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My car is a 69 427 big block with an upgraded carburettor system. Engine is stock with a mild cam.
I just had the rear main replaced and the mechanic took the car for a test drive and said the carburettor secondaries were not opening. The mechanic suggested I take the car into a carb speciality place to get looked at.
What causes the secondaries not to open? Is there some sort of trouble shooting guide or steps a novice could follow to narrow the problem?
I also notice a somewhat inconsistent idle, idling at about 5-600 rpm with a lumpy idle and then with a bit of distance driving the car idles at 900 rpm.
I will grab pictures of my set-up tomorrow and post.
Sounds like you need to rebuild your carb. Buy a rebuild kit and get it done. Hard to say what causes it not to open. Bound linkage, crud, vacuum diaphragm leaking, etc. If you have it rebuilt like your mechanic indicated it should be corrected.
Have you had somebody press the accelerator to the floor while you observe the throttle shaft? If it is not rotating all the way to the stop, you have a bent accelerator linkage. This is the 15 minute C3 tune up.
Very common, especially with a 427. Hard to resist stomping on it for the last 49 years...
My car is a 69 427 big block with an upgraded carburettor system. The mechanic took the car for a test drive and said the carburettor secondaries were not opening. What causes the secondaries not to open?
What type of "upgraded carburetor system" do you have..? It sounds like you have a standard Tripower system. Are you running something different?
Assuming you have the stock Tripower with 3 Holley 2-barrel carbs with vacuum secondaries, the first question would be, how did the "mechanic" determine that the secondaries were not opening? It would be very difficult to determine that from a simple test drive. The vacuum secondaries should transition into opening so smoothly that there would be very little actual noticeable effect as they tip in at elevated rpm.
That said, there is very little that can prevent the secondaries from actually opening: They sense venturi vacuum from the primary carb (center carb), and once venturi vacuum overcomes the spring pressure in the diaphragms, the diaphragms will pull the secondaries open. Nothing can prevent that, unless the diaphragms are ruptured, the vacuum passage is plugged, or some mechanical defect is preventing the secondaries from opening. I would suggest you do the "paper clip" test on the diaphragm secondary actuation rods and verify the "problem" before you assume that they are not working.
If you have a single 4-barrel carb (your thread topic heading indicates you have 2-barrel carbs, so I'm confused...), the same issues apply, and the vacuum secondary operating principle is the same. Once you verify that you have full travel on your throttle linkage allowing the primary throttle to open all the way, you can test the operation of the secondary actuation rod with a paper clip.
What type of "upgraded carburetor system" do you have..? It sounds like you have a standard Tripower system. Are you running something different?
Assuming you have the stock Tripower with 3 Holley 2-barrel carbs with vacuum secondaries, the first question would be, how did the "mechanic" determine that the secondaries were not opening? It would be very difficult to determine that from a simple test drive. The vacuum secondaries should transition into opening so smoothly that there would be very little actual noticeable effect as they tip in at elevated rpm.
That said, there is very little that can prevent the secondaries from actually opening: They sense venturi vacuum from the primary carb (center carb), and once venturi vacuum overcomes the spring pressure in the diaphragms, the diaphragms will pull the secondaries open. Nothing can prevent that, unless the diaphragms are ruptured, the vacuum passage is plugged, or some mechanical defect is preventing the secondaries from opening. I would suggest you do the "paper clip" test on the diaphragm secondary actuation rods and verify the "problem" before you assume that they are not working.
If you have a single 4-barrel carb (your thread topic heading indicates you have 2-barrel carbs, so I'm confused...), the same issues apply, and the vacuum secondary operating principle is the same. Once you verify that you have full travel on your throttle linkage allowing the primary throttle to open all the way, you can test the operation of the secondary actuation rod with a paper clip.
Lars
good points but not only those issues keep secondarys closed , an air restriction will not allow vacuum secondarys open , I went through 3 carbs and rebuilding etc to stumble upon my issue I left the hood ajar by accident and went for a ride when I nailed it the back tires smoked ! I pulled over to look at the carb and noticed the hood ajar when I closed it no secondarys again unless I opened the hood. This was with my 69 Camaro rs forced induction the 2" Cowel hood is to close to the top of the carb ,
maybe he has the wrong air filer housing or a never cleaned K&N filter?
2020 Corvette of the Year Finalist (performance mods)
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mine was a bent foot pedal throttle arm. Once I straightened it out my fuel mileage went way down but the fun factor QUADrupled....!!!! Anyone, anyone.......
What type of "upgraded carburetor system" do you have..? It sounds like you have a standard Tripower system. Are you running something different?
Assuming you have the stock Tripower with 3 Holley 2-barrel carbs with vacuum secondaries, the first question would be, how did the "mechanic" determine that the secondaries were not opening? It would be very difficult to determine that from a simple test drive. The vacuum secondaries should transition into opening so smoothly that there would be very little actual noticeable effect as they tip in at elevated rpm.
That said, there is very little that can prevent the secondaries from actually opening: They sense venturi vacuum from the primary carb (center carb), and once venturi vacuum overcomes the spring pressure in the diaphragms, the diaphragms will pull the secondaries open. Nothing can prevent that, unless the diaphragms are ruptured, the vacuum passage is plugged, or some mechanical defect is preventing the secondaries from opening. I would suggest you do the "paper clip" test on the diaphragm secondary actuation rods and verify the "problem" before you assume that they are not working.
