tri-power issues 1968 corvette
Thread Starter
2nd Gear
Joined: Oct 2007
Posts: 2
Likes: 0
tri-power issues 1968 corvette
I am wanting to rejet my two outer carbs but I am not sure where the jets are. I know the center carb has a metering block with the jets in it, but how about the secondary carbs? I understand they have a vacum diaphragm but I don't beleive I am getting the performance I should have.
c3 owner
c3 owner
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
If you are not sure that the 2ndarie carbs open up at high power demand, you should 1st assure this B4 attempting to rejet the carbs.
You could install some kind of breakaway thing ( like paper or tape ) to the 2ndary carb linkage and do a high power test drive. If it is torn, the carbs open.
Or you disconnect the 2ndary carb linkage, do a high power test drive, re,member the feeling. Reconnect, do the same test drive and if there is a hell of a difference, they open and work.
Only now, after assuring they open ( not having a damaged vacuum pod ) , you can look for other problems, like rejetting or ?????
Günther
If you are not sure that the 2ndarie carbs open up at high power demand, you should 1st assure this B4 attempting to rejet the carbs.
You could install some kind of breakaway thing ( like paper or tape ) to the 2ndary carb linkage and do a high power test drive. If it is torn, the carbs open.
Or you disconnect the 2ndary carb linkage, do a high power test drive, re,member the feeling. Reconnect, do the same test drive and if there is a hell of a difference, they open and work.
Only now, after assuring they open ( not having a damaged vacuum pod ) , you can look for other problems, like rejetting or ?????
Günther
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Once everything is proven in working order, change out the secondary diaphragm springs to weaker and weaker ones until you bog (under load), then go back to the last previous set. Go one step at a time, always keeping both springs the same rating. I've done this on several tri-power setups, and have never found the original springs to deliver the best performance.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Sorry, I've found a half dozen 2300 renew kits, but not a vacuum secondary diaphragm spring kit. If I have any left, they must be buried in storage. There's only one listed on Holley's site (# 20-13), so you might want to verify it works with the 3x2's before ordering.
Burning Brakes
Joined: Jul 1999
Posts: 922
Likes: 164
From: Hudson NH
Why do you feel you need to re-jet the outer carburetors? The design of these carbs is basically the secondary side of a 4160 style carb. They have a metering plate with fixed main (jet) and idle orifices. You would need to change the secondary metering plate. If you decide to go this route, select a metering plate with similar sized idle feed passages so as not to disrupt your idle mixture. Although not externally adjustable, the outer carbs do have an idle circuit.
A second option is to simply increase the main (jet) orifice in your existing metering plate using a pin vise. Increase only in very small increments keeping in mind it is the total area and not the diameter that is critical. The area increases as the square of the diameter:
Area = (Radius)^2 X pi or (pi X (Dia)^2) / 4
As correctly posted above, you should first make sure the end carburetors are fully opening, and open as fast as possible without causing a bog or hesitation. While you are checking the opening rate of the outer carburetors with different springs, you’ll probably want to inspect and replace the diaphragms as well. A torn or improperly installed diaphragm will not allow the end carburetors to open.
The correct spring kit is 20-13. The correct diaphragm kit is 135-6.
Good luck,
Mark
A second option is to simply increase the main (jet) orifice in your existing metering plate using a pin vise. Increase only in very small increments keeping in mind it is the total area and not the diameter that is critical. The area increases as the square of the diameter:
Area = (Radius)^2 X pi or (pi X (Dia)^2) / 4
As correctly posted above, you should first make sure the end carburetors are fully opening, and open as fast as possible without causing a bog or hesitation. While you are checking the opening rate of the outer carburetors with different springs, you’ll probably want to inspect and replace the diaphragms as well. A torn or improperly installed diaphragm will not allow the end carburetors to open.
The correct spring kit is 20-13. The correct diaphragm kit is 135-6.
Good luck,
Mark
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Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Why do you feel you need to re-jet the outer carburetors? The design of these carbs is basically the secondary side of a 4160 style carb. They have a metering plate with fixed main (jet) and idle orifices. You would need to change the secondary metering plate. If you decide to go this route, select a metering plate with similar sized idle feed passages so as not to disrupt your idle mixture. Although not externally adjustable, the outer carbs do have an idle circuit.
A second option is to simply increase the main (jet) orifice in your existing metering plate using a pin vise. Increase only in very small increments keeping in mind it is the total area and not the diameter that is critical. The area increases as the square of the diameter:
Area = (Radius)^2 X pi or (pi X (Dia)^2) / 4
As correctly posted above, you should first make sure the end carburetors are fully opening, and open as fast as possible without causing a bog or hesitation. While you are checking the opening rate of the outer carburetors with different springs, you’ll probably want to inspect and replace the diaphragms as well. A torn or improperly installed diaphragm will not allow the end carburetors to open.
The correct spring kit is 20-13. The correct diaphragm kit is 135-6.
Good luck,
Mark
A second option is to simply increase the main (jet) orifice in your existing metering plate using a pin vise. Increase only in very small increments keeping in mind it is the total area and not the diameter that is critical. The area increases as the square of the diameter:
Area = (Radius)^2 X pi or (pi X (Dia)^2) / 4
As correctly posted above, you should first make sure the end carburetors are fully opening, and open as fast as possible without causing a bog or hesitation. While you are checking the opening rate of the outer carburetors with different springs, you’ll probably want to inspect and replace the diaphragms as well. A torn or improperly installed diaphragm will not allow the end carburetors to open.
The correct spring kit is 20-13. The correct diaphragm kit is 135-6.
Good luck,
Mark
Yes, you need 2 each for a set of trips.
Racer
Joined: Mar 2007
Posts: 278
Likes: 2
From: Wellington Florida
AED makes a spring kit for this also. They also make a billet aluminum replacement metering plate that allows the use of regular jets, making tuning much easier. Pay very close attention to the clearance between the bottom of the float and the protruding jets from the bottom of this new plate. They are very close and can hang up if not checked first.
