Mid range miss can also be ignition related.
Check the spark quality at each plug wire. You can use an old spark plug, break the ground electrode off, put the plug in one wire at a time, ground the base, should give a blue hot spark from the center electrode to the plug base.
Bring the RPM up and observe that the spark continues through the full range.
How are the plugs, wires, cap and rotor?
Electronic ignition is forgiving of a worn distributor, but has it's limits. Check the shaft for excessive side play. Also check the centrifugal advance, be sure it moves freely and snaps back when released.
A plug reading may help determine which cylinder is dropping. If one is a different color that can be a clue as too what is going wrong.
Back to the carb...
A good test for fuel problems is to try a partial choke when the problem occurs. If choking it helps, good chance it is too lean. But, weak spark can mimic lean mix, because rich fuel ignites easier.
If the choke makes it worse, chances are it is not a carb problem.
You can take a fuel sample from the drain plug on the bottom of the carb. With the engine off, fuel valve open, get a clean glass ready. Take the plug out and catch the flow in the glass. It should have a full flow, then slow to a trickle as the bowl empties down. It should continue to trickle indefinitely. If it slows to a drip or stops, there is a fuel delivery problem. Most likely a restriction upstream at the fuel valve, the separator screen, or the screen inside the fuel inlet fitting at the carb.
Whatever you caught in the glass will also be inside the carb. If it's cloudy, it has water in the fuel. If there is rust or dirt, the tank is contaminated and will need to come off and be rinsed out. I discourage inline filters, they can be too restrictive for a gravity flow system.
If the fuel was clean, the carb may be OK. If you want to open it up and take a look, you can usually ease it apart without tearing the gasket. Then it can be cleaned up and reassembled without buying a kit. Those are very simple carbs, easy to service and understand. Just work over a clean surface so not to loose anything.
Once back together, be very careful to keep the fuel supply clean. I like to open the fuel valve before connecting the line to the carb. That will flush anything that may have gotten into the line while it was open.
Keep in mind, the carb can only work as well as the rest of the engine. If there are compression or valve problems, restricted air cleaner, vacuum leaks, exhaust restriction, ignition problems, the carb can't do what it needs to do, and is often blamed for something that it shouldn't be.
More about the air cleaner, up inside the canister, above the oil bowl, there is a wire mesh filter that is often overlooked. It needs to be washed out with solvent periodically. If it has been neglected, the mesh will need to come out and be cleaned or replaced. Not an easy job, but if it's caked with dirt the engine can't breathe.
Now, to adjust the mixture... Get it running and up to operating temperature. If it doesn't come up to temp, replace the thermostat. It needs some heat to run properly.
Start with the idle mixture, the small screw up by the mount flange. The engine must be idling down around 400-500 RPM. Any faster and it will be on the intermediate circuit and the idle will not adjust. The idle mix is backward from modern carbs, turning the screw in enriches the mix, backing it out leans it. You are actually adjusting air bleed, not fuel.
Back the screw out until the engine falters, then slightly back in to best idle, then slightly more. That will give the best mix for easy starts.
Then adjust the high speed, the larger screw down lower, some are on top of the bowl, others are under the bowl. Turning the screw in leans the mix, out enriches it.
Start with the screw about 1 1/2 turns off the seat. When turning the screw in to seat, be gentle, stop as soon as it bottoms, don't tighten it.
With the engine at idle, fully open the throttle a fast as possible. The engine should falter, possibly die. Start backing the mixture screw out 1/2 turn at a time until the engine will take full sudden throttle without hesitating. This will take several attempts to get it right, keep adjusting until you get instant throttle response. A single puff of black smoke is desirable, but no continued smoke once up to speed.
Hope this helps, let us know...
