coming in contact with the fiber gasket .
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
I had a similar problem but the engine was new. Turned out the weights flew out at such a low RPM that adjusting the idle pulled the weights in, retarded the timing. There was a huge dead spot where the engine wouldn't run. Just like you got.
After it broke in the problem went away but its worth checking if your mechanical timing is all in about this rpm point.
If this is the problem it also means your timing is wrong. Your static timing would probably be retarded.
Rev it up and see if your getting any mechanical timing. If you are then this isn't the problem.
If you aren't then you need more static timing to get it to idle without mechanical timing.
What normally would be static timing is for you total timing. If this is the prob.
After it broke in the problem went away but its worth checking if your mechanical timing is all in about this rpm point.
If this is the problem it also means your timing is wrong. Your static timing would probably be retarded.
Rev it up and see if your getting any mechanical timing. If you are then this isn't the problem.
If you aren't then you need more static timing to get it to idle without mechanical timing.
What normally would be static timing is for you total timing. If this is the prob.
Last edited by Techno; Aug 3, 2005 at 10:25 PM.
Burning Brakes
Joined: Feb 2005
Posts: 1,235
Likes: 0
From: Mesa AZ
i had the same problem on my 75 l48. check if there are big gaps where the butterflies meet the bores and check if there is play on the shaft that controls them. those little gaps can let a lot of air through increasing the idle.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Again thanks for the suggestions and the replies are as follows:
Yes it is the stock manifold.
I am using the upper base plate gasket as shown.
I tried changing the timing and it still won't idle.
I checked the vacuum advance and it works.
The springs are installed on the mechanical advance.
I pulled the EGR and it's clean without debri, gasket is ok.
I tried 2 different carbs, where the butterflies meet the bores and the play on the shaft are good.
If I've missed anything please let me know and Please! keep the suggestions comming.
Thanks
Frank
Yes it is the stock manifold.
I am using the upper base plate gasket as shown.
I tried changing the timing and it still won't idle.
I checked the vacuum advance and it works.
The springs are installed on the mechanical advance.
I pulled the EGR and it's clean without debri, gasket is ok.
I tried 2 different carbs, where the butterflies meet the bores and the play on the shaft are good.
If I've missed anything please let me know and Please! keep the suggestions comming.
Thanks
Frank
Race Director
Joined: Dec 2004
Posts: 15,012
Likes: 2
Hand over the primary (carb) idles approx. 700 rpm more.
You just have to get out and find the leak.
Thread Starter
Safety Car
Joined: Mar 2005
Posts: 4,255
Likes: 82
From: Brampton Ont.
Originally Posted by nitronick
This right here tells you there's a vacuum leak. You have changed everything else, including two carbs, and still have the same problem.
You just have to get out and find the leak.
You just have to get out and find the leak.
Exactly......now how do I ( get out and find the leak )? That's my problem nitronick, how do I find it??????
It's so severe that it should be so obvious and easy to locate!
HELP!
Frank
Melting Slicks
Joined: Sep 2004
Posts: 2,655
Likes: 19
Have you tries a small propane cylinder with a hose on the end. Turn on the propane and start pushing the hose around possible leakage points.
Cap off the brake booster & headlight vacuum system first.
Also check your vacuum advance canister holds vacuum with a hand pump.
Gary
Cap off the brake booster & headlight vacuum system first.
Also check your vacuum advance canister holds vacuum with a hand pump.
Gary