1985 120 Johnson shudder at 1100-1900 RPM

I have a 1985 120 Johnson that will shudder/vibrate at 1100 to 1900 rpm, once out of that rpm range the engine runs fine. But as soon as you throttle back into that rpm range it comes back. I have been chasing this problem since last year when I bought it. In the process of this I found #1 piston to have leaned out and scored the cylinder. I have rebuilt the motor cleaned and rebuilt the carbs. Thinking that this would fix my problem, but it hasn't the problem is still there unchanged. I noticed that the low oil wire was not hooked up from the time I bought the boat. When I hook it up I get a beeping noise, which In the manual is the low oil alarm. I have filled my oil tank full and it still sets the alarm. Does anyone know if the low oil alarm can cause a problem like this. Thanks

Re: 1985 120 Johnson shudder at 1100-1900 RPM

I know for a fact that the low oil alarm will not cause the shuddering that you describe.

But, I do not know what is causing your issue. I have a V6 looper powerhead, (1996 200) just rebuilt that is doing the shuddering thing at 2700-3000.
When it is shuddering, I have pushed the key in to flood it with some extra gas to see if it was a lean issue...NOPE.
The engine immediately bogs.

So, I have come to the conclusion that it is a rich condition at that particular speed.

My plan of attack is to read the plugs now, if i can tell anything, re-check compression, lean out the carbs by one full turn of the idle screws. (The idle screws also control to some extent the amount of fuel at the intermediate point in throttle range. At least they do on my motor.) Your 1985 will not have the idle screws I am talking about.

Then, new plugs, run it, read plugs. If I cannot tell by that, I will put a timing light on each wire when it is shuddering to see if it is ignition related.

I will let you know the outcome, if you want. It will take me a while, as I don't go out regularly, and I will also need a second person...one to drive while i hang off the back....

Re: 1985 120 Johnson shudder at 1100-1900 RPM

I have checked my compression, spark and carbs all is ok. Does anyone know what could be causing this shudder/vibration?