Merc 15HP Super/Tohatsu 18HP

Hello All
I have just bought a Mercury 15 Super here in Australia and I understand that it is the Tohatsu 18Hp
My query is that the motor is not getting full throttle, It will leap out of the hole and pull hard but does not seem to have much top speed
I had a look at the timing and carb linkages and while it is getting to the 20 degree timing mark the carb butterfly is only getting to half way
Now it is only a simple adjustment to take the timing to 25 degrees and this will also give me more butterfly opening, But should I do that or just adjust the throttle linkage instead
I am thinking that to derate this motor from an 18 to a 15 this is how they've done it but if there's 3 more ponies available I would like to have them
I have the factory service manual and it shows some 18's as having 25 degree timing but I would like to be sure before doing the adjustment
Link to both engines
http://www.tohatsu.com.au/content/m18e2
http://www.mercurymarine.com.au/home/outboards/twostroke/-15-hp---6-hp/15-hp-super.aspx
Thanks for any advice

Re: Merc 15HP Super/Tohatsu 18HP

You must free stopper A nut CW and turn screw CCW to advance timing from 20? to 25? while advancing throttle, adjust nut tight once reached that posiion. Roller will move to extreme left high hill end achieving 180? butterfly opening at full throttle. Assume you have same cam, right ? Don't expect to fly with an 18, will have better hole shot and slight better top end speed at wot than with any 15.

Don't know why Mercury puts funny names, should be called just 18 HP...

Happy Boating

Re: Merc 15HP Super/Tohatsu 18HP

Thanks very much Sea Rider
I have the same cam but it is only just hitting the beginning of the flat part if that makes sense, It can go further easily enough
So I gather that 25 degree timing is not going to hurt the engine at all?

Re: Merc 15HP Super/Tohatsu 18HP

Yes both Merc Super and Tohatsu 18 are near 300 CC. When advancing timing from 20? to 25? cam will advance accordingly, you should have carb roller/cam resting slightly passing begining of high hill. You are just letting tiller to throtle more with +5? timing compensation for correct wot operation. Place an induction tach and check if with delivered prop engine is inside safe working parameters (4750-5500) at wot.

Let us know if worked for you...

Happy Boating

Re: Merc 15HP Super/Tohatsu 18HP

Thanks heaps Sea Rider
I won't be near the boat for a week or so and will let you know then
Thanks again, Excellent info