2GC Tuning
March 13th, 2024, 05:11 AM
#1
Registered User
Thread Starter
Join Date: Aug 2022
Posts: 13
2GC Tuning
if anyone can recommend a guy that can tune a carb properly, I’d love to meet him. I’m having a hell of an issue getting my Rochester 2bbl and rocket 350 tuned properly. I’m in Utah
Last edited by joe_padavano; March 13th, 2024 at 07:03 AM.
March 13th, 2024, 05:39 AM
#2
Rocket Renegade!
Join Date: Dec 2014
Location: Vancouver Island
Posts: 4,464
March 13th, 2024, 07:06 AM
#3
Old(s) Fart
Join Date: Mar 2007
Location: Northern VA
Posts: 47,444
Here's your own dedicated thread. Once again, posting a question about a 350 Olds in an old thread about EFI in a big block is not your best choice for getting useful information. It would also help you get a more useful answer if you could elaborate on exactly what "tuning" you think you need. Is the engine stock or modified? Is the carb doing something that it shouldn't? The mind reading thing really isn't working.
March 13th, 2024, 11:55 AM
#5
Registered User
Join Date: Aug 2014
Location: Western North Carolina
Posts: 572
The 2GC is a great carb and really simple to rebuild and dial in. The only thing you'll really be able to "tune" on it is your mixture. You'll need to describe in more detail what it is you're expecting to get out of it. My advice is; rebuild it with quality gaskets and set your float level and mixture screws to the prescribed values in the CSM. Once you have it on the car and it's running you can fine tune the mixture with a vacuum gauge (simple and easy). You can also do this with an 02 sensor but maybe you're not that sophisticated yet. To be honest, that is probably overkill for the 2GC. A simple vacuum gauge will get it purring like a kitten.
Don't expect Holley double pumper performance out of it. It's an economical 2 barrel. Rebuilt properly it will give you great hot and cold starts, a smooth idle and great drivability. If you want more than that, consider a 4 barrel conversion.
Don't expect Holley double pumper performance out of it. It's an economical 2 barrel. Rebuilt properly it will give you great hot and cold starts, a smooth idle and great drivability. If you want more than that, consider a 4 barrel conversion.
March 14th, 2024, 11:03 AM
#6
I agree with acavagnaro about rebuilding and tuning. If you go on YouTube there is a channel called Uncle Tony's Garage and he talks about tuning with a vacuum gauge. I just did this to my 67 last month and I haven't played with a carb in about 40 years.
April 28th, 2024, 10:57 AM
#8
Registered User
Join Date: Sep 2010
Posts: 472
If it's the original 2GC it should be very easy to tune. Original, meaning it has the right body, bleeds, jets, power valve etc.. If it is not original, it is possible that you will never be able to get it to run correctly because the idle mixture can't compensate for the wrong jets etc...
First make sure everything else is in good shape, points, plugs, wires, cap and rotor, fuel filter. Set the timing and dwell correctly.
Once that's done, find the idle mixture screws they are in the front of the carb near the very bottom pointed towards the radiator, they should have a spring between the head and the carb body. People muck with these and I have rarely seen them set correctly. Both should be adjusted exactly the same number of turns out.
Hook up a tachometer to the engine and set the idle fairly low in park (like 600rpm). Shut off the engine.
Now with a flat bladed screwdriver, while counting the turns (everytime the flat blade reaches the same position it's 1/2 turn) turn one idle mixture screw in until it's gently seated. Write down the number of turns (say 2 1/4). Then turn the other one in, until it's gently seated, while counting the turns and write down the number (say 3 1/4). Now average the two counts of turns (in my case 2 3/4). Now unscrew each idle mixture screw the average number of turns (in my case 2 3/4).
Start the car and set the idle speed back to around 600rpm. If during the adjustment process the RPM changes significantly (say by more than 50RPM), readjust the idle speed to keep it around 600rpm.
There are two ways to adjust the mixture, one is using the lean fall off (meaning that you have a bit of a lean mixture for better gas mileage) the other is using the rich fall off (meaning that you have a bit of a rich mixture for better performance). You can choose which one you prefer, and can always experiment to find something you like. Basically there is a certain range of turns on the mixture screws where the RPM will stay steady (say 1 3/4 to 2 3/4 turns out). This is where you want to start your adjustments. If you stay closer to the least amount of turns it will have a leaner idle, closer to the higher amount of turns it will have a richer idle).
So look at the RPM on the tach. Turn each mixture screw in 1/4 turn (90 degrees). After adjusting both screws did the idle speed go down?
If the idle speed didn't go down turn both in another 1/4 turn, repeat until you see the idle speed go down. Once you see the idle speed go down, turn both back out 1/4 turn, that's the lean fall off point. Now turn it out 1/2 turn you now have set the idle mixture near the lean fall off point. You might want to know how many turns out from seated is this setting. Shut off the motor. Turn each idle screw in counting the turns. Write the number of turns down. The two should be identical. Now unscrew them that amount of turns to get back to where you want to be.
If the idle speed did go down, then you are past the lean fall off point. So turn both out 1/4 turn, if the idle speed increases or stays the same, repeat. Do this until the idle speed actually drops. This is the rich fall off point. Screw them in 1/4 turn to get back to the high idle and then turn them in 1/2 turn. This is near the rich fall off point. I would suggest you follow the procedure above to figure out how many turns out this is.
