The idle tubes are the two small tubes that poke out the bottom of the venturi. Run thin wire down these tubes to make sure they are clear. Ethanol will leave residue behind that cleaners will not get out.
There is a very small hole that leads from the idle needle to the inside of the bore. You can blow air through this hole to make sure it isn't blocked.
- Connect a vacuum meter to one of the ports coming off the carburetor, or intake manifold.
- The engine should be at operating temperature.
- Screws should be about 1 1/2 turns out for starters.
- Set the idle speed to manufactures specifications. When the RPM is too high, the idle circuit is bypassed and adjustments will not affect anything.
- Turn the 2 screws out watching the vacuum. Stop when the vacuum doesn't increase.
- Taking turns with the 2 idle mixture screws, turn the screw in 1/4 of a turn and wait for a second so the engine will catch up. Do this until the vacuum starts to drop. You should also hear the RPM drop.
- Now turn the screw back 1/4 of a turn.
That is it, your done.
Don't have a vacuum gauge (not sure you should be working on your engine without one)? Follow the same sequence but instead of vacuum increase and drop, listen for the RPM to increase, or drop.
Adjusting the screws don't make any difference?
- The idle speed is higher than the recommended idle speed. This bypasses the idle circuit.
- There is a vacuum leak. Use carburetor cleaner and spray around any vacuum area, base of carburetor, intake manifold. If the RPM smooths out, you found your problem.
- Look at the passages in the illustration. Make sure all of the passages are open.
- You may be getting too much fuel. This brings up a whole other list of possible problems.
- You may not be getting enough fuel. Just the opposite of the last bullet point. This could be from fuel pump, plugged passage, float level, sticky needle & seat.
- Take special notice of the throttle body gasket. You may have the incorrect gasket.
This is either a vacuum leak, or lack of fuel.
- Spray carburetor cleaner around the the throttle body gasket and mounting gasket. If the idle changes then you found your leak.
- Intake manifold, vacuum hoses
- Replace or clean your fuel filter. Some of these carburetors have a filter inside the fuel inlet.
- Adjust the idle mixture. Start by turning it out slowly. If the RPM gets better then you may have found the problem. Make sure the engine is at operating temperature and the idle is at specification. Turn the idle mixture screw out 1/4 turn at a time, wait a second for the engine to catch up, then turn out again. Do this until the RPM doesn't increase. Start turning it in 1/4 turn at a time until the RPM starts to drop. Turn back out 1/4 turn and leave it.
- Test the fuel pump. Should be about 4 lbs. If too low it will starve for fuel.
- The needle may be sticking in the seat and not allow enough fuel to enter.Dirty passages will cause fuel starvation.
- Make sure the fuel bowl vent tubes are clear.
- Check the venturi gaskets that they fit correctly. The wrong gaskets may have been installed blocking one of the air vents. Also be sure the gasket fits flat. They sometimes need to be trimmed.
Idle Air Compensator
The idle air compensator is used on some carburetor models to offset enrichment effects caused by excessive fuel vapors from fuel percolation, during extreme hot engine operation.
The compensator consists of the thermostatically controlled valve usually mounted in the area above the main venturi or at the rear of the float bowl. The valve closes off an air channel which leads from above the carburetor venturi to a point below the throttle valves.
The compensator valve is operated by a bi-metal strip that senses temperature. During extreme hot engine operation, excessive fuel vapors entering the engine manifold cause richer than normally required mixtures, resulting in rough engine idle and stalling. At a certain predetermined temperature, when extra air is needed to off-set the enriching effects of fuel vapors, the bi-metal strip bends and unseats a valve which uncovers the air channel leading from the carburetor venturi to below the throttle valves. At this time, just enough air is added to the engine to offset the richer mixtures and maintain a smooth engine idle. When the engine cools and the extra air is not needed, the bi-metal strip doses the valve and operation returns to normal mixtures.
In order to ensure proper idle adjustment, the value should always be closed when setting engine idle speed and mixtures.