If you have a single 4-barrel carb (your thread topic heading indicates you have 2-barrel carbs, so I'm confused...), the same issues apply, and the vacuum secondary operating principle is the same. Once you verify that you have full travel on your throttle linkage allowing the primary throttle to open all the way, you can test the operation of the secondary actuation rod with a paper clip.
Lars
Agree. We need more info about the carb system upgrade. Pictures too. Close-ups of the carb opening, linkage position at idle, half and full throttle (engine off).
This is pretty funny. We're all offering suggestions on how to fix the carb, and the OP hasn't even told us what type of carb he has... Since he was talking about 2-barrels in his post title, I'm assuming he has a Tripower. Everyone else is assuming it's a 4-barrel with vacuum secondaries, but he's not providing any info. In the end, we're going to find out it's an Edelbrock Performer, which doesn't even have vacuum secondaries at all...
What type of "upgraded carburetor system" do you have..? It sounds like you have a standard Tripower system. Are you running something different?
Assuming you have the stock Tripower with 3 Holley 2-barrel carbs with vacuum secondaries, the first question would be, how did the "mechanic" determine that the secondaries were not opening? It would be very difficult to determine that from a simple test drive. The vacuum secondaries should transition into opening so smoothly that there would be very little actual noticeable effect as they tip in at elevated rpm.
That said, there is very little that can prevent the secondaries from actually opening: They sense venturi vacuum from the primary carb (center carb), and once venturi vacuum overcomes the spring pressure in the diaphragms, the diaphragms will pull the secondaries open. Nothing can prevent that, unless the diaphragms are ruptured, the vacuum passage is plugged, or some mechanical defect is preventing the secondaries from opening. I would suggest you do the "paper clip" test on the diaphragm secondary actuation rods and verify the "problem" before you assume that they are not working.
If you have a single 4-barrel carb (your thread topic heading indicates you have 2-barrel carbs, so I'm confused...), the same issues apply, and the vacuum secondary operating principle is the same. Once you verify that you have full travel on your throttle linkage allowing the primary throttle to open all the way, you can test the operation of the secondary actuation rod with a paper clip.
Lars
Apologies, I tried to change the title of the thread but couldn't edit it! It's a Holley 4 barrel carburettor (Not sure of part number) but I am going to put the original Rochester 4 barrel carburettor back on.
I think the problem was the throttle wasn't opening anywhere near full with the pedal to the floor.
Apologies, I tried to change the title of the thread but couldn't edit it! It's a Holley 4 barrel carburettor (Not sure of part number) but I am going to put the original Rochester 4 barrel carburettor back on.
I think the problem was the throttle wasn't opening anywhere near full with the pedal to the floor.
You might luck out with something simple - check if the throttle cable bracket on the carb moves.They can. FWIW - I'm a newbie and rebuilt my Holley 3310-4 4160 carb. It really was easy and cost less than $50. If you go that route as a time saver let me suggest a new set of floats & springs. Also get a 'clutch bit'. Weird bit I never heard of used to remove the secondary plate. I just filed down and old screwdriver bit. Car runs fantastic and I learned a lot doing it.
Apologies, I tried to change the title of the thread but couldn't edit it! It's a Holley 4 barrel carburettor (Not sure of part number) but I am going to put the original Rochester 4 barrel carburettor back on.
I think the problem was the throttle wasn't opening anywhere near full with the pedal to the floor.
I am sure some shops have made a fortune over the years "fixing" the lack of power problem with C3s...bend the accelerator linkage and when the customer takes it for a ride, cannot believe how much faster the car is...and then charge for a whole bunch of work that was never done.
BTW, that does not look like a 7029215 Q-Jet to me. My middle name happens to be Lars, but that does not mean I know what I am talking about.
in your picture the choke is partially closed, the top secondary air valve will not open with your finger unless the choke is fully open.
you are also missing the choke cover and it is not clear in the picture as to whether the rod from the choke coil is actually attached to the coil.
Actually looking at the picture in this link it seems your choke coil is installed backwards https://www.corvetteforum.com/forums...-question.html
I am curious what you did to fix this. I have a 1971 C3 and the second Carbs are not opening. The throttle is not pulling all the way through and I feel that the pedal is already half way to the floor. Would love some advice if you have any
I am curious what you did to fix this. I have a 1971 C3 and the second Carbs are not opening. The throttle is not pulling all the way through and I feel that the pedal is already half way to the floor. Would love some advice if you have any
what do you mean by second Carbs?
If you have a stock Quadrajet the secondary throttles on the bottom of the carburetor are opened by the throttle linkage.
The secondary air valve on the too of the carburetor only opens when air is flowing through the carburetor and the engine is under load.
what do you mean by second Carbs?
If you have a stock Quadrajet the secondary throttles on the bottom of the carburetor are opened by the throttle linkage.
The secondary air valve on the too of the carburetor only opens when air is flowing through the carburetor and the engine is under load.
sorry. Yes quadrajet. Front two open back two are not. Throttle does not seem to have full range of travel.
06-01-2018, 03:19 AM
Carb Secondaries not opening
