Mariner / Mercury / Tohatsu 2.5hp - 3.3hp outboard

[Fire99]

Evening People,

I've been looking at the differences between the 2.5 and 3.3hp 2-stroke Mariner/Mercury/Tohatsu outboards.

Forgetting the gears aspect and purely the powerhead, they are identical bore/stroke and after going through the workshop manual, they even use the same jet / needle in the carb.

So how does the 3.3 gain it's 0.8hp extra? Does anyone know the definitive answer to where the restriction is in the 2.5?

It's not engine capacity, carb, needle or jet. Anyone know the answer?

cheers,

Nick

 

[Alrob]

I believe it is the gear ratio

 

[Fire99]

I believe it is the gear ratio

That may affect performance characteristics but not HP output I would say.

 

[VicS]

various tricks are use to get different power outputs from the same basic engines.

Restrictor plates, limiters on the throttle opening, different porting.

I have a 6 hp engine which at first glance is the same as its 8hp cousin. Detailed investigation reveals that the two have different crankcases with different inlet manifolds and reed valves!

You ll probably find that the higher power is achieved at higher rpm. Gear ratio and prop size may play a part if that is the case.

 

[misterg]

It's not engine capacity, carb, needle or jet. Anyone know the answer?

Yes.

As I've posted several times in the past, the throttle slide of the 2.2 is too long for the carb which restricts it to 50-60% opening (because the top of the slide hits the top of the carb).

The throttle slide for the 3.3 is shorter which allows the throttle to open fully.

It's obvious if you take of the plastic around the air inlet and look in when you open the throttle.

The slide from the 2.2 can be cut down to allow it to open fully.

The gear ratio in the specs appears different, but in reality the ratio is little changed. I suspect that the gears are just different for the F/N gearbox on the 3.3 which the 2.2 doesn't have.

HTH

Andy

 

[Fire99]

Yes.

As I've posted several times in the past, the throttle slide of the 2.2 is too long for the carb which restricts it to 50-60% opening (because the top of the slide hits the top of the carb).

The throttle slide for the 3.3 is shorter which allows the throttle to open fully.

It's obvious if you take of the plastic around the air inlet and look in when you open the throttle.

The slide from the 2.2 can be cut down to allow it to open fully.

The gear ratio in the specs appears different, but in reality the ratio is little changed. I suspect that the gears are just different for the F/N gearbox on the 3.3 which the 2.2 doesn't have.

HTH

Andy

Cheers Andy,

Must of missed your comments previously but that sounds pretty logical and no doubt the cheapest option for the manufacturers to vary the power outputs.

 

[Fire99]

Quick Update

Hi All,

For all those that are interested, I overhauled the Mariner 2.5 yesterday. In reality it didn't need it as after inspection all the waterways were perfectly clear and gaskets good.
Anyway, I replaced the powerhead to leg gasket and Head Gasket, changed leg oil, new waterpump impeller and waterproof greased everything.

Funnily enough the parts were Johnson parts as they were cheaper. (Thanks Walkers Chandlery)

Anyway, for those still curious, I followed Andy's tips and checked the Carb Slide. Yep full throttle on a 2.5 has the slide a good 30-40% closed.
So I cut around 10mm off the top of the slide and filed down the sharp edges.
Then I lengthened the slot in the plastic where the throttle leaver runs to allow full throttle. You can see in the moulding where that should be.

Anyway, after testing it revs much higher and goes like the proverbial clappers. I've checked mixture etc and all is fine since the needle/jet settings are the same for the 2.5 and the 3.3

So ultimately I still don't have a neutral gear of the 3.3 but i'd guess I now have the power.

Mystery solved and a good afternoons work.