Carb help please...
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
Carb help please...
Hi all,
So i have a quick question, i noticed the other day that on my stock 76 L48 the secondaries never open! You can push them open when the engine is off but as soon as its on the vacuum wont allow you to open them manually and they wont open on their own if you just open the throttle.
I found this article on the net http://www.carburetor.ca/carbs/tech/...jet-index.html
And to shorten things thats what it says
"For example, on a mild 350 Chevy the air valve will never open all the way - because the motor can't use more than 600 cfm or so."
Is that true and is my carb acting normally?
Cheers
So i have a quick question, i noticed the other day that on my stock 76 L48 the secondaries never open! You can push them open when the engine is off but as soon as its on the vacuum wont allow you to open them manually and they wont open on their own if you just open the throttle.
I found this article on the net http://www.carburetor.ca/carbs/tech/...jet-index.html
And to shorten things thats what it says
"For example, on a mild 350 Chevy the air valve will never open all the way - because the motor can't use more than 600 cfm or so."
Is that true and is my carb acting normally?
Cheers
Last edited by Carlos840; Jan 13, 2010 at 08:37 AM.
Le Mans Master
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From: Butler Pa
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
Since i never managed to see the secondaries open im wondering if they are...
Team Owner
Joined: Jul 2008
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
I'll admit that what follows is on the outer edges of my understanding of carburetors, and I haven't owned a car with vacuum secondaries in a couple of decades, so between my bad memory and lack of expertise, I hope that if I really dork something up, someone more knowledgable will correct me.
That said, vacuum is inversely related to load, as in, when the load is low on an engine, the vacuum is high, and when the load is high, the vacuum is low. Given that, if I understand this correctly, you you want your vacuum secondaries to open when the vacuum is low, because that is when the load is high and you want to get more gas into your motor. So, when you are in your garage and open the throttle, your vacuum will go down somewhat (since you opened the throttle plate), but the engine will be under essentially zero load, so the vacuum should still be high enough to keep your secondaries closed.
If I completely missed the boat here, just ignore the muttering man in the corner....
That said, vacuum is inversely related to load, as in, when the load is low on an engine, the vacuum is high, and when the load is high, the vacuum is low. Given that, if I understand this correctly, you you want your vacuum secondaries to open when the vacuum is low, because that is when the load is high and you want to get more gas into your motor. So, when you are in your garage and open the throttle, your vacuum will go down somewhat (since you opened the throttle plate), but the engine will be under essentially zero load, so the vacuum should still be high enough to keep your secondaries closed.
If I completely missed the boat here, just ignore the muttering man in the corner....
Race Director
Joined: Jun 2000
Posts: 10,037
Likes: 668
From: FL
Rememebr, the air valve on top of the carb are not the secondary throttle blades. The secondary throttle blades are at the bottom the carb and are mechanically operated. You can see the lockout down there. To check operation of the lockout:
1) Have helper floor gas with engine off to set choke. Floor again to see see if lockout is engaged.
2) Start engine and let fully warm up. Make sure choke is warmed up and disengaged.
3) Engine off. (very important)
4) Floor gas again and see if lockout is disengaged. You can manually open the air valve and look down the secondary bores to see if the throttle blades open.
1) Have helper floor gas with engine off to set choke. Floor again to see see if lockout is engaged.
2) Start engine and let fully warm up. Make sure choke is warmed up and disengaged.
3) Engine off. (very important)
4) Floor gas again and see if lockout is disengaged. You can manually open the air valve and look down the secondary bores to see if the throttle blades open.
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
I'll admit that what follows is on the outer edges of my understanding of carburetors, and I haven't owned a car with vacuum secondaries in a couple of decades, so between my bad memory and lack of expertise, I hope that if I really dork something up, someone more knowledgable will correct me.
That said, vacuum is inversely related to load, as in, when the load is low on an engine, the vacuum is high, and when the load is high, the vacuum is low. Given that, if I understand this correctly, you you want your vacuum secondaries to open when the vacuum is low, because that is when the load is high and you want to get more gas into your motor. So, when you are in your garage and open the throttle, your vacuum will go down somewhat (since you opened the throttle plate), but the engine will be under essentially zero load, so the vacuum should still be high enough to keep your secondaries closed.
If I completely missed the boat here, just ignore the muttering man in the corner....
That said, vacuum is inversely related to load, as in, when the load is low on an engine, the vacuum is high, and when the load is high, the vacuum is low. Given that, if I understand this correctly, you you want your vacuum secondaries to open when the vacuum is low, because that is when the load is high and you want to get more gas into your motor. So, when you are in your garage and open the throttle, your vacuum will go down somewhat (since you opened the throttle plate), but the engine will be under essentially zero load, so the vacuum should still be high enough to keep your secondaries closed.
If I completely missed the boat here, just ignore the muttering man in the corner....
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Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
Rememebr, the air valve on top of the carb are not the secondary throttle blades. The secondary throttle blades are at the bottom the carb and are mechanically operated. You can see the lockout down there. To check operation of the lockout:
1) Have helper floor gas with engine off to set choke. Floor again to see see if lockout is engaged.
2) Start engine and let fully warm up. Make sure choke is warmed up and disengaged.
3) Engine off. (very important)
4) Floor gas again and see if lockout is disengaged. You can manually open the air valve and look down the secondary bores to see if the throttle blades open.
1) Have helper floor gas with engine off to set choke. Floor again to see see if lockout is engaged.
2) Start engine and let fully warm up. Make sure choke is warmed up and disengaged.
3) Engine off. (very important)
4) Floor gas again and see if lockout is disengaged. You can manually open the air valve and look down the secondary bores to see if the throttle blades open.
thanks
Drifting
Joined: Apr 2008
Posts: 1,565
Likes: 2
From: Sparta Mo.
No you're kinda right. The secondaries open under zero vacuum. (or close to it) If the car is warmed up and idling, and you dump the throttle wide open all at once, you should see the secondaries start to open. If we're talking about a quadrajet, (and I assume we are) you should also hear thwwuuuuaaaaa sound bellowing from the top of the engine. ......Very unmistakable.
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
No you're kinda right. The secondaries open under zero vacuum. (or close to it) If the car is warmed up and idling, and you dump the throttle wide open all at once, you should see the secondaries start to open. If we're talking about a quadrajet, (and I assume we are) you should also hear thwwuuuuaaaaa sound bellowing from the top of the engine. ......Very unmistakable.
I was just worried cause i never saw the air valve on top of the secondaries move and i thought it should...
I can open it with engine off but it wont move with engine on even if i floor the throttle!
Last edited by Carlos840; Jan 13, 2010 at 10:37 AM.
Drifting
Joined: Apr 2008
Posts: 1,565
Likes: 2
From: Sparta Mo.
You're probably ok then. Just not on it long enough to make the top plates open. They have a delayed reaction to the bottom plates. If you rev it to 4500 from a dead idle, the top plates may only move a quarter to half an inch.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Once the choke lockout lever is released, and the secondary throttle valves are mechanically open, then this creates a low pressure area under the secondary air valve (engine manifold vacuum) and the higher atmospheric pressure above the valve forces the air valve to open, raise the rods and feed extra air and fuel to the intake against the forces of the air valve spring and dashpot.
The nechanical throttle valves are just a part of the process, without the manifold vacuum, no extra air or fuel is realized.
The nechanical throttle valves are just a part of the process, without the manifold vacuum, no extra air or fuel is realized.
Thread Starter
Racer
Joined: Sep 2009
Posts: 310
Likes: 0
From: London
Cheers