Nissan 90 idle roughness

[rogerwa]

I need some ideas. I have a 89 Nissan 90hp that is in great condition. I have cleaned the carbs, adjusted them, checked the linkages, changed plugs, etc and I still get uneven running or roughness from the 1000-2000 rpm range. Once it gets to 2000 it is smooth. The motor does not respond well to throttle changes in this range, it seems like I have to give it more throttle than I should and the motor runs jerky.

I am thinking that it is still a carb adjustment issue, but cannot seem to get it straight. I do not see a lot of response when I tweak the mixture screws (which are new).

It does not kill and will run all day, but to get to the lake I have to run in this range for around 15 minutes and it is not as pleasant as it could be.

Any ideas of where I should look?

 

[TOHATSU GURU]

Re: Nissan 90 idle roughness

I would soak the carbs. Chances are you have a partial blockage of the low speed circuit. Other than that it could be anything from ignition to mechanical.

 

[rogerwa]

Re: Nissan 90 idle roughness

Thanks Guru, but that is exactly what I did this spring. I put in new kits and went over the passages real good with cleaner and compressed air after it had soaked overnight. I paid special attention to the idle speed and low speed circuits.

I am begining to think it is just the motors personality as it runs superbly from 2000 to WOT (~5300).

 

[TOHATSU GURU]

Re: Nissan 90 idle roughness

The next step would be to look at the ignition system while it's screwing up. Inline spark indicators will tell you if you are getting an ignition problem. If you have good regular spark then your back to the fuel system and/or looking at the block.

 

[rogerwa]

Re: Nissan 90 idle roughness

Just a little update on this one. I took my book over it and went over it and decided to try the adustments from the beginning.

Timing checked out just right and so I went to the carb adjustment. With the air baffle cover off I adjusted the butterfly to 5 degrees as stated and set the idle mix needle at just over 1 turn (I have two books by tohatsu/nissan - they don't agree on the initial setting so I went just a little over 1 turn). I then tuned it to where the carbs were set at about 1.25 turns and was able to bring the idle down. It did not want to settle where it should but I was able to get it just about right. I took it out for a spin and it did run better. Not perfect, but better.

When I go back to the dock, I put the baffle cover on and put the motor hood on and it would not stay running. it would run a little bit until it gave a cough and died. it seems that the baffle cover changed something. I did check it and the motor hood for obstructions, but there were none.

In order to get it running at idle, I need to set the needles at 2+ turns, which is out of spec.

I must be running rich which would also explain why it kind of loads up.

Any ideas?

 

[TOHATSU GURU]

Re: Nissan 90 idle roughness

Just go back through the fuel system again if you think its a fuel system issue.