Yamaha 25 hp

[Hayden]

Hey guys I have a Yamaha 25hp 2 stroke motor and I am wondering can you change props to get more speed I average around 22 knots now
Just a thought I would ask

Cheers
Haydo

[mickeytmc]

Originally Posted by Hayden

Hey guys I have a Yamaha 25hp 2 stroke motor and I am wondering can you change props to get more speed I average around 22 knots now
Just a thought I would ask

Cheers
Haydo

Sure can Haydo, it's like changeing gears in a car; the pitch(which is written on the prop) = distance travelled per rotation. You can go up or down, go up, no weight she'll fly downside, load up, instant pig.

[forsterfisho]

So if your running a heavy rig and want to help speed when under load - go up a prop size and down to increase hole shot and top speed when unloaded?

[BREAM MACHINE]

Get hold of a tacho and see what revs you get at W.O.T check against engine specs.

[quintrex101]

If it's a 25hp Yammy 2 cylinder i think it's the same motor as the 30hp Yammy 2 cylinder but it is downgraded somehow ?

Not to sure ..

Maybe a part could be replaced and you could gain 5hp?

[smokindrags]

Originally Posted by quintrex101

If it's a 25hp Yammy 2 cylinder i think it's the same motor as the 30hp Yammy 2 cylinder but it is downgraded somehow ?

Not to sure ..

Maybe a part could be replaced and you could gain 5hp?

Very good advice
it could just be the intake plate
and then some mixture adjustments

or even just a needle and jets change to up the hp

[vaporboy]

Originally Posted by smokindrags

Very good advice
it could just be the intake plate
and then some mixture adjustments

or even just a needle and jets change to up the hp

wouldn't it be a smaller capacity bore?

[smokindrags]

NO not in all cases
some motors a Detuned version of it,s bigger brother is select hp ranges


but all homework and specs need compared
Really need to get year and compare the motor specs
then if there the same check the carb diam etc and so on

[CK]

The 30 & 25hp yamahas are different beasts with different bores.

[MR_T]

Originally Posted by Coonta Kinta

The 30 & 25hp yamahas are different beasts with different bores.

any idea on 60 and 80 yamas 07 build?

[Hayden]

Hey guys cheers for the info my motor is a 2003 twin carb motor if you need any info cheers

[vaporboy]

I've only worked on motorbikes but heres a few late night creative power ideas

a 30hp cylinder and head might bolt right onto your gearbox/shaft, whatever its called.

on pump gas you could shave a little off the base gasket for more compression.

open the airbox up a bit will need a carby tune.

get handy with a grinding tool, a port job can really wake some motors up .

some kind of stroker kit?
shorter crank, bored out, wider but thinner piston it'll give heaps more capacity.

props definitely can make a big difference but in theory what top speed you gain you lose acceleration?

a good carby tune and clean, if it runs reed cages then they could do with a clean.

have perfect oil/ petrol mixture.

mayb even bore out the carby a mill or two

check all your ignition wires, earths for corrosion, a new plug is always good

i don't know how difficult it could be but take exhaust off (if its got an exhaust/ muffler) and feel how much slop is in the piston and rings, might be time for rebuild. clean the exhaust while its out couldn't hurt.

maybe even make the muffler (if its got one) a smoother flow through it, remove all obstructions, baffles, etc.

evenly shave the flywheel a little = a lot more acceleration.

bore it out to the max

run some really high octane fuel in it but you'll probably have to add another base/head gasket to lower compression, stop detonation.

haha there's a few ideas if you get desperate

[leachy]

Hi Hayden

If you have the CV25 as I do it exactly the same motor as the CV30. Same capacity motor and the same carb. I have posted here before looking for the answer to this. From what I believe its something real simple. Apparently a few years ago Yamaha was only charging $50 difference between the 25 & 30so there cant be a great difference if any, I believe that it is a restrictor plate but if that is the case you need to check the fuel jets as it may run lean.

I too am looking for a taco to fit to the motor to try and work out the correct size prop. Mine is fitted to a Ally Craft 395 Shadow with casting decks. I am sure it is hitting top revs on the weekend it was doing 25 knots / 46 km/h by GPS with me a three kids in the boat, buit the engine sounded like it was over reving. It gets to that speed with about 20% left to go, before you can feel that easy throtle opening last stage.

I was told that there is a taco that just clips onto the high tension lead of the spark plug and works of the inductance of the charge, but I have not been able to find one. There would be the low voltage side of the coil but without a work shop manual it a litle hard to know exactly whtas what. I am also sure the full workshop manual will be the same for the CV25 & CV30, this will also list the different parts.

Regards
Leachy

[leachy]

Hi Hayden

Sorry I didnt see that yours was the twin carb model. Its the better one of the two 25's that Yamaha make, it has the blue stripe on the motor. If I was you I would just leave it as is, my experience is that manufactures build a certain amount of reliability in to motors but the downside is they give up a little performance. I think I would prefer the reliability.

[Justin Boyle]

The 25 and 30hp yamahas are exactly the same. 25kn is pretty good and I don't thing you would get any more out of that with different props. I reckon even 22kn isn't bad either. From what I have read the only difference is that there is a restrictor on the throttle of the 25hp. I'm pretty sure it is just a pin that stops the throttle opening up full. I saw one advertised on ebay once and that was their description anyway. Worth a look to see if that is it.

If you want a tacho i bought a multimeter from dick smith that is an automotive one and that has an induction thing that you use for a tacho. Put it on number one lead and pick up the revs from that. Only problem is if it is too close to another lead it gets a dirty signal and doesn't work properly.