1998 90hp 2 stroke idle vibration

[Poco2]

i just purchased this motor on a 24 foot pontoon boat. I have rebuilt all three carbs and adjusted them to specs. Replaced fuel filter. I have replaced prop with a four blade Mercury prop. I have replaced plugs. I have adjusted spark advance and idle speed. Idle speed is about 800 rpm. Compression is even. My problem is that it vibrates excessively at idle and lower speeds. It jumps around at idle like it is missing. It cruises nicely at 2500 rpm and up. Is this normal for a three cylinder engine? Anything else I should check?

 

[ondarvr]

Did you adjust the low speed screws on each carb to where it runs the best?

 

[Poco2]

I set them at the specified 1.5 turns out and tried to adjust for best running but it made little difference. Right now I have them a little less than 1.5 because I run at 2500 feet altitude. What is the procedure for making a fine adjustment? I usually adjust all three the same then run awhile. I never go more than 1/2 turn in either direction.

 

[ondarvr]

Adjust each one independently, they may be very different. Adjust as needed, not really any limits. At 2500 ft you will be starting out very rich, it needs to go leaner.

 

[Silvertip]

Adjust them one at a time, Idle speed will change. Readjust the mixture screws again and adjust the idle speed again. This is not a one time procedure.

 

[Poco2]

I will try adjusting again. Thanks