Sounds like you're still a little rich on the upper midrange, (the degree of steepness of your needle
The last thing I intend is to insult the intelligence of anyone reading this
, but many of us aren't very conversant with the inner mysteries of carburetors, that said;
In the float chamber, the main jet controls the amount of fuel sent to a tube called the needle jet. The needle jet opens into the main bore of the carburetor and allows the fuel into the intake manifold by means of the negative pressure formed by the intake air rushing through the venturi. This is the very same principle witnessed by blowing across the top of a soda straw and drawing up the liquid. The recommended diameter of this venturi is approximately 80% of your inlet port diameter.
The piston carries the 'jet needle' that fits into the needle jet (aren't you just loving this?). The jet needle is straight for approximately 1/3 of its length; the rest is tapered. At idle and low speeds, the piston is nearly all the way down, pushing the needle into the needle jet most of the way. In this position, the straight, large-diameter part of the needle, (the root
), fills up most of the space inside the needle jet tube, restricting the fuel flow to a narrow annular space around the needle. As the piston rises with increased engine speed, the needle is withdrawn from the jet. Because the needle is tapered, the annular space through which the fuel can travel increases, allowing more fuel to match the increased airflow.
From idle up to about 1/8 throttle (i.e. moderate / cruising speed), the major source of fuel is the pilot, or slow circuit. This includes the pilot jet and the pilot screw adjustment. On most carburetors, turning the pilot screw out results in a richer mixture at smaller throttle openings.
In summary; you can see, there are a number of variables involved in proper fuel delivery:
• From zero to 1/8 throttle
, the pilot circuit is the major contributor, influenced somewhat by the root diameter of the jet needle.
• In the lower midrange
, the starting point of the needle taper is crucial.
• In the upper midrange
, the degree (steepness) of the needle taper is significant.
• In the high speed range
, the main jet is the biggest factor.