15 to 20 HP upgrade

Bigplumbs · 30 September 2016, 08:07

Any one upgraded their Merc/Mariner/Tohatsu 4 Stroke from 15 to 20 HP

These are all the same engine and all you need is a new carb which costs £350 (ouch) I was wondering if any of you have done this and if you felt/feel it is worth the dose mine is a 15 hp mariner 4s

Dennis

kaman · 30 September 2016, 11:20

Hi I bought a new Tohatsu 15hp 4 stroke in 2010.
After a few years I decided to upgrade it to 20hp as I found it to be a bit of a struggle getting my Honwave 3.5ae on the plane with over 200 kgs onboard. I bought a brand new carb think it was around £260 mark.
At the time the 20hp was around £250 more than the 15hp - so I was not massively out of pocket with the upgrade
There's not a massive difference in performance from the 15hp to 20hp in terms of top speeds achieved typically around 3 - 5 mph gains.
Where I do notice it most is getting the Sib on the plane. I no longer need to shift weight forward to get on the plane when my mate and a load of camping and fishing gear are onboard.
In summary there's no a massive leap in power but I'm glad that I made the modification.
If I could go back to date of purchase I would buy the 20hp from the outset though.

Bigplumbs · 30 September 2016, 15:51

Thanks for the info nice to hear from someone who has done it. 3-5 mph is well worth it I think. Was it easy to do and was there any setting up needed

And also much change in fuel consumption

Dennis

kaman · 30 September 2016, 16:34

Howdy Bigplums
Can't say I've noticed any great change in fuel consumption.
Recently covered 30 mile trip out to Ailsa Craig and back. I filled the tank up upon return and found I had burned just over 5 litres. I don't believe I would have burned any more less when it was a 15hp unit.
I tend to do a lot of trolling when I'm out fishing.
When out exploring I tend to get on the plane and throttle back down - maintaining the minimum amount of revs to maintain planing. I don't like revving the guts out of it for prolonged periods.
The engine now noticeably revs higher at WOT under load, it spins up a bit quicker when getting on the plane now too.
In my opinion it's worth doing.
I ordered a new gasket set with the carb. My mate who is a mechanic kept me right when doing the upgrade.

kaman · 30 September 2016, 19:46

Just noticed your question regarding set up.
No set up of the carb required.
Straight out of the box.
Like for like swap of carbs.
It runs like a dream - no issues what so ever.
That was my main concern - reliability.
My dealer spoke to Tohatsu directly who confirmed it was a straight forward swap of carbs for the 5hp power upgrade, and that the only difference between the two engines was the carb.
I've not encountered any issues regarding reliability.
In summary well worth doing but next time around I'd go fo a 20hp from the outset ☺

Bigplumbs · 30 September 2016, 20:14

Many thanks Kaman I think I will go for it

Dennis

kaman · 30 September 2016, 21:29

Good stuff.
Look forward to hearing how you get on.

Northern Pike · 08 October 2016, 19:09

Cross-reference part numbers. It could be the same carb with a bigger main jet.

Bigplumbs · 08 October 2016, 19:50

I am afraid that my particular engine cannot be upgraded so will just have to be happy with how it is which is ok
Dennis

kaman · 10 October 2016, 17:04

Howdy.
A few questions if i may......
How old is your engine Bigplums?
Whereabouts in the country are you?
Lastly who informed you that you couldn't upgrade it to 20hp?

Northern Pike · 10 October 2016, 17:11

Some years ago I made an OMC 20 into a 25.
I had to remove the restrictor between the carb and the manifold, and install a larger main jet. That's all. More air/fuel, but in balance.
I cross referenced parts lists to be sure.

Bigplumbs · 10 October 2016, 17:47

Quote:

Originally Posted by kaman

Howdy.
A few questions if i may......
How old is your engine Bigplums?
Whereabouts in the country are you?
Lastly who informed you that you couldn't upgrade it to 20hp?

Age is July 2007

I am in Suffolk near Ipswich

It was Barrus UK who informed me based on the Seriel number

kaman · 10 October 2016, 18:54

Had a quick look in the garage
The part number for the 15hp carb as far as I can see is
3BHF A5 016
The 20hp according to the box it came in is
3BH-03100-0
If your engine is the same model / stable mate of my Tohatsu and it has the same 15hp carb I don't see why the upgrade wouldn't be possible.
Hope this helps.

Bigplumbs · 11 October 2016, 05:18

Quote:

Originally Posted by kaman

Had a quick look in the garage
The part number for the 15hp carb as far as I can see is
3BHF A5 016
The 20hp according to the box it came in is
3BH-03100-0
If your engine is the same model / stable mate of my Tohatsu and it has the same 15hp carb I don't see why the upgrade wouldn't be possible.
Hope this helps.

Thanks for the help but I think my engine was the block which was made for the 9.9, 13.5 and 15 hp and is less cc than the current model which is a 15/20 hp. Mine I am told has less cc than the current model.

It actually goes quite well with the little valiant 3.4 so I will be happy with what I have which is very unlike me

Dennis

Fenlander · 11 October 2016, 07:57

I think the advice BP has been given may well be correct. Currently as Kaman has found both 15hp & 20hp are 351cc and basically the same.

Back around 2006/7 the 15hp was a 323cc... there was no 20hp but they jumped straight up to a 526cc 25hp.... hence no simple carb upgrade 15hp to 20hp.

kaman · 11 October 2016, 08:10

Yup that will be right enough then if it's the older version of the 15hp. Oh well no harm trying.
Next time around I'll be going straight for the highest HP that I can get for the given model of engine rather than changing carbs etc later on to up the HP.

Bigplumbs · 12 October 2016, 07:02

I am a little diaspointed that I cant do the upgrade but it was going to cost over £350 and I was not sure if it would have given me much of an advantage. As I said I love the way the 3.4 handles as it is and I have much bigger boats if I want to go power mad

Dennis