How to adjust a 650
Turn each of the four (two if not a double pumper, on some) screws in just till they stop (not tight). Turn each out about one and a half turns.
Attach a vacuum gauge to any available port that carries full vacuum (in base of carb or on manifold).
Attach a tach to the ignition system (hand held, attaches to negative side of coil or slot marked "tach" on dist cap).
start the engine and turn throttle linkage adjustment to the lowest rpm the engine will run at.
adjust either of the front screws out so therpm and vacuum increases. As soon as it moves, do the same at each of the other screws. Repeat this till you have reached the highest rpm and vacuum reading. If rpm exceeds 800 while doing this, lower it at the throttle linkage. If any screw stops responding, don't adjust it any further. (happens on old carbs sometimes).
Usually after you reach the highest point, it's best to turn all the screws back in around a quarter turn.
Adjust the linkage so the rpm is at the factory setting for the engine you have or, if modified, generally best is between 800 and 900 at idle in gear (if automatic).
If it's on a Chevy engine, timing should be at 32 degrees total advance, base timing at around 12 degrees.
Adjustments cannot be made if there are any vacuum leaks or if fuel bowl floats are not properly set. Highest idle rpm and highest vacuum reading does not always happen at the same time. You are looking for the highest vacuum reading with the idle being smooth.
Holley carbs have many adjustments. Accelerator pump stroke and curve, squirter size and jet size determine how well the carb is "fine tuned" to your usage. On 85 and up automatics, transmission control cable MUST be correctly adjusted or you can quickly kill the transmission.
Sep 11, 2011 |
Chevrolet Silverado Cars & Trucks