70HP Nissan, moments of 100%, otherwise 75%

[shellbackII]

After a 4 hour soaking of the two dissassembled carb's and new needle valves on my 70hp Nissan I now have both cylinders but it still has a nagging issue. When you think the motor is max'd out, and performing, it's really at about 75-80%, then, something like "switches" and the RPMs kick up too a higher level and there's a slight improvement in performance, and now running like 100%. She's really running sweet! Then it switches off and you are back to 75-80% performance. These "two power/rpm modes" are consistent. The duration of the 100% level can vary, but it seems to be inspired when topping out at high rpm. Once it "happens", you can be there for only a brief time before it switches back, or it can run several minutes at 100% even if you bring down the throttle, idle, and throttle back up. Then all of a sudden it "switches" off and your back to 75-80%. When it's running 'right' it can pull up an adult skier, when it's not, there's not quite enough there.....safety governor? or... what's that check valve on the crankcase with a hose going up to the magneto? timing advance or something? possibly that check valve is faulty? please help me.

 

[pvanv]

Re: 70HP Nissan, moments of 100%, otherwise 75%

I take it you have an NS70A. You may need to go through the carbs again. You may have a slightly sticky float/needle. Install fresh plugs. Also verify the link and sync. Slop in the advance linkage could contribute to you symptoms. The lubrication tube on the stbd side of the motor just redistributes oil, so should not be a factor.

 

[rogerwa]

Re: 70HP Nissan, moments of 100%, otherwise 75%

I don't recall if the 70 is a 2 or 3 model, could he be dropping a cylinder at top speed? Potential electrical/spark issue? If it were fuel I wouldn't think it would a switch type of effect, more of a surge effect.