yamaha 75 2 stroke becoming a 90?

[diver 1]

Resurrecting an old thread but does anyone know what parts need to be changed to transform a Yamaha 75 2 Stroke to 90 Hp?

[phantom 19]

75A or 75C? There is slightly different transfer porting between the A and C blocks which affects whether you can get a true 90hp from them.

The exhaust tuner is the main difference - You can get 85hp out of the 75A changing for the free flowing 85hp/90hp exhaust and rejetting the carbs, but the missing 5hp is from the block transfer ports and low compression head.

The 75C is basically the old 80hp which has the high compression head and transfer ports tuned to that head so can get a true 90hp out of it by changing to the 85/90 exhaust and rejetting the carbs.

Trying to change the exhaust on an elderly 75A is likely to end in many broken bolts and tears as you need to completely strip the leg down and remove the shock mounts.

[diver 1]

Hi Thanks for reply. I've since realised the engine is a 70, not 75 and it dates to 1989 so perhaps as you say not worth the hassle. I've re-built engines of similar age before and the yams especially seem prone to everything seizing.

[phantom 19]

70 is entirely different engine from 75.

The main issue with Yamaha’s is seized powerhead mounting bolts- because Yamaha use proper bolts with a rolled thread not cut thead like other manufacturers. When in use the unthreaded part of the bolt forms a slight void in the power head mounting holes which when corrosion builds up grips the bolts. They cured it on later engines by using fully threaded rolled thead bolts