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Old 25 July 2013, 18:12   #1
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Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
Yamaha 20hp tuning advice

Hi there thanks for reading I have a yamaha 2cyl 20hp 2 stroke engine, it's the 20d model
I know it's the same block as the 25n only differences from the parts lists are the carbs and exhaust extension, though the main jet remains the same

The engine is on a 3m zapcat so sticking a heavier 3cyl motor on isnt an option

The carbs are 250 each and the exhaust 79

Was just wondering what I could do to increase the hp cheaper can I up jet it and fit the 25 exhaust?

Opinions ideas other than pay 500 for a pair of carbs?

It's already propped correctly running a power tech sea worked prop at the correct wot for the motor

Many thanks
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Old 25 July 2013, 21:44   #2
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Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
Are main and slow jets same ? If 25 HP carbs happens to have wider internal fuel passages than same 20 HP carbs, you're done, will enevitably need to buy 25 HP carbs along exhaust and consequently kick in 500. Check both advance timings, probably same carbs for both, and + timing + exhaust on the 25 HP model ?

Upping jets on a restricted carb doesn't work and engine will perform awfully. Encountered same issue upgrading a Evinrude 9.9 to 15. $350 after, worked beautifully...

Happy Boating
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