Simple HP upgrade

[callmaker]

The 2007 F6MSH, (6hp) and the F8MSH, (8hp) parts listings seem to be identical from plug to prop! I have not seen an F8MSH or its carburetor but since the full throttle RPM is listed as 500 above that of the F6MSH, I will assume the only difference is the partial or full removal of the throttle plate stop (aluminum protrusion facing outside of carb.). The throttle control rod adjustment (VIA - brass screw) can then easily be done, thus increasing rpm and HP. In my mind it makes sense. The carb and its components part numbers are identical. I just bought this outboard, price was right, but I would of rather found the 8 HP. I found the outboard weights to be the same and parts were the same so I theorized the situation. I have done this, the throttle plate does open wider. This would cause more air and fuel to enter the engine. Now thinking about spark advance, but electronics appear same!
I once had a Johnson which was changed from a 10 to a 15 with only a carb. adjustment.
Can any Yamaha folks chime in on this? It is my first Yamaha and it will be used on a Grumman Sport Boat.

[almetelo]

There are a few diferences but mainely on jets

[callmaker]

I went to our local Yamaha dealership and talked to two techs there. They told me the difference in the 8hp is the carb. The purchase of the F8 carb. $300 plus! Just wondering if they are trying to make a sale? I do think I will take the outboard to the shop and have them do an rpm check. After I cleaned the fuel system (the float bowl had the syrup like green varnish) this motor purrs.

[almetelo]

I dont think you'll need the complete carb, just jets.
However I never had these 2 carbs side by side.
I know that on some carbs (using the same block but diferent HP) are wider (intake) but I dont think is the case for such low HP improvement.
I know are a lot mods around made for the F6A/B to F8

Have a look on attachment and take you conclusions

[callmaker]

Thanks for the input. I looked on this site and found my model and year and wrote down the name and number of the "critical" parts that may be changed to affect performance ie; jets and so forth. Part numbers are listed identically on the f6 and the f8 carb. I really think its a matter that makes business sense to the manufacturer. The company would rather sell outboards than jets! Cost of jets, needles etc is about half of what the entire carb costs.

[madharchod]

just remove the plate and it will do the job ,during my time i have changed many plates to make the hp more the last one i did was F80DETl to F100DETl as the only difference was the plate remove it and bingo you ah new horsepower,some people will give you lot of crap but iam an expert on changing these motors while i was in business so now iam semi-retired my children are managing the business but occasionally help out
cheers

[madharchod]

according to the parts manual the part number 9-68T-14271-00 will be in 6hp and it wont be in 8hp or the inside diameter will be different ,rectify that and you have a new hp motor
cheers

[callmaker]

according to the parts manual the part number 9-68T-14271-00 will be in 6hp and it wont be in 8hp or the inside diameter will be different ,rectify that and you have a new hp motor
cheers

***, I must be an idiot! that part has been used for over a decade on everything from a 6hp to the 9.9. (the phenolic part between the carb. and the intake manifold) And what do you mean by removing the "plate"? Surely not the throttle plate. I guess you mean the insolator. I strongly disagree! It is used in the f8 model! I can see modifying the opening in the "insolator" to increase airflow but only for larger horsepower gains. To increase airflow and greater fuel delivery through the main jet.. (venture), all one needs to do is open the throttle plate wider. The internal carb. throttle plate shaft rotates just a bit over a quarter of the possible opening distance on the f6 carb. The f8 carbs. increased rotation widens the throttle plate gap, increasing air flow, creating more (suction) of fuel to mix with higher velocity air. (more air, more fuel, more power) For the increase shouldn't be hard to do.
As I see it the only difference between the f6 and the f8 is the carb and throttle linkage.... the f8s exterior throttle link configuration is different than an f6, no top end stop, the f8 allows for only the idle adjustment, also a longer throttle rod on the f8, this then allows for more velocity of incoming air, creating greater suction of fuel; bottom line, more power = more fuel & air consumption.