Vacuum Leak
Thread Starter
Drifting
Joined: Feb 2005
Posts: 1,583
Likes: 5
From: Richardson Tx.
Vacuum Leak
When Lars worked on my Carb it was able to idle at a nice 700 rpm in his shop. When I install it on my car I cannot get it to idle below 1100 rpm. He also set up a new electric choke too - so the Carb seems unlikely as the leak source.
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
Advanced
Joined: Dec 2013
Posts: 52
Likes: 2
From: Brunswick OH...IO!
When Lars worked on my Carb it was able to idle at a nice 700 rpm in his shop. When I install it on my car I cannot get it to idle below 1100 rpm. He also set up a new electric choke too - so the Carb seems unlikely as the leak source.
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
Instructor
Joined: Apr 2016
Posts: 104
Likes: 7
From: Denver Colorado
When Lars worked on my Carb it was able to idle at a nice 700 rpm in his shop. When I install it on my car I cannot get it to idle below 1100 rpm. He also set up a new electric choke too - so the Carb seems unlikely as the leak source.
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
I capped the "T" coming out of the manifold for vacuum - no change in rpm
I put a vacuum pump on the vacuum advance can at the distributor - it holds vacuum steady.
I capped the metal vacuum line coming out of the Carb - no change.
All of the unused ports on the carb are capped.
I replaced the Carb mounting gasket. Bolts are lightly snug.
I sprayed carb cleaner around the intake and base of the carb- no change
I took a hose and tried to listen for a hiss around the intake.
I'm at a loss - any other suggestions on what to check?
I just solved a vacuum leak/fast idle problem on an '82 and it turned out to be a leaking rod seal on the right headlight actuator. Another possibility would be distributor flyweight springs that are too light and allowing some mechanical advance to come in when it's not needed.
Thread Starter
Drifting
Joined: Feb 2005
Posts: 1,583
Likes: 5
From: Richardson Tx.
Melting Slicks
Joined: Dec 2009
Posts: 3,229
Likes: 656
From: Thunder Bay
The carb is not going to perform the same on your engine. Timing alone would make the two engines idle completely different. You need to set up the carb for your application.
Advanced
Joined: Dec 2013
Posts: 52
Likes: 2
From: Brunswick OH...IO!
Don't jack with the carb! I've read here and other places most idle and performance issues are ignition related.
Lars set that carb up on a mule engine which is what these carb guys do. You mess with one adjustment it can throw off other adjustments... Call the guy that worked on it!
Good luck
Lars set that carb up on a mule engine which is what these carb guys do. You mess with one adjustment it can throw off other adjustments... Call the guy that worked on it!
Good luck
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Melting Slicks
Joined: Apr 2005
Posts: 2,777
Likes: 105
You need to adjust the carb on your engine.
Don't be afraid to adjust it and see if you can get the idle lower. Also you can tweak the mixture screws as well to dial it in. No 2 engines are alike.
Don't be afraid to adjust it and see if you can get the idle lower. Also you can tweak the mixture screws as well to dial it in. No 2 engines are alike.
Last edited by babbah; Apr 15, 2016 at 01:50 AM.
Racer
Joined: Apr 2011
Posts: 471
Likes: 3
you can record Lars setting on the carb and revert back the setup if playing around doesn't change the situation...
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Do you have power brakes? Have you removed (and blocked off) the vacuum feed to the booster?
In general, you need to disconnect/block-off ALL vacuum sources from the carb and manifold (except for distributor vacuum advance, which you have already checked out). Then, see how it idles. If different, then you need to chase down where your vacuum leak is located.
P.S. Did you install the gasket/spacer under the carb as recommended by Lars? Sometimes these Q-Jets require specific pieces so that manifold vacuum isn't lost. Check back with Lars, if not sure.
Any possibility that the intake manifold gasket[s] is leaking (sucking in excess air)?
In general, you need to disconnect/block-off ALL vacuum sources from the carb and manifold (except for distributor vacuum advance, which you have already checked out). Then, see how it idles. If different, then you need to chase down where your vacuum leak is located.
P.S. Did you install the gasket/spacer under the carb as recommended by Lars? Sometimes these Q-Jets require specific pieces so that manifold vacuum isn't lost. Check back with Lars, if not sure.
Any possibility that the intake manifold gasket[s] is leaking (sucking in excess air)?
Last edited by 7T1vette; Apr 15, 2016 at 08:26 AM.
Thread Starter
Drifting
Joined: Feb 2005
Posts: 1,583
Likes: 5
From: Richardson Tx.
Do you have power brakes? Have you removed (and blocked off) the vacuum feed to the booster?
In general, you need to disconnect/block-off ALL vacuum sources from the carb and manifold (except for distributor vacuum advance, which you have already checked out). Then, see how it idles. If different, then you need to chase down where your vacuum leak is located.
P.S. Did you install the gasket/spacer under the carb as recommended by Lars? Sometimes these Q-Jets require specific pieces so that manifold vacuum isn't lost. Check back with Lars, if not sure.
Any possibility that the intake manifold gasket[s] is leaking (sucking in excess air)?
In general, you need to disconnect/block-off ALL vacuum sources from the carb and manifold (except for distributor vacuum advance, which you have already checked out). Then, see how it idles. If different, then you need to chase down where your vacuum leak is located.
