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Old 10 September 2010, 08:19   #11
Country: UK - England
Town: macclessfield
Boat name: Reach Out
Make: Quicksilver
Length: 4m +
Engine: 30hp Tohatsu EFI
Join Date: Aug 2010
Posts: 301
My 20 hp 2005 yamaha outboard opens fully 90 deg on both carbs.

I think the bore of the cards is bigger on the 25 hp....

What sized bore does a stock 25 hp yamaha 2 smoke have?? (around 2005)

simsy is offline   Reply With Quote
Old 13 September 2010, 03:54   #12
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,538
The fact they are carrying differnet assy part numbers would indicate something is different. The down side is that the spares lists will not go down to the individual componet depths, so if it opens all the way, you probaby have different castings, or as you say, a bigger / smaller bore, although putting my manuifacturing efficiencty hat on, I would be surprised if it were two totally differnet castings, purely dwn to economies of scale. Having said that, Yam also mke bikes,so goodness knows whether one of them is shared with a two wheeled machine or not.

Remember you also have differences in the exhast side. I still reckon if that's the case, you may be as cheap just trading in for a bigger engine.

Only way to find out for sure is go measure one.

9D280 is offline   Reply With Quote
Old 21 September 2010, 09:13   #13
Country: Canada
Town: GTA
Boat name: Zodiac
Make: Zodiac Futura MkIII
Length: 4m +
Engine: 50 HP ETec Tiller
Join Date: Aug 2010
Posts: 184
I don't know anything about Yam's but I went through this with a '89 Evinrude 20. Its all about fuel/air in, exhaust out, and maintaining the proper balance. I spent hours at a cooperative dealer examining and cross referencing microfiches and part #s. I also examined a same year 25. When looking into the carb throat of my 20 I saw a restricted diameter casting behind the carb. 25 didn't have that: manifold casting dia. equaled inner dia. of carb. Only part # differences were manifold and main jet. Using a Dremel I carved the hole in the manifold to equal that of the 25 (after stuffing the bore with a grease coated rag to prevent chips from entering the engine), then I installed the bigger main jet specified for the 25. Voila! Noticeable 25% increase in HP.
Same block was also indicated for 30, but exhaust had a different part #. Through this process I also consulted with a mechanic friend who used to race and hotrod outboards. He showed me 2 exhausts from similar Mercs, but with different HP ratings. The only difference was a couple of 1/4" holes drilled into the higher HP one.
Balance: More fuel air in; more exhaust out.
To up my 20 to 30 I would have had to remove the powerhead and alter the exhaust. Too much work, but I happily ran my new 25 for several years with no problems.
Northern Pike is offline   Reply With Quote
Old 23 September 2010, 09:07   #14
Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
il have a lil look and c if the bore is reduced but like the parts list suggests summat is different with the carb bodies must be diam, the main jets are the same on the 25 and the 20 so im assuming reduced flow at both ends i know from my bike experience when u have a bike restricted to the 33bhp limit for ur test its normally washers between the carb and manifold? and on mopeds tht have to be restricted to 30mph they usually have the oppisate end restricted a washer in the exhaust

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