Once you have the number of turns for the near lean fall off point and near the rich fall off point, you can decide if you want gas mileage or power. And set them to that number of turns. Or it's acceptable to adjust it anywhere between these two points.
One last thing, once you set both mixture screws (the equal number of turns) to the desired setting. Reset your idle speed to the factory setting.
First make sure everything else is in good shape, points, plugs, wires, cap and rotor, fuel filter. Set the timing and dwell correctly.
Once that's done, find the idle mixture screws they are in the front of the carb near the very bottom pointed towards the radiator, they should have a spring between the head and the carb body. People muck with these and I have rarely seen them set correctly. Both should be adjusted exactly the same number of turns out.
Hook up a tachometer to the engine and set the idle fairly low in park (like 600rpm). Shut off the engine.
Now with a flat bladed screwdriver, while counting the turns (everytime the flat blade reaches the same position it's 1/2 turn) turn one idle mixture screw in until it's gently seated. Write down the number of turns (say 2 1/4). Then turn the other one in, until it's gently seated, while counting the turns and write down the number (say 3 1/4). Now average the two counts of turns (in my case 2 3/4). Now unscrew each idle mixture screw the average number of turns (in my case 2 3/4).
Start the car and set the idle speed back to around 600rpm. If during the adjustment process the RPM changes significantly (say by more than 50RPM), readjust the idle speed to keep it around 600rpm.
There are two ways to adjust the mixture, one is using the lean fall off (meaning that you have a bit of a lean mixture for better gas mileage) the other is using the rich fall off (meaning that you have a bit of a rich mixture for better performance). You can choose which one you prefer, and can always experiment to find something you like. Basically there is a certain range of turns on the mixture screws where the RPM will stay steady (say 1 3/4 to 2 3/4 turns out). This is where you want to start your adjustments. If you stay closer to the least amount of turns it will have a leaner idle, closer to the higher amount of turns it will have a richer idle).
So look at the RPM on the tach. Turn each mixture screw in 1/4 turn (90 degrees). After adjusting both screws did the idle speed go down?
If the idle speed didn't go down turn both in another 1/4 turn, repeat until you see the idle speed go down. Once you see the idle speed go down, turn both back out 1/4 turn, that's the lean fall off point. Now turn it out 1/2 turn you now have set the idle mixture near the lean fall off point. You might want to know how many turns out from seated is this setting. Shut off the motor. Turn each idle screw in counting the turns. Write the number of turns down. The two should be identical. Now unscrew them that amount of turns to get back to where you want to be.
If the idle speed did go down, then you are past the lean fall off point. So turn both out 1/4 turn, if the idle speed increases or stays the same, repeat. Do this until the idle speed actually drops. This is the rich fall off point. Screw them in 1/4 turn to get back to the high idle and then turn them in 1/2 turn. This is near the rich fall off point. I would suggest you follow the procedure above to figure out how many turns out this is.
Once you have the number of turns for the near lean fall off point and near the rich fall off point, you can decide if you want gas mileage or power. And set them to that number of turns. Or it's acceptable to adjust it anywhere between these two points.
One last thing, once you set both mixture screws (the equal number of turns) to the desired setting. Reset your idle speed to the factory setting.
April 28th, 2024, 12:37 PM
#9
Phantom Phixer
Join Date: Dec 2011
Location: Apopka, FL
Posts: 4,693
Here's another tuning tip for best performance.
These carbs were built during the era of pure gasoline.
Today. ethanol gas has taken over and pure gas is hard or impossible to find.
The difference between the two is the heat content of the fuel.
Ethanol has a lower heat content than gasoline, so a mixture of the two, will require richer mixture of fuel and air.
The idle mixture screws adjust only that, the idle mixture.
The fuel mixture above 1000 RPM is controlled by the size of the jets and the level of the gas in the float bowl.
The best way to richen the mixture is to increase the jet size, Leave the float level at factory specs.
There is a number stamped on the jet.
I would recommend one jet size larger, maybe two, to maintain the proper mixture for ethanol laced gasoline.
Jets are available at Daytona Parts.
Daytona Parts Home
These carbs were built during the era of pure gasoline.
Today. ethanol gas has taken over and pure gas is hard or impossible to find.
The difference between the two is the heat content of the fuel.
Ethanol has a lower heat content than gasoline, so a mixture of the two, will require richer mixture of fuel and air.
The idle mixture screws adjust only that, the idle mixture.
The fuel mixture above 1000 RPM is controlled by the size of the jets and the level of the gas in the float bowl.
The best way to richen the mixture is to increase the jet size, Leave the float level at factory specs.
There is a number stamped on the jet.
I would recommend one jet size larger, maybe two, to maintain the proper mixture for ethanol laced gasoline.
Jets are available at Daytona Parts.
Daytona Parts Home
April 29th, 2024, 09:02 AM
#11
Registered User
Join Date: Apr 2018
Posts: 276
My wife's is the original owner of a 71 S 350 2bbl which hesitated off idle since day 1. The cold start idle was way high, maybe 1400 and the hesitation was awful. My final solution a few years ago was the drill out the idle tubes .001 or .002. The cold starts are perfect now and no more hesitation. I couldn't be happier with the results. The long numbered drill bits were sourced from McMaster Carr.
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