Single Merc Power UP

**microbream** · 05-29-2011

Hi Guys,
I have the following setup here in OZ.
Hull : Fibreglass shallow 5deg V hull, 8ft x 4ft beam . hull weight 55lb (25kg)
Motor: 2010 5hp Mercury, 2 stroke, 6.2ci. (102 cc), 44lbs (20kg), 6000rpm ,8.9:1 CR, 115 PSI comp. cold, 55 bore x 43 stroke.
Total mass: 280lbs (including me)
Prop: Black Max 8 3/8” x 9”
Top Speed: 25mph (yes it does plane!)
Other mods include motor jacked 1.5”

Now I want more power out of the single cyl merc , I don’t want to upgrade as I love the portability and internal fuel tank (also no rego required in OZ). There is no boat ramp near me so I have to lift everything into the water.
I would like to start with the compression raised to 155psi.
I will run it on PULP or E10 (95 octane here in OZ) and 50:1 fuel oil.
Note: Manual say 10% Ethanol is OK to use ?
I will also look at getting the prop repitched and cupped to 11” (some testing required with tiny tach)
After this I am stuck on what to do next. I would like to keep it reliable as I use it to run 2-3miles up the river to get to my fishing spot FAST!
What about carb jets, porting, reed valves (carbon?), timing(advance) , exhaust tuner .
Any help would be greatly appreciated.

Cheers
microbream

**clough** · 05-29-2011

i know you said you wanted to stck with what u got but i know in the late eightys they made a two cylinder 8hp that was the same as the 6hp my brother has its easy to lift and with 8hp youd be laughing for speed...motor my brother has is a 89...best of luck! there are some smart dudes on here theyll get you fixed up

**microbream** · 05-29-2011

Originally Posted by clough

i know you said you wanted to stck with what u got but i know in the late eightys they made a two cylinder 8hp that was the same as the 6hp my brother has its easy to lift and with 8hp youd be laughing for speed...motor my brother has is a 89...best of luck! there are some smart dudes on here theyll get you fixed up

over here we can get a 9.8hp tohatsu