Home › Forum › Ask A Member › 70hp idling characteristics? › Re: 70hp idling characteristics?
September 1, 2016 at 12:54 am
#43098
quote t2stroke:
There were a host of things done to that motor from its introduction in 1986 (56 cu in) till the ultimate change in 1994. The early motors (’86 to ’93) had bridge ports (exhaust). Many changes (bulletins) were done to improve reliability. In 1994 the powerhead picked up oval exhaust ports which was the cure for cylinder scoring.
Now for the problem you have at the moment. Yes the motor is very sensitive to exhaust back pressure. You will quickly learn that trimming the motor up and putting it in gear is normal. After it is moving thru the water you can trim down and get on plane. Just part of learning to run this motor. Do you plan to keep this motor on 25/1? It doesn’t need that kind of oil. 50/1 is normal. If you don’t like that 40/1 would be more than enough.
Your idle timing problem can be cause by a center hub magnet that has moved. Remove the flywheel and look at the center hub. You can usually tell if it has moved. (Glued in place). Before you put it off too long you might pull the cylinder head and check for cylinder scoring? If you have scoring then of course anything else you do won’t help. There are modifications that can be done to the exhaust port when the motor is bored to prevent the same problem again. These motors called for a slightly fatter H.S. jet. .002 I believe from stock depending on what the model calls for. Moving the water elbow to top of the block insures the block is full of water before it pees. I think they dropped the top end timing about two degrees. I may be leaving something out. Been a long time ago since we were modifying all of these motors.
Now for the problem you have at the moment. Yes the motor is very sensitive to exhaust back pressure. You will quickly learn that trimming the motor up and putting it in gear is normal. After it is moving thru the water you can trim down and get on plane. Just part of learning to run this motor. Do you plan to keep this motor on 25/1? It doesn’t need that kind of oil. 50/1 is normal. If you don’t like that 40/1 would be more than enough.
Your idle timing problem can be cause by a center hub magnet that has moved. Remove the flywheel and look at the center hub. You can usually tell if it has moved. (Glued in place). Before you put it off too long you might pull the cylinder head and check for cylinder scoring? If you have scoring then of course anything else you do won’t help. There are modifications that can be done to the exhaust port when the motor is bored to prevent the same problem again. These motors called for a slightly fatter H.S. jet. .002 I believe from stock depending on what the model calls for. Moving the water elbow to top of the block insures the block is full of water before it pees. I think they dropped the top end timing about two degrees. I may be leaving something out. Been a long time ago since we were modifying all of these motors.
Dan in TN
I recommended the heavy oil mix in an effort to avoid one of those mystery powerhead failures, maybe I over did it a bit…Dan makes a good point about the pesky loose inner flywheel sensor magnets, and another good point about ensuring your powerhead is in good condition now…And yes, the jet update kit recommended backing off WOT timing a few degrees.