any tips on modifying a 25hp Yamaha outboard? Wanna boar it out to a 30 or 35.

I would be thinking modifying fuel supply first, the Yamaha small block is actually the same block for 15hp or 25hp just with different carby/fuel supply, maybe this could be increased further but I am sure it would take the engine out of it spec range, would definitely decrease engine life.

Not possible, just trade up..

The 15hp block is also different than the 25hp. (Im talking the twin carby version).

I would suggest the first post is about the single carb 25hp (CV) it is the same physical outside block as the 30hp but porting is different, so you need to strip motor and the re tune, bigger jets, throttle and timing links are different.

The best you can do is try and get a power pack that has the rev limiter removed, or not installed. Thats what the Surf clubs boys do.

HTH

our yammy dealer told me they use the same blocks with different carby, my 40hp could go to 60, a 60hp block up to 90 and same in smaller sizes, he was highly regarded so id say he was onto it, i was quoted $1500 ish for my 40 to 60 hp upgrade,

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cauncy said..
our yammy dealer told me they use the same blocks with different carby, my 40hp could go to 60, a 60hp block up to 90 and same in smaller sizes, he was highly regarded so id say he was onto it, i was quoted $1500 ish for my 40 to 60 hp upgrade,

Who ever told you that is wrong sorry.

Id love to know what Yamaha dealer said that

For starters the 40hp (3 cylinder) is the same size as the 50hp. 88 kilos but the 60 and 70hp are both 116 kilos. The 90hp is a different block again and total weight of 127 kilos..

Some models use the same casing, but the ports are different. A bigger carby will let more fuel in, but the engine needs bigger exhaust to allow that burnt fuel out. EDIT: actually they may be able to allow for more exhaust by using different exhaust plates and on checking they are also different part numbers between the 40 and the 50..Same crank and piston part numbers but lots of parts are totally different numbers between the two..

From model to model their will be different changes, but issues that need considering are things like reeds, carbies, electrical packs (CDI) linkages for timing advance.

The only real way to get better performance out of a 25 would be like i said to source and old power pack that was never made with a rev limiter, maybe you could also shave the head and try increase the compression but im not sure why..


Oh and im bored stupid trying to not walk around on an injured leg..So yes i actually did just look up boats.net and did a parts comparison..

Is it possible you're both talking about both 2 and 4 stroke models? ie a 25 comes in both stroke variants.

JB must be on about 2 strokes, if your talking about Ports and Reeds.

(Did the OP specific somewhere was variant it was??)

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rod_bunny said..
Is it possible you're both talking about both 2 and 4 stroke models? ie a 25 comes in both stroke variants.

JB must be on about 2 strokes, if your talking about Ports and Reeds.

(Did the OP specific somewhere was variant it was??)

Yep 100% im talking about two strokes. The 25hp gets modified a fair bit, especially for racing IRBs and thunder cats.. Thats why im guessing it was a 2 stroke. Actually in the Yamaha range their is even two Yamaha 25hp models. A twin carby and single carby version, but two completely different motors again. The 25hp (cv) is the same block as the 30hp (cv) thats why i assume he's asking about this model.

4 stroke would be even harder..Actually a mate tried it with a 80hp four stroke to a 100hp. Again same block, (actually its the European ford mondio motor) but it actually went well worse..

Back in the same day we would race jet skies. Completely financed and backed by Yamaha. We worked out a cost ratio and it worked out to be around $1000 per 1 mph extra in speed..Those days we were taking 160hp ski's to 180 hp.. Today they come out of the box at nearly 300 hp

Get the prop right. Everyone looks for more power without looking to lose drag a well shaped appropriately pitched stainless steal prop will be a lot cheaper.

Has anyone run av gas???

is this to go in a tinny race???

Dunno about increasing power, but we used to decrease 9.9s to 6s by changing the sticker. Made them legal as a kid.

related:




Honestly one of the best machines I've ever owned; a tinny. Super light and responsive. Could trim it that it's just planing on the last 1cm. Park on beach by driving up beach. I learnt a lot from that little boat.