carburetor
- Thread starter 56yblock
- Start date
Ole Don
Well-known member
A two throat, or two barrel carburetor, is adjusted by turning the two screws down low on the main body. If the fuel is fresh, and the motor warm, turn one in until the motor looses RPM or stumbles slightly, then turn it back out until it smooths out. Then to the second, then set idle rpm. This can also be done with a vacuum gauge. The air filter should be clean and in place for best results.
Daves55Sedan
Well-known member
The two idle-fuel adjustment screws are located at the back of the carb valve base and they are side-by-side (one for each venturi). In the photo below you can see one of the screws at the back of the carb.
If the carb was rebuilt, I would approach like this: With engine turned off, turn both screws clockwise all the way in (do not tighten), just snug, then back them both out counterclockwise 1-1/2 turns.
Then start engine and fine tune using a vacuum guage. You can connect the guage to that same vacuum tubing seen in the photo below by temporarily removing the vacuum windshield wiper hose from the metal tubing and connect the guage to the metal tubing. With engine idling (after engine is warmed up to normal operating temperature), turn the screws until you can achieve a steady reading of around 19.
After that is done, readjust engine idle screw at the passenger side of the carb to about 475-500 RPM's.
If the carb was rebuilt, I would approach like this: With engine turned off, turn both screws clockwise all the way in (do not tighten), just snug, then back them both out counterclockwise 1-1/2 turns.
Then start engine and fine tune using a vacuum guage. You can connect the guage to that same vacuum tubing seen in the photo below by temporarily removing the vacuum windshield wiper hose from the metal tubing and connect the guage to the metal tubing. With engine idling (after engine is warmed up to normal operating temperature), turn the screws until you can achieve a steady reading of around 19.
After that is done, readjust engine idle screw at the passenger side of the carb to about 475-500 RPM's.
Daves55Sedan
Well-known member
So, with the two screws in the back of the carb unscrewed 1-1/2 turns (approx.) and the vacuum guage reading somewhere around 18-19 inches of mercury (steadily), the carburetor suddenly develops a "miss" upon attempting to accelerate???
Daves55Sedan
Well-known member
Let's try this:
Tools NEEDED to do this; a vacuum test guage with rubber hose connection, screwdriver, dwell/tachometer with RPM guage.
1) Engine must be warmed up to normal operating temperature and carburetor choke must be fully did-engaged prior to ANY adjustments.
2) Turn engine off.
3) Disconnect the windshield wiper vacuum rubber hose from the metal tubing on top of the engine and insert the vacuum guage rubber hose into the metal tubing.
4) turn both idle-fuel mixture screws at the back of the carb clockwise until they stop, then turn both counterclockwise 1-1/2 turns.
5) Now start engine.
6) Read the vacuum guage. It should read somewhere between 18-20 inches mercury.
7) turn both idle fuel mixture screws to achieve the highest steady vacuum reading at the guage. Ideally, it should be the same amount at both screws. NOW, engine idle fuel mix should be properly set.
8) Engine still running, read the RPM meter on your dwell/tachometer. Turn the IDLE speed screw until the RPM meter reads 475 RPM.
9) Turn engine off and remove test equipment. RECONNECT your windshield wiper vacuum hose to the engine vacuum tubing.
10) It would be best to also get a timing light and check your engine timing and set it to factory specifications, and AFTER it is adjusted, go back and re-set your idle speed screw to 475 RPM's.
Tools NEEDED to do this; a vacuum test guage with rubber hose connection, screwdriver, dwell/tachometer with RPM guage.
1) Engine must be warmed up to normal operating temperature and carburetor choke must be fully did-engaged prior to ANY adjustments.
2) Turn engine off.
3) Disconnect the windshield wiper vacuum rubber hose from the metal tubing on top of the engine and insert the vacuum guage rubber hose into the metal tubing.
4) turn both idle-fuel mixture screws at the back of the carb clockwise until they stop, then turn both counterclockwise 1-1/2 turns.
5) Now start engine.
6) Read the vacuum guage. It should read somewhere between 18-20 inches mercury.
7) turn both idle fuel mixture screws to achieve the highest steady vacuum reading at the guage. Ideally, it should be the same amount at both screws. NOW, engine idle fuel mix should be properly set.
8) Engine still running, read the RPM meter on your dwell/tachometer. Turn the IDLE speed screw until the RPM meter reads 475 RPM.
