Mercury 4hp 2 stroke - increase power to 5hp?

[corinthmistral]

Hi. I have purchased a late model (2003) Mercury 4 hp two stroke outboard. The capacity and weight is identical to the equivalent 5hp model and i think i read somewhere that the 4hp can be uprated to 5hp quite easily. When it runs it "feels" as if it has a bit more to give, and doesnt rev quite as high as i would imagine it could. Hence it has the feel of a restricted motor. Has anyone derestricted one of these (or tohatsu / mariner / yamaha, which are all the same.)??? If so, what is required??? (Why do this? ... i would simply like a bit of reserve power, 4hp is enough to get my tender planing, but it doesnt have any in reserve when loaded with baggage etc. And i wasnt able to find a decent 5hp two stroke motor within 200 miles!!!) Many thanks.

 

[Lakesailor]

I think it is the carb.

Frightening price for the alternative.

I wouldn't bother. Probably cheaper to sell and buy again.
But is it that much of an issue?

 

[VicS]

As far as I can see the slow running jets and adjusting screws are different.
http://www.boats.net/parts/search/Merc/Mercury/2003/1005201AB/CARBURETOR/parts.html

Also and probably the most significant are different stop plates for the reed valve

http://www.boats.net/parts/search/M...B/CYLINDER BLOCK-CRANKSHAFT-PISTON/parts.html

Probably looking at about £40 of parts

But, no you cannot buy them from boats.net. Mercs dont allow them to export to the UK


OTOH the carb bodies might be different...!

 

[agurney]

How about adding a nitrous kit?
Extra oomph when you need it.

 

[Seven Spades]

I bought a 4hp 2 stroke from Fairway in Fareham and they told me they could convert it to 5hp easily, but I did not ask the price at the time because it would have invalidated the warranty.

 

[duncan]

I think you will find it's the hp increase is produced by an increase in revs, and that's achieved through the jetting and set up.

Courtesy of some active t-leaves I have had the 4 and 5 versions, as well as the 4hp/4stroke, all Merc badged. Used on a Quicksilver 310 airdeck, inflatable keel, tender.

Whilst memory is a tricky thing at times I really can't remember a whole lot of practical difference in terms of performance - the real differences come about with weight distribution as you will know. The other is that the 4 stroke is heavier to handle!

Inevitably I ended up with the 4hp stroke, and the one change I did make was to enable the use of a remote tank (left behind by the t-leaves on one of their visits).