Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
The real truth about fixing 'nozzle drip'
Cliff Ruggles:
Nozzle drip is directly related to the throttle angle of the primary throttle plates at idle speed.
Things that cause the throttle plates to be high enough to start pulling fuel off he boosters include low compression combined with big camshafts (low vacuum at idle).
Idle mixture set too lean.
Retarded ignition timing, or at least not as much as the engine wants/needs for the combination of parts.
Not enough idle bypass air.
Combinations of all of the above.
The "sucking" sound at idle is often produced by the air traveling across the two slots in the airhorn that lead down to the idle bypass air holes in the baseplate. Nothing abnormal with that, some will make a high pitched sound, and it varies some for different applications.
The first thing to determine with nozzle drip, is if you have control of the idle A/F ratio with the mixture screws?
The mixtures screws should slow the engine down considerably when turned in the clockwise direction. They may not kill it clear out, but going lean with them should show some control.
Make sure the engine is fully warmed up at idle. Adjust the idle mixtures screws for the best idle quality. We do this here by backing them out, 4-5 turns for the 10-32 screws and 8-9 turns for the fine pitched metric screws. Most carburetors will end up about 2 1/2 to about 4 turns out with the 10-32 screws, and 4 1/2 to about 7 turns out with the fine pitch metric screws. This "normal" range can be effected by the size of the holes under the mixture screws and how much taper and lead the idle mixture screws have.
Late model carburetors use small holes under the screws, and long/tapered mixture screws, so they require quite a few turns out from seated to get a lot of idle full flowing.
Slowly turn them in one at a time until the engine speed changes or slows, then back about 3/4 to 1 full turn. Repeat for the other screw, then balance them out if/as needed. Set the idle speed screw so the engine is around 700-750 rpm's.
If the car is has an automatic transmission, hold the brake and place the transmission in gear, and observe the drop in rpm's. If it drops more than about 200rpm's, raise the idle speed and add some more idle fuel, or both until you get the best idle quality in and out of gear. Hope this helps some?......Cliff
bigvette1:
Thx Cliff, explains the situation and where to go, JP
Cliff Ruggles:
In any and all cases, set the idle mixture screws for the best possible idle quality in and out of gear. Lean settings will cause nozzle drip, and really "stinky" exhaust, which is more eye burning that a richer setting.
This often leads hobbyists to believe the mixtures are too rich. Lean mixtures are simply very difficult to burn, and when we don't put in enough fuel in PPM for complete combustion, a LOT of stinky fumes exit the tailpipes. A slighly richer mixture, in contrast will have a more complete burn, and a slightly "rich" smelling exhaust. A slightly richer mixtures also requires less spark lead, and works better with blocked heat crossovers, or none at all.
A common mistake made when tuning, is not allowing the engine to complete heat-soak before setting the mixture screws. The engine should be fully up to temp, and have been driven considerable before making final adjustments.
Ignition timing plays a BIG role for idle fuel requirements as well, just 1 or 2 degrees with some engine/cam combinations can make a tremendous difference in idle quality, in and out of gear, as each set-up has a "sweet spot" for the timing and fuel curves, expecially at idle speed.
Also keep in mind, that nearly an aftermarket camshaft is going to have different lobe positions, LSA, and duration than a factory camshaft. They most often reduce idle quality, just a by-product of their designs, as it takes more duration and more off seat valve timing to make more power in the upper rpm ranges. When doing so, idle quality and low speed power suffers, no way around it. With this in mind, fully expect to have to increase ignition timing beyond factory specs, and add some idle fuel, or both, for best results in all areas........Cliff
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