technical demand

[costinmusat]

I am the user of an Yamaha F15 CMH Short shaft built 2006 or later
I need your oppinion for increasing the power to 20 Hp.
Do you believe is possible to optain more power by changing some jets
or by changing the entire carburettor
On the front part of the carburettor I have found the code: 6AG10VD25
In advance : Thank you for the answer

[panasonic]

Can you please provide all the information on the data tag which is on the transom bracket.

[boscoe99]

The differences between the F15C (6AG prefix) and the F20 (6AH prefix) seems to be

1) the carburetor itself or

2) perhaps just internal parts of the carburetor such as a) a jet needle, b) the main jet, c) the idle jet and d) the plunger spring

I would suspect that the differences between the F15C and the F20 carburetors ($382.00 for the F20 by the way) are just the four items mentioned in 2) above. Cost for these four is approximately $50.00. More work than just replacing the carburetor but considerable savings.

For fifty bucks and a small amount of work, with a possible 20% increase in power, I would do the modification and see what happens.

To confirm all of the above here is what you do. Go to the Yamaha parts catalog and open up the parts book for your F15C. Then open up the parts book for the F20 using a separate browser. You can then compare all of the parts used in the F15C to the same parts used in the F20. Install the F20 peculiar parts on your F15C and you should have the equivalent of the F20.

[fairdeal]

[QUOTE=boscoe99;138403]The differences between the F15C (6AG prefix) and the F20 (6AH prefix) seems to be

1) the carburetor itself or

2) perhaps just internal parts of the carburetor such as a) a jet needle, b) the main jet, c) the idle jet and d) the plunger spring

/QUOTE]

Just curious - having no experience with this -

how does the extra hp manifest itself?

does the engine then run up to higher rpm?

or does "more fuel" allow a higher hp output - at the same rpm?

[boscoe99]

[QUOTE=fairdeal;138404]

The differences between the F15C (6AG prefix) and the F20 (6AH prefix) seems to be

1) the carburetor itself or

2) perhaps just internal parts of the carburetor such as a) a jet needle, b) the main jet, c) the idle jet and d) the plunger spring

/QUOTE]

Just curious - having no experience with this -

how does the extra hp manifest itself? Would have to be more torque is my guess.

does the engine then run up to higher rpm? WOT RPM red line is the same for both models.

or does "more fuel" allow a higher hp output - at the same rpm? More fuel at the same load/RPM would tend to indicate one is running richer than the other.

I am not saying that I understand it. All I can do is to compare the differences between the two models using Yamaha part numbers. It is a bit bewildering to me, I must admit.

What I can't determine is if the F20 carburetor throttle body has a slightly larger diameter, which it could very well have. Yamaha does not provide a part number for that part of the carburetor. More air, more fuel = more torque and HP.

I was expecting to find either a different CDI or an intake restriction. Did not find either.

The more I think about it, the more I am of the belief that the carburetor throttle body has a larger diameter for the F20. Still cheap for the amount of HP % gain.

[costinmusat]

Transom bracket information

Can you please provide all the information on the data tag which is on the transom bracket.

two different damaged stamps , as I can see
1. YAMAHA- F 15CMH
6AGK S 1006896
H,

2. 2007 11.0
53-60
6 AG-GO model 08

[99yam40]

I too would think it needed more air with the more fuel to get more HP

[panasonic]

I would think that carb has a larger bore then the original. It may have a restricter plate in the exhaust manifold as well.

To get more HP you need more fuel and air moving through the engine.

A fellow I know did it to a 2 stroke a good while ago. The new carb was bigger and I believe there was a restrictor plate in the exhaust he had to remove.

I can't for the life of me remember if it was a 9.9 to a 15 or a 15 to a 20..

I will ask him.

[Captn Dave]

I looked into doing this to an early 90's 150 since the 175 and 200 all had the same block. I found out the 175 and 200's had larger bores in the carburetors but used the same carb body. I believe those two were jetted different. The 175 and 200's also had a different tuner and exhaust plate than the 150 but don't know if they were different between each other.

[panasonic]

[QUOTE=boscoe99;138407]

I am not saying that I understand it. All I can do is to compare the differences between the two models using Yamaha part numbers. It is a bit bewildering to me, I must admit.

What I can't determine is if the F20 carburetor throttle body has a slightly larger diameter, which it could very well have. Yamaha does not provide a part number for that part of the carburetor. More air, more fuel = more torque and HP.

I was expecting to find either a different CDI or an intake restriction. Did not find either.

The more I think about it, the more I am of the belief that the carburetor throttle body has a larger diameter for the F20. Still cheap for the amount of HP % gain.

You have to more Air and fuel to get more HP out of a same block (cc). I know one guy who did this with a small outboard. Bigger carb and remove either a intake or exhaust restrictor, can't remember which.

My experience with this is with motorcycle.Had a KZ1000 and put bigger 28mm carbs on it thinking to get more juice, no noticeable difference. Except running really rich and fouling plugs... Opened up intake manifolds and exhaust manifolds with die grinder quite a bit. Noticeable difference!

Put high dome oversize pistons in 11.5 to 1 and a bit more juice...Some hard to tune after that ..don't know how many times I changed jets and needles..

[boscoe99]

I looked at the exhaust system of the F15C and the F20 and damn if I can find any difference at all.

For all I know (not really known actually) Yamaha might have sized a motor for 20 HP and then simply downsized the carburetor to limit the output to 15 HP. Sorta like putting a restrictor plate on a NASCAR motor. Have not heard of them changing the exhaust of anything else when they do that.

In the NASCAR truck series wiki says they simply installed a smaller carburetor to decrease the output. Maybe Yamaha is following NASCAR.

[panasonic]

I looked at the exhaust system of the F15C and the F20 and damn if I can find any difference at all.

For all I know (not really known actually) Yamaha might have sized a motor for 20 HP and then simply downsized the carburetor to limit the output to 15 HP. Sorta like putting a restrictor plate on a NASCAR motor. Have not heard of them changing the exhaust of anything else when they do that.

In the NASCAR truck series wiki says they simply installed a smaller carburetor to decrease the output. Maybe Yamaha is following NASCAR.

I think you are exactly right. Yamaha made a 20 hp then derated to a 15 by changing the carb to a smaller size. NASCAR restrictor plate races..good example.

I have a 20hp which is really a 25hp, just smaller carbs. I checked all the parts on both and the only difference is the carbs.

I got my eye on a set of 25 carbs on Ebay...might have a 25 soon enough.

But I know in the past there are some engines with an exhaust restrictor as well. I will look and see what Mr Google says about that.

[99yam40]

I think the 2 stroke Yamaha 9.9 is a detuned 15 with a carb and reeds different

[panasonic]

Next time I have my carbs off I going to measure the carb bores and the intake manifold bores and see if they are the same size


I suspect the intake manifold is going to be bigger than the carbs bore. Meaning my 20 is a 25 detuned to a 20.

Choke of the air and less fuel/air from a smaller carb, less juice.

[rodbolt17]

the difference between most of the two stroke 9.9 and 15 motors is the reed box.

that is all on that motor.

some of the old jonnyrude 20,25 and 30's the only difference was the carb venture,main jet and intake.

the only way to tell on the F15 is to simply look at what parts are different and why.