C08
Well-known member
The 4hp 2 stroke outboard block looks to be the same cc as the 5hp. Any idea whether it is possible to uprate the 4 hp to 5 hp on the mercury engine and if so what the conversion consists of?
Uprate a 4HP Mercury 2 stroke?
- Thread starter C08
- Start date
LONG_KEELER
Well-known member
C08
Well-known member
The two that I have done, a 25hp Tohatsu was just a restrictor plate removal and a 9.9hp Yamaha was a bigger jet and a ticle on the ignition timing. Hopefully a real expert will be along soon.
macd
Active member
I daresay that someone will be along with the precise 'fix'. In the meantime, if the restriction is either a restrictor plate or a slide which cannot completely open, a peek through the carb venturi with the throttle wide open should quickly reveal it. If it is either of these, it's possible that re-jetting would be necessary once the restriction's removed. A flick through the parts book for the 5hp version should show this. (It might also show a different slide, although it may be possible to modify the original if it's this that restricts the venturi.) As CO8 wrote, there may also be a timing tweek. Ignore such a possibility at your peril. That said, even the 5 won't be in a giddy state of tune.
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VicS
Well-known member
The carbs are different but the difference(s) may only be due to 2 or3 components which can be identified from the the parts breakdowns. However I cannot actually confirm that the carb bodies are the same .
The reed valve stop plates are also different.
C08
Well-known member
Thanks vicS for your (as usual) informed comment. That is a pity-what I am trying to get is a back up 5hp 2 stroke long stroke engine and these are now pretty rare so I was thinking that as the block is the same maybe I could get a 4hp engine as an alternative and uprate it. I will have a look at the parts list but it does not look promising if the reed valves are different anyway.
VicS
Well-known member
If the whole carb has to be changed then its not a realistic proposition as that would be whatever $200 translates to. I doubt if that will be necessary and probably you are only looking at the equivalent of $25 for the carb parts and another $5 for the reed stop plate.
Changing the reed stop plate though will involve splitting the two halves of the crank case!
macd
Active member
This thread may throw a bit of light on the subject: http://www.ybw.com/forums/showthread.php?285971-Mercury-4hp-2-stroke-increase-power-to-5hp
It includes links to parts diagrams, courtesy the ever-estimable Vic.
It includes links to parts diagrams, courtesy the ever-estimable Vic.
fdimu
New member
I want to uprate 4hp mercury 2 stroke to 5hp and checking parts list I need;-
5HP;-
Carb 3303-812648T
Idle adjustment screw 10-812935
Pilot jet #52 16251
Reed valve stops 34-17666
It is a pain that I will need to split the crankcase to replace the reed valve stops but makes sense that the carb jet lets more fuel in and the reed valve opening wider allow more fuel/air mixture in.
Not aware of any other differences.
So will post progress here.
5HP;-
Carb 3303-812648T
Idle adjustment screw 10-812935
Pilot jet #52 16251
Reed valve stops 34-17666
It is a pain that I will need to split the crankcase to replace the reed valve stops but makes sense that the carb jet lets more fuel in and the reed valve opening wider allow more fuel/air mixture in.
Not aware of any other differences.
So will post progress here.
ChromeDome
Well-known member
Old thread revival - 8 years actually!
Still:
The biggest difference is probably the self-deception when you think you notice a difference.
I don't know what the change involves or costs, but self-deception is hard to put into a business case
Still:
The biggest difference is probably the self-deception when you think you notice a difference.
I don't know what the change involves or costs, but self-deception is hard to put into a business case
Mark-1
Well-known member
Gotta love a thread revival.
I once restricted a 62hp Motorcycle to 33hp. It was the same model as mine own which I'd been riding for years so I was used to the bikes. As you say I didn't notice the difference, turns out all those years I was never at the very top end of the Rev range where that extra 30hp was.
I'm sure it would be the same with a Mariner 4hp 2T. I've got a 5hp 2T with the high thrust (low geared) prop and I'm sure I have never had it absolutely revving it's nuts off where that extra 20% of horsepower is lurking.
