19_Chevelle_69
06-19-2006, 11:56 PM
TOOLS:
-- Test tachometer. Your dash mounted one is usually not sensitive enough to see small changes in engine speed, and you can't see it from under the hood.
-- Screwdriver(s) appropriate to your carbs idle and mixture screws.
-- Vacuum gauge.
(The following is taken from an article that can be found at www.chevyhiperformance.com)
To begin, make sure the engine is up to operating temperature and the choke is completely off. A cold engine requires more fuel than a fully warmed engine even if the choke is off. Remove the air cleaner and connect the tach to the engine. Typically, one lead connects to the negative side of the coil, and the other hooks to a suitable ground. Connect the vacuum gauge so it reads manifold vacuum.
Remember that working around the engine is plenty dangerous and can chew up fingers, catch cords from the tach or timing light, and generally cause havoc if you’re not careful. Always work slowly and thoughtfully when tuning a running engine. Rushing to the hospital for stitches takes all the fun out of working on cars.
Before you fire up the engine, slowly turn each idle-mixture screw in until it gently bottoms out and count the number of turns it takes. Do this for each idle-mixture screw and then return them to their original positions. If all the idle-mixture screws are not adjusted the same, do that now. Remember to always adjust both screws the same amount. This will help balance the idle mixture and allow the engine to run as smoothly as possible. If the carburetor is new to the engine, start at 1½-turns out (counterclockwise) from full in.
Now start the engine and set the idle speed to around 850 rpm with the idle-speed screw. If your engine has a big cam and must idle at a higher speed, that’s OK. Note the reading on the vacuum gauge. Next, turn one idle-mixture screw in ½-turn and note the change on the vacuum gauge. If the vacuum reading increases—say, from 14 inches to 14-½ inches—move around to the other side of the carburetor and turn that mixture screw in ½-turn as well. Again, note the vacuum gauge reading; if the gauge continues to climb, then adjust each idle-mixture screw in another ½-turn. On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture.
Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a ½-turn and evaluate the results. The goal of adjusting the idle-mixture screws is to achieve the highest possible idle vacuum at a set idle speed. If the idle speed increases after you adjust the idle-mixture screws (which is likely), be sure to adjust the idle speed back to the base speed. This is important because a higher idle speed will increase the vacuum reading. Maintaining a standard idle speed will make it easier to evaluate changes to the idle-mixture screws.
-- Test tachometer. Your dash mounted one is usually not sensitive enough to see small changes in engine speed, and you can't see it from under the hood.
-- Screwdriver(s) appropriate to your carbs idle and mixture screws.
-- Vacuum gauge.
(The following is taken from an article that can be found at www.chevyhiperformance.com)
To begin, make sure the engine is up to operating temperature and the choke is completely off. A cold engine requires more fuel than a fully warmed engine even if the choke is off. Remove the air cleaner and connect the tach to the engine. Typically, one lead connects to the negative side of the coil, and the other hooks to a suitable ground. Connect the vacuum gauge so it reads manifold vacuum.
Remember that working around the engine is plenty dangerous and can chew up fingers, catch cords from the tach or timing light, and generally cause havoc if you’re not careful. Always work slowly and thoughtfully when tuning a running engine. Rushing to the hospital for stitches takes all the fun out of working on cars.
Before you fire up the engine, slowly turn each idle-mixture screw in until it gently bottoms out and count the number of turns it takes. Do this for each idle-mixture screw and then return them to their original positions. If all the idle-mixture screws are not adjusted the same, do that now. Remember to always adjust both screws the same amount. This will help balance the idle mixture and allow the engine to run as smoothly as possible. If the carburetor is new to the engine, start at 1½-turns out (counterclockwise) from full in.
Now start the engine and set the idle speed to around 850 rpm with the idle-speed screw. If your engine has a big cam and must idle at a higher speed, that’s OK. Note the reading on the vacuum gauge. Next, turn one idle-mixture screw in ½-turn and note the change on the vacuum gauge. If the vacuum reading increases—say, from 14 inches to 14-½ inches—move around to the other side of the carburetor and turn that mixture screw in ½-turn as well. Again, note the vacuum gauge reading; if the gauge continues to climb, then adjust each idle-mixture screw in another ½-turn. On most carburetors, turning the mixture screw in (clockwise) leans the mixture, while counterclockwise (out) enriches the mixture.
Initially, if the engine stumbles or the vacuum drops when turning the mixture screw in, turn both screws out about a ½-turn and evaluate the results. The goal of adjusting the idle-mixture screws is to achieve the highest possible idle vacuum at a set idle speed. If the idle speed increases after you adjust the idle-mixture screws (which is likely), be sure to adjust the idle speed back to the base speed. This is important because a higher idle speed will increase the vacuum reading. Maintaining a standard idle speed will make it easier to evaluate changes to the idle-mixture screws.