building 40 yamaha powerhead???

[Dillonwalling09]

i have an 1989 yamaha 40 powerhead that i wanting to beef up!! any help guys? where do i start?? would like to turn it into a 70

[evilrude 1]

i have an 1989 yamaha 40 powerhead that i wanting to beef up!! any help guys? where do i start?? would like to turn it into a 70

You need to find a tuner from a 50hp,carbs from a 70,(you could use 90hp carbs if ported) shave the head (stock comp is only 130 or so) and if you really want to spin it--weld the crank pins. There is also a wire to the cdi that needs to be snipped or it will limit the rpm (its governed) I can't remember the colour but will find out if you would like me too.
I would also disconect the oil inj (runs too lean anyways) and always remember to jet it
Fantastic little engines (just like its big brother the 70hp OMC)

Good Luck

[BC Black Max]

Dont forget a set of Chris Carson or Boysen Reeds, and you can also shave weight off the flywheel.

[olboatman]

Dont forget a set of Chris Carson or Boysen Reeds, and you can also shave weight off the flywheel.
Is there a rule as to the max percentage of weight that can be shaved from the flywheel and still idle ok? Does it depend on the amount of moving mass (cylinders,etc)?Thanks Gary

[BC Black Max]

We took about 4 lbs off the Tohatsu we did. Its really up to the machinist.



Is there a rule as to the max percentage of weight that can be shaved from the flywheel and still idle ok? Does it depend on the amount of moving mass (cylinders,etc)?Thanks Gary

[olboatman]

We took about 4 lbs off the Tohatsu we did. Its really up to the machinist.

Thanks for gettin back to me! Gary

[BC Black Max]

Gary,

One more thing I should tell you. The flywheel needs to be balanced after is machined. Only a high performance engine shop around here had the equipment to do it. The ring gear alone is about 1lbs. If you are pull start only, bounce that right away.
It makes the single biggest difference in acceleration.



Thanks for gettin back to me! Gary

[olboatman]

BC That is the first thing I do to my small twins(pull the ring gear). I would rebalance the flywheel if I had it cut. Was really interested about how far I could have them cut. These are wheels on 25 and 35 hp OMCs -late 70s vintage. Gary