1990 Mariner 60 lag at mid throttle

[Walnut Farm]

I’m a new member to the forum. Motor is a 1990 3 cylinder 60hp Mariner. I’ve previously had a problem with a weak idle and dying right off of idle but I believe I now have that under control. I corrected it by changing the idle timing to 6 degrees btdc from 9 degrees bdtc and adjusting the idle screws from about 1 ¼ turns out to ¾ of a turn out. I still have an rpm lag at mid throttle, that is, I’ll push the throttle forward and it’ll take a couple of seconds for the rpm to catch up to the throttle position.

Here’s what I’ve done so far with the motor, rebuilt carbs and checked float settings, checked compression (135 psi in all 3 cylinders), checked fuel pump pressure ( 4 to 5 psi), confirmed TDC markings and checked max timing position, used fresh fuel, set idle timing (a couple of times).
I suspect I need to make a subtle timing adjustment. I had suspected that the carb setting of only ¾ of a turn out was somehow a symptom of the problem but anothr source has said that this setting is not unusual on older motors. There is a very small range on the idle screw settings where it will run. Another item I considered were crankshaft seals but from what I’ve read, that would result in more of a starting issue. Any thoughts and ideas are appreciated.

 

[Walnut Farm]

I’ve been doing some additional work on trying to diagnose what the problem is with the flat spot by checking the basics. I took a look at my idle speed and timing at idle. Idle speed is supposed to be 550 to 600 rpm in forward gear. Mine was at 850 rpm in forward, 1250 in neutral. Since my throttle plates are fully closed and synchronized, there are 2 ways I could lower idle speed and I tried both.
1. I can adjust the idle stop. Adjusting the idle stop also changes the timing. By adjusting the idle stop I was able to get the idle down to 750 rpm in gear, 1100 rpm in neutral. That put the idle timing at 3 degrees before TDC. I tried going lower but the engine would die when in gear.
2. I can turn the idle mixture screws out. Turning the idle screws out at all from their ¾ of a turn out weakens the idle to the point where the engine will die when put in gear. This still seems suspicious to me because the manuals and on line forums that I’ve read say find the best idle and then turn the screws out a quarter of a turn from there. I can’t do that without the engine dying in gear.
I don’t have any ideas as to what to do next. I am going to replace the fuel pump diaphragms just as a preventative measure and a long shot but I don’t think that’s the problem. I believe I checked the idle jets when they were out to make sure they were the recommended size, but I’m going to check that again today.