Triple carb motors with different jet sizing?

[Tiller guy]

Why wouldn't all the cylinders need the same air/fuel ratio?

[Fastjeff57]

Fuel tends to "run down" into the lower cylinders.

On the earlier Merc triples (that fried their top pistons regularly) Merc used a .0785 jet in the top carb and a .072 in the bottom (it uses two carbs).

Jeff

[LittleCharger]

Each cylinder typically has a different temps and exhaust pulse as well, tune each cylinder on it's own based on piston wash and plug color

[champ20B]

Why wouldn't all the cylinders need the same air/fuel ratio?

That is what I would expect! But perhaps there is a difference in design from one cylinder to the other that warrants such a change?......ex.( one of the crankcases powers a fuel pump or has an oil circulation set up or something, that lowers crank case compression, thus requiring a bigger jet for weaker intake)???

[Tiller guy]

I figured it was temp related, but so far all of the above sound like very good answers.

[Revengable]

Tiller if you run that monster 25 past 6k keep the bottom hole one step rich.

Per this: http://www.groupk.com/tec-rearseize.htm

[Tiller guy]

Tiller if you run that monster 25 past 6k keep the bottom hole one step rich.

Per this: http://www.groupk.com/tec-rearseize.htm

Good read and thanks for sharing.

[Fastjeff57]

Ah-hah! That answers a question that's been bugging me for years: One of Yamaha's triples has a different compression ratio on one of three cylinders. (It's in the sale literature.) Now I know why!

Jeff

[champ20B]

That is what I would expect! But perhaps there is a difference in design from one cylinder to the other that warrants such a change?......ex.( one of the crankcases powers a fuel pump or has an oil circulation set up or something, that lowers crank case compression, thus requiring a bigger jet for weaker intake)???

Well, after reading that one cylinder has lower compression by design and remembering this older reply here of mine, looks like I got the closest if not exact guess! But I figured it was something to this nature. Awsome!

[Powerabout]

One year of the OMC 56 had a cooling issue
the fix was a bigger jet in the middle cylinder

[Fastjeff57]

With the infamous early Merc triples (that I run and love) the top cylinders tend to fry pistons on a regular basis: Of the dozen plus to come through my shop, only two had a good top piston. Merc's solution was to put .0785 jets in the top carb (there are only two carbs) and .072s in the lower. (Normal jetting was .074 in both.)

From running hell out of them, I find that the test plugs run about the same with this unbalanced jetting.

Jeff

[Powerabout]

I'd say its usually exhaust tuning that determines the final mixture in a 2 stroke combustion chamber regardless of what went in via the carb.

[Tiller guy]

I'd say its usually exhaust tuning that determines the final mixture in a 2 stroke combustion chamber regardless of what went in via the carb.

Do you think cutting a tuner down would cause a lean condition? My plugs look lean after modding my exhaust on my yamaha 30hp 3 banger.

[Powerabout]

Do you think cutting a tuner down would cause a lean condition? My plugs look lean after modding my exhaust on my yamaha 30hp 3 banger.
Yes!

[Tiller guy]

Does any anyone know were I can buy a jet kit for a small Yamaha with Nikki carbs?

[ice_spy]

I would try Ebay, you guys seem to have all the parts available in the US.
On the note of exhausts and tuning, why do 2 stroke outboards have a simple flared tube exhaust and housing and not develop an expansion chamber similar to that of 2 stroke motorbikes and jetski engines etc. The amount of power gained from a tuned expansion pipe giving reverbing sound waves holding in the mixture is phenomenal. Is the mid section just too narrow or does the exhaust exit have something to do with it also ? Could you not rid of the exhaust housing and tube and then squash in a power pipe that could be water cooled to reduce heat if needed ? I know it would sound great ! Ring ding ding ding. Power band !!

[Tiller guy]

MX bikes make a lot more power for sure! A Kawasaki KX 500 probably makes twice as much power compared to my 496cc Yamaha 30hp.