'89 Nissan 90Hp interesting problem

[rogerwa]

Here is the backgorund..

The motor is on a 25' pontoon.

WOT is at spec or above empty but I can do 5200+ with a normal load of 6 people. It runs great at RPMs over 2K and can run all day.

Compression on motor is an even 120 across all three cylinders, I have soaked and put carb kits in last year. I have checked all linkage lenghts.

The Carb needles are set at approximately 1 3/4. This seems to be the best setting. Any less and it does not idle and will sometimes caugh and die (lean sneeze). It idles out of gear at around 1k to 1.1k and 800 in gear.

Here is the problem..

My cabin is on a river about a 20 min boat ride from the lake. When I sart it cold, I wll lift the idle lever up to get it started, while giving it choke. I run it at aournd 2000 RPM to let it get warmed up and slowly reduce the RPM until it idles without the idle lever raised.

When I idle to the lake at no wake speeds, which we cheat a little bit and usually idle at about 1500-2000 RPM, the motor seems to be bogged, needing to be cleared out. it runs like this until I get to the lake and then when I increase speed, it "clearsout" and runs great. Even at idle.

Also, I need to run it tilted up so that it is not so deep inthe water. When I put the motor in the down postion it smokes terribly from the exhaust relief port. I can only guess that is because of exhaust back pressure.

From my observations, it would seem to me that this is running rich at idle, yet when I adjust the idle mix screws to the lean, it doesn't run well at idle.

I am looking for idea on where to look to resolve this. I just replaced the plugs this past weekend and they were pretty oily.

Ideas??

 

[pvanv]

Re: '89 Nissan 90Hp interesting problem

Tricky speed to run, just above idle. Fuel dispersion in the airstream is not ideal, as the velocity is on the low side. Promotes fuel dropout, which can lead to the other issues (acting rich when over 1500, acting lean under 1500). The could also happen if you are a little rich in oil. 50-to-1 with dino TCW-3 oil is plenty. Sometimes the 100-to-1 Amsoil helps running below the "sweet" RPM range. A little less smoke and less buildup of oily carbon. Does have an unusual smell as it burns, which is objectionable to some.

Likewise, as you surmised, the extra exhaust at the relief port at slow speeds is probably from back pressure, which is exacerbated by running at 1500-2000 with the LU very deep. Not much water rushing by to scavenge the exhaust. Is the height correct at WOT? The heavy toon boats usually run with the prop pretty deep -- sometimes too deep. Are you able to lower the motor at WOT?

If this condition has been a daily thing for a long time, it may be time to de-carbon the motor. There's FAQ info on that procedure. Also, Tohatsu/Nissan has a spray can "engine tuner" which helps dislodge carbon buildup. HTH.

 

[rogerwa]

Re: '89 Nissan 90Hp interesting problem

I can manage the smoke issue by raising it up. It is the loading up that I cannot seem to solve. I am going to try a different warm up routine by not raising the lever as high on startup but I don't think that will help.

This is oil injected btw, which gives a 50:1 mixutre at low speeds and lowers it from there on up.

Can this be an oil injection problem of injecting too much oil at low speeds? Does this happen on these engines? From what I have heard the OI on these are pretty good and maintenance free from an adjustment standpoint.

 

[pvanv]

Re: '89 Nissan 90Hp interesting problem

I can manage the smoke issue by raising it up. It is the loading up that I cannot seem to solve. I am going to try a different warm up routine by not raising the lever as high on startup but I don't think that will help.

This is oil injected btw, which gives a 50:1 mixutre at low speeds and lowers it from there on up.

Can this be an oil injection problem of injecting too much oil at low speeds? Does this happen on these engines? From what I have heard the OI on these are pretty good and maintenance free from an adjustment standpoint.

I doubt that setting the warmup lever differently will aid in the loading situation. It's a throttle-only control.

The oil injection on these is indeed reliable, and actually uses less oil at idle, then increases a bit under load. A dealer could check it to see if it's a bit oil-rich.

As your title noted, it IS an interesting issue. When Elvin gets back on the board, he may have your solution.

 

[rogerwa]

Re: '89 Nissan 90Hp interesting problem

FWIW I think I resolved this problem. I was just about ready to dump the thing in the water.. I pulled my manual out and went over all the steps in the carb tuning section and the one I did not reset after I had pulled the carbs was setting the middle and lower throttle plates in relation to the top.

Once I did this it ran like a top. I think the lower and middle were set closed without the top one closed, and then when I set the idle speed screw it caused some weird behavior.

Go figure..