yamaha performance problems!

[andyangler]

Hi all,Im having problems with performance on my yamaha 70hp,When i first put my boat in the water it was doing about 22knots WOT.This then reduced after a few trips out to about 10-14knots WOT.First of all i put it down to the fact that i have a plastic prop on there which needs replacing and with the boat being in the water for some time now maybe there is a build up on the bottem of the boat slowing me down.But i went for a good run on sunday(1 and a half hours)and about a hour into it after doing about 12knots she just flew back up to 22knots without touching the controler.This was great untill i turned the engine off in the lock and when i turned it back on to head to my mooring it was back to 10 knots again.Im sure it has nothing to do with the fuel side of things as this has all been checked.Someone has mentioned maybe i should check the coil,if so can someone please tell me which coil they mean.Any help would be great.
Thanks all
Andy

 

[spannerman]

It could well be an ignition problem, I can't remember if Yam use the same system as Mercury, but they have a low speed stator winding and a high speed stator, these produce the electricity to power the ignition system and if one goes down so does performance.
They sit under the flywheel, but the leads come out to the ignition module, there is a procedure to ohms test the stator windings and ignition trigger pickup winding, and the coil itself but you need the manual to do this.
With outboards its a process of eliminating the various components as you can't test the module without a test rig which even dealers don't usually have.