P.S. Did you install the gasket/spacer under the carb as recommended by Lars? Sometimes these Q-Jets require specific pieces so that manifold vacuum isn't lost. Check back with Lars, if not sure.
Any possibility that the intake manifold gasket[s] is leaking (sucking in excess air)?
I have sprayed carb cleaner on the intake gasket areas - I didn't notice a change.
there aren't many other things left to check
Drifting
Joined: May 2012
Posts: 1,353
Likes: 44
From: Calgary Alberta
Don't waste your time on another thing until you adjust your idle speed screw.
It's a carb, not a spaceship. Don't be afraid to tune it a little. All you need to do is back out the idle screw a half a turn and see what happens. If you don't like what happens, turn it back in. If it lowers the speed, which it likely will, turn it another half turn, and so on.
If you email Lars, he'll likely tell you the same thing.
It's a carb, not a spaceship. Don't be afraid to tune it a little. All you need to do is back out the idle screw a half a turn and see what happens. If you don't like what happens, turn it back in. If it lowers the speed, which it likely will, turn it another half turn, and so on.
If you email Lars, he'll likely tell you the same thing.
Safety Car
Joined: Oct 2007
Posts: 4,364
Likes: 1,724
From: Seattle Area Washington
You now have an electric choke. Did the metal line from the hot air choke housing get capped off? After car has reached normal operating temp unhook throttle linkage, manually give the linkage on carb a short pull, let linkage on carb snap back to its home position restart engine and see if idle RPM changes.
Thread Starter
Drifting
Joined: Feb 2005
Posts: 1,583
Likes: 5
From: Richardson Tx.
Not trying to start a war, but I did email Lars.
His response was that there should not be more than a 50 rpm difference between my carb on his test engine and on my engine.
On his engine:
Cold-start fired right up at 1250 rpm. Choke opened far enough after 4 minutes to drop the fast idle off the fast idle screw, putting the carb on the hot idle screw control at 750 rpm initially. After 5 minutes, the choke was fully open. After 6 minutes, the secondary lockout lever was fully retracted and the carb was hot-stabilized at 840 rpm hot idle.
On my engine:
1600 rpm cold start
1100 rpm hot idle
Question: Would adjusting the idle screw be masking a vacuum leak problem ?
His response was that there should not be more than a 50 rpm difference between my carb on his test engine and on my engine.
On his engine:
Cold-start fired right up at 1250 rpm. Choke opened far enough after 4 minutes to drop the fast idle off the fast idle screw, putting the carb on the hot idle screw control at 750 rpm initially. After 5 minutes, the choke was fully open. After 6 minutes, the secondary lockout lever was fully retracted and the carb was hot-stabilized at 840 rpm hot idle.
On my engine:
1600 rpm cold start
1100 rpm hot idle
Question: Would adjusting the idle screw be masking a vacuum leak problem ?
Last edited by 79vetter; Apr 15, 2016 at 10:27 AM.
Race Director
Joined: May 2006
Posts: 18,690
Likes: 2,572
If you have a vacuum leak you will experience other symptoms than just a high idle, hesitation etc. Note the position of the idle speed screw and than start turning it counter clockwise and your problem will go away. This is basic adjustment of a carburetor.
Melting Slicks
Joined: Feb 2006
Posts: 3,105
Likes: 96
From: Sunrise FL
Did you check throttle linkage to see if its holding the throttle blades open?
Disconnect it, see what happens.
Another thing is the PCV HOOKED UP? This is a big "leak" and will affect idle speed!
I agree with Lars, the idle speed should be close. But it is easy to reset it!
Disconnect it, see what happens.
Another thing is the PCV HOOKED UP? This is a big "leak" and will affect idle speed!
I agree with Lars, the idle speed should be close. But it is easy to reset it!
Last edited by mikem350; Apr 15, 2016 at 12:56 PM.
Advanced
Joined: Dec 2013
Posts: 52
Likes: 2
From: Brunswick OH...IO!
Not trying to start a war, but I did email Lars.
His response was that there should not be more than a 50 rpm difference between my carb on his test engine and on my engine.
On his engine:
Cold-start fired right up at 1250 rpm. Choke opened far enough after 4 minutes to drop the fast idle off the fast idle screw, putting the carb on the hot idle screw control at 750 rpm initially. After 5 minutes, the choke was fully open. After 6 minutes, the secondary lockout lever was fully retracted and the carb was hot-stabilized at 840 rpm hot idle.
On my engine:
1600 rpm cold start
1100 rpm hot idle
Question: Would adjusting the idle screw be masking a vacuum leak problem ?
His response was that there should not be more than a 50 rpm difference between my carb on his test engine and on my engine.
On his engine:
Cold-start fired right up at 1250 rpm. Choke opened far enough after 4 minutes to drop the fast idle off the fast idle screw, putting the carb on the hot idle screw control at 750 rpm initially. After 5 minutes, the choke was fully open. After 6 minutes, the secondary lockout lever was fully retracted and the carb was hot-stabilized at 840 rpm hot idle.
On my engine:
1600 rpm cold start
1100 rpm hot idle
Question: Would adjusting the idle screw be masking a vacuum leak problem ?
Lars has a slick way to set it.