Rough mid throttle

Have 1987 70 hp 2 stroke Yamaha etlh. Have been trouble shooting for 2 seasons. Problem is with rough mid range acceleration. Idles fine. From 80 percent to wide open throttle fine. In between throttle response sluggish and seems to vibrate. So far I have cleaned and rebuilt carbs(cleaned and went thru three times actually). Changed fuel pump twice. Checked compression, synced carbs, checked timing, changed fuel lines and filter, did a Seafoam decarb. I think it might be running a little rich as when I did Seafoam decarb engine really smoothed out about 15 seconds before it ran out of fuel. The idle mixture screw, per manual, is 1 7/8 out +_ 1/4 turn. I honestly can not tell much difference between 1 1/2 out to the 2 1/4 range. Do I rush running to lean if I go down to the 1 to 1/4 out range? I am thinking that it will still get plenty of fuel but do not want to risk burning piston. Any other suggestions appreciated.

Spec idle mix is a starting point....when engine is warm turn 1 screw out at a time. Listen to the engine. It will indicate the mix it likes best.

Is there any concern of getting it too lean and burning a piston as long as I seem to find the “sweet spot”? Even if I am a full round from specs?

Sounds to me by the way that the motor is operating that there is still some debris or restriction within the carburetors that is affecting the mid-range air/fuel circuitry.

The fact that you are also having to make an excessive adjustment to the fuel/air mixture indicates that you are trying to overcome a problem within the carburetors.

If some gasoline is misted into the air intake system when the motor is bogging down/getting sluggish does it get better or worse?

My data indicates the base idle mixture setting is 1 1/4 turn outward from being lightly seated. Plus or minus 1/4 turn.

If carburetors are not cleaned in a heated ultrasonic cleaner, using a good cleaning solution, and are not rodded out and then the circuits verified to be free and clear and flowing correctly, it can be all a waste of time. Leading to frustration with the motor.

Thanks for your reply. I did carefully clean them using heated ultrasonic cleaner then used air to blow passageways out. I am comfortable that they are cleaned properly As for the setting I am using what I believe to be the correct Yamaha service manual supplied to me by a member on this site. That does not mean it is correct thou. To be honest I can tell very little difference between 11/2 turns and 2 turns. I was going to crank down further but first wanted to see if I was risking engine damage by making too lean.

I pulled plugs and the third cylinder is not burning properly. Thought maybe weak fire. Pulling one wire at a time rpm drops about same no matter which wire I pull. The top cylinder seems to have more fire as I get a “bite” when I pull that wire compared to middle and bottom cylinders. Do could ever just get weak? Simply way to test?