Differences between Mariner 5HP and Yamaha 4HP outboards

[Even Chance]

Hi all,
I have an E-Boat, and she is powered by a long shaft outboard.
I have been using the same 5HP Mariner 2st for years, but this year she went bang when the rings gave way, and a new engine was sought.
I purchased a 4HP 2st Yamaha that was in incredible condition, and it outwardly appears to be the same engine.
The stroke and bore of these engines are the same, the only discernable difference being the prop on the 5HP Mariner is 1/2" more pitch.
Where is the difference in the HP coming from? Is the carb jetting possibly the only difference? Im not a 2st genius, so dont know.

Im asking because the 4HP Yamaha prop has a ding in it, and I would also like the power that the 5HP Mariner had back again. Would swapping the 5HP carb onto the 4HP along with the prop give me the 5HP power in the 4HP outboard?

I hope someone knowledgeable out there can help me out before I just try it out and see.

Many thanks,

Barry

 

[Even Chance]

More info:-

4HP Yamaha serial No- 6E0 L 353854
5HP Mariner serial No- 6E3 L 420264

There is some confusion as to the bore and stroke of the Mariner. 1981 and 1982 versions of the same engine had different cc.

Im now lost, and without lifting the heads off and getting the verniers out I have no idea what the differences are.
can the bore/stroke/differences be found out by the serial number? I hope so.

Thanks,

Barry

 

[MonniotC]

This may be a red herring, as I'm not sure if it's the same engine, but 3 years ago I bought one of the last of the 4hp 2-stroke Mercurys. The dealer told me that to convert it to 5hp, all I had to do was remove a small removable throttle-stop on the throttle control arm (the butterfly actuator on the front of the carb). Naturally I did. I seem to remember that it was nylon, and had a single locking screw, but my memory is a little vague. Take a look. If your engine has a removable throttle-stop, that'll be it. Otherwise, swapping the carbs should do it.

 

[VicS]

What monniotC says about the throttle limiter has been mentioned before. I think maybe tuning it round to allow the throttle to open fully rather than discarding it.

It would be worth looking closely at the carbs to see if they have the same diameter throats. If the old Mariner one is larger then it will be worth swapping them, assuming that is feasible

If you fit a larger carb or simply allow the throttle to open fully it would be worth experimenting with the props not just because of the damage to the Yamaha one but because with the increased power the lower pitch prop might allow the engine to rev too highly. The higher pitch prop might slow the engine somewhat. On the other hand the higher revs would be producing more power just by virtue of the increased speed.

 

[Even Chance]

Hi Chris,
Its sure worth a look, thats for sure. Im only using around 1/3rd throttle max anyway, due to the pitch of the prop trying to push the E-Boat. A larger dia, less pitch prop would also be a better idea, but Im tight!!

 

[Even Chance]

Hi VicS, just posted at the same time as you!
Aye, the prop would make a difference for sure, as I said in my last post.

If the only difference is the carb, the higher pitch prop from the 5hp would also be getting transferred.

 

[aquaplane]

It was about 3 years ago that I bought a 4Hp Tohatsu 2 stroke which was the same engine as the Mariner and Mercury, the Yamaha wasn't mentioned so I don't know about that.

I had the restrictor taken out to turn it into a 5Hp and the bloke played with the mixture to make it run right. I also had a high thrust prop fitted as it was for a displacement hull, that improves performance in astern as well as going ahead.

HTH.

 

[VicS]

I had the restrictor taken out

That's something else to look at. The possibility that the 4Hp has a restrictor plate between carb and manifold.

It is also a possibility that the manifold and porting is different. I have a 6hp Evinrude. It is almost the same as the 8 hp version. The carbs only have different jets I think but the manifold, the reeds and the porting in the cylinder block are different. Externally everything looks the same but internally far from it.

 

[gandy]

The web site "www.ribsmarineparts.co.uk" has parts listings and exploded diagrams for all the Mercury and Mariner engines. A quick look suggests that at least some models have different main jet and slow jets for the 4 and 5HP variants.

 

[Even Chance]

Thanks for the info guys.
I will have a better look at the ribsmarineparts site and see if I can get more info.

Cheers,

Barry

 

[KellysEye]

Don't know if this helps but the only mechanical difference between a 4HP Manriner and a 5 is the carburetor jet has a slightly larger diameter, you can see the diameter difference in the parts list