15 HP 2 Stroke will not return to idle

[Ginger James]

Hi,
I have been given an old 15HP 2 stroke outboard, when i start the engine and open the throttle the motor picks up rpm's but when I return the throttle to the idle position the motor will not return to idle.
I have run the motor with out the cover on and have noted the carb linkage returns to the "idle" position. Also there is no air filter.
This is my first motor and first fishing boat.... all second hand.

Any pointers.
GJ

[Hamfisted]

Do the carb "butterfly" plates look like they're returning to the closed position? If they are then it's getting air from some other area. Check the idle air adjustment screws, they should be about 1 1/2 turns out from a light seat. Look for a vacuum leak around the base of the carb, maybe spray some WD40 around the carb base and see if the idle changes to locate the leak, might just need a carb rebuild (cleaning out and new gaskets...). Do you have a service manual for the motor? You can go to www.Yamahapubs.com and order one online and they'll mail it to your door. Do you know what year the motor is? Look on the mounting bracket plate and it should say something like C15MHR or something like that. Good luck.


Mike

[Ginger James]

Thanks for your reply.

T have taken another good look at the carb and at the back or nearest to the motor there is a brass coloured tube/pipe. I checked my mates motor (8hp) and on his this tube/pipe attaches to an outlet on the piston block, his motor has 2 one for each piston I have 1 outlet on the top piston. This currently connects to an outlet under the bottom piston. On his motor this bottom connection (under the bottom piston) returns to the other side of the engine at about the same point, on mine i do not have this possibility.

What I am thinking is that these 2 should be attached together and returned to the carb.

I have no book to go with the motor, on the plate I have "yamaha 15A - 615S".

Hope you can help futher.
GJ

[Hamfisted]

The 15hp motor shares the same block as the 9.9hp motor, but the 8hp motor is different so the tubes would be different. I think what you're describing are vent tubes that run from the upper crankcase to the intake manifold. If either of them is lose or dangling that could be your problem. I sent you a PM if you want the service manual for it that better describes the plumbing.


Mike