9) Turn engine off and remove test equipment. RECONNECT your windshield wiper vacuum hose to the engine vacuum tubing.
10) It would be best to also get a timing light and check your engine timing and set it to factory specifications, and AFTER it is adjusted, go back and re-set your idle speed screw to 475 RPM's.
dmsfrr
Well-known member
56yblock,
If your distributor has original style points, double check that they are adjusted correctly (dwell/gap) before readjusting and setting the timing. (with the vacuum line to the distributor disconnected and plugged) Also keeping in mind that y-block engines usually like a few extra degrees of initial advance.
.
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Crankster
Well-known member
One thing I've noticed from reading different forum postings on this subject is that it helps if people understand exactly what it is they are trying to accomplish and why. There is for some reason much confusion or misunderstanding. The idle mixture screws are just the final trimming or "cut" of the otherwise fixed idle air fuel circuit. Hardly any air flows through a carburetor at idle, so engine manifold vacuum is utilized through a special circuit.
Consequently ignition timing - which affects the amount of engine manifold vacuum - has an affect on the end idle mixture setting. In other words anytime you make changes in the ignition timing, the correct idle air mixture setting will change. Float height too will affect the fuel level in the bowl, which will change idle fuel mixture (and possibly jetting) as well.
It has a direct effect on how the engine behaves just off idle and the transition, which is where any street driven motor spends a lot of time. At the same time a lot of people seem to get this adjustment wrong for whatever reason.
The procedure itself is right in every manual or carburetor instruction sheet on the planet and has been for years, but. Keep in mind this "1.5 turns out" business is the starting point or a temporary bench setting, but it's not the end point. I see this over and over. "I have them set about 2 turns out and it runs like crap".
Each engine is different, if they were all set the same they wouldn't need the adjustment to begin with and the carmakers would have saved themselves .50c on each unit on the screws. All this initial setting does is ensure that the idle mixture is on the high side, extremely rich - and enough so that the engine will always at least start and idle, and can then be tuned to the best adjustment for that particular engine. If they are left that way it won't run right.
This is always done by turning the mixture screws in equal amounts clockwise, to lean out the mixture. (Unless you've got some smog era atrocity carb) The engine RPM will increase up until a definite, delineated point. This is the area where you are interested in. The idea is to lean out the idle mixture as far as possible consistent with a smooth idle. Very small adjustments - as little as 16th of a turn - make BIG changes in AFR and how it will run.
A mixture that is too rich won't have a crisp idle and will tend to load up and foul plugs, wastes gas, dilute crankcase oil. It's also kind of a moving target. Seasonal temperature variations and elevation play a role in the optimum setting.
Consequently ignition timing - which affects the amount of engine manifold vacuum - has an affect on the end idle mixture setting. In other words anytime you make changes in the ignition timing, the correct idle air mixture setting will change. Float height too will affect the fuel level in the bowl, which will change idle fuel mixture (and possibly jetting) as well.
It has a direct effect on how the engine behaves just off idle and the transition, which is where any street driven motor spends a lot of time. At the same time a lot of people seem to get this adjustment wrong for whatever reason.
The procedure itself is right in every manual or carburetor instruction sheet on the planet and has been for years, but. Keep in mind this "1.5 turns out" business is the starting point or a temporary bench setting, but it's not the end point. I see this over and over. "I have them set about 2 turns out and it runs like crap".
Each engine is different, if they were all set the same they wouldn't need the adjustment to begin with and the carmakers would have saved themselves .50c on each unit on the screws. All this initial setting does is ensure that the idle mixture is on the high side, extremely rich - and enough so that the engine will always at least start and idle, and can then be tuned to the best adjustment for that particular engine. If they are left that way it won't run right.
This is always done by turning the mixture screws in equal amounts clockwise, to lean out the mixture. (Unless you've got some smog era atrocity carb) The engine RPM will increase up until a definite, delineated point. This is the area where you are interested in. The idea is to lean out the idle mixture as far as possible consistent with a smooth idle. Very small adjustments - as little as 16th of a turn - make BIG changes in AFR and how it will run.
A mixture that is too rich won't have a crisp idle and will tend to load up and foul plugs, wastes gas, dilute crankcase oil. It's also kind of a moving target. Seasonal temperature variations and elevation play a role in the optimum setting.
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