1981 Yamaha 9.9/15hp deceleration problems

[johnny81]

I have a 1981 Yamaha 9.9hp (two stroke) that has been upgraded to a 15hp. The motor will accelerate with no issues, I can accelerate as slowly or fast as I like and up to about 90% of WOT I can decelerate smoothly again as fast or slow as I like. However, if I take the motor to WOT it will then not decelerate until the throttle is returned to about 30% open and consequently I get a massive deceleration throwing everyone forwards in the boat. I hope I have explained the problem properly. If I take the motor to WOT then reduce throttle to approx 60 or 70% and hold it there I still get no deceleration - it continues running at WOT - its only once I return the throttle to approx 30% that it will slow (and it does this very quickly). Once slowed I can immediate throttle up however much I want. As long as 90% of WOT is not exceeded the motor will accelerate and decelerate with no problems. I have recently cleaned the carburettor (although I appreciate that this could still be a problem) and changed the spark plugs. Many Thanks in advance for any help.

 

[99yam40]

have you looked at the carb plates to see if they move with the throttle cable/ linkage

 

[johnny81]

I haven't - will do so first time I get a chance and tell you what I find - many thanks

 

[johnny81]

I haven't - will do so first time I get a chance and tell you what I find - many thanks

 

[johnny81]

I think you may have hit the nail on the head!! They do seem a little sticky when reducing throttle - I will have a proper look later and see if I can improve things with a bit of WD40. Many Thanks

 

[johnny81]

Have managed to test this out on the water and I certainly now have a vast improvement on deceleration - not quite perfect - but no longer being thrown forward violently every time i try and reduce speed.