Top Side Internals
From OpenContent Curriculum
The two major components accessible from the top side of the carburetor are the throttle slide (valve) and the jet needle. There is no adjustment which can be made to the slide other than with the idle adjustment screw which is accessed from outside. It is important to note that the slides are specific to each engine type and size and cannot be arbitrarily interchanged. The cut-away angle varies and with it the air-fuel mixture. There is a number stamped on the slide's bottom to indicate differences. Typically the zinc-bodied carbs have an anodized aluminum slide (pictured above) while the aluminum-bodied carbs use a plated brass slide.
The jet needle is stamped with a number that indicates characteristics such as the angle of the taper. It is important that the needle is correctly matched to the needle jet. Both have been selected for a specific engine and cannot be arbitrarily exchanged with needles from other carbs and engines. The jet needle has five grooves which are not numbered, but are counted from the top down: 1-2-3-4-5. When reading the jet needle designation for a particular engine, the number stamped on the needle: 5DP7 is written like this "5DP7-2". The number following the hyphen indicates the standard placement groove for the E-clip seen above. The mid-range performance of the engine can be affected slightly by raising the clip (leaner) or by dropping the clip (richer). Sometimes an engine "stutter" which appears due to temperature (or altitude) can be tuned out by moving the clip.
This pic shows the relationship between the two parts with the most confusing nomenclature, the jet needle and the needle jet. The two parts work together to produced smooth operation through correct mixing of fuel which is supplied through the main jet. The needle jet can be seen from the top side of the carb, but can only be removed from the bottom. (It is is retained by the main jet and a brass washer.)
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