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]

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]

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]

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

[Coonta Kinta]

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

[MR_T]

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 :D

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 :D 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? :p
shorter crank, bored out, wider but thinner piston :D 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 :D

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. :D

bore it out to the max :D

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

haha there's a few ideas if you get desperate :p

[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.

[bender]

jaydon...its nt worth the cash to change the reed block etc in your motor...plus you would neeed the re jet the carbys etc...i went to alexander and son in myree and got the prop on my merc 25 repitched and it got me from 20 knots to 23 on my tinny....redoing the prop on any motor maked a difference...why else would the avon boys spend hours doing it to theor 15 hp motors

[Mandurah Marine]

The 25/30 CV is a 496 cc engine and the 25 standard is 395cc

Before modifying, the most important thing to do is make sure you have the correct prop for the boats purpose. The engine needs to rev at it's upper end of Wide Open Throttle range (set out by the manufacturer) with it's fair average load (fuel, people gear) for what you are using the boat for. You need a Tacho. I use a Tiny Tach - the commercial one as it has max rpm memory so you don't have to keep looking at it. I mount the unit (with cable ties)on the tiller arm of my race motor and cable tie the one wire under the bracket which is under the tiller arm. This thing is installed in less than 5 mintues, race ready and water proof (it has been under a couple of times).
I sell these units for $130 if anyone is interested (remember it is not the standard unit and I give as much advice/assistance as needed with it to get peak performance out of your setup)

Once you know your WOT you can then fine tune the prop, engine height and engine trim - refer to my assistance above.
You might get 2 - 3 knots with just height and trim!

[leachy]

Thanks for the info. I wish mine was the 25Hp blue stripe with twin carbs at least you know its working more effeciently.

I am borrowing one of these tachos of my mate so that I can see what is going on. I had a look at the throttle butterfly last night and it seems that it not opening the whole way but I could be wrong, its a little hard to tell because the bolts that hold the air filter one also hold the carb on. I believe that the motor height is spot on, right at the to of the water, the cav plate is just below and the hydrofoil tips are just visable when running flat out. The reason that I feel it not got the correct prop and is topping out on maximum RPM is I have added front and rear casing decks, three kids and stuff and it still doing max RPM and the same speed according to the GPS. Its on a AllyCaraft 395 Discovery fairly light boat.

Regards
Leachy