Originally Posted by Shane Hethering
Thanks for the post.
I also have a 20hp Yamaha 2 stroke outboard. Runs ok but I could do with more hp. The serial no 6A-9S-016773 it is mid to late 80's and I am trying to get more hp . It is the single carburettor model and I have put a 25hp carburetor main jet in which is slightly bigger from #135 to #140 , I also advanced the timing but no real difference, How easy is it to remove the washer from the the exhaust do I need to take the engine head off
I had a fairly recent discussion with a local, but very helpful and knowledgable outboard mechanic (one with real experience doing this sort of stuff rather than just servicing engines). My engine is twin carb and more modern but I suspect the same applies:
1. The carb body needs replaced not just the jets.
2. The exhaust manifold needs replaced (it may be possible to widen it out, but they are two different parts not just one part with a washer fitted loose in it. After many years of use in a hot salty environment, I am not sure I'd expect to attack it with a drill and not damage it.)
3. The timing needs adjusted.
4. Its a "relatively easy job" assuming everything unbolts easily. If any bolts shear then its just become a major effort. On a 30 yr old engine that is a very real risk for you.
Off the top of my head the parts (at Yam prices) were c. £600 GBP [can't remember if there was VAT on top]. Labour would be more on top, and for a motor of my age wasn't going to be worth it. For a motor of your age it definitely won't, unless you can find secondhand parts and DIY the work.
These engines hold their value well, but there is not much difference is price between 20 and 25, and you'll get no extra resale on a converted 20-25 (unless you happened to have a test rig to prove it was giving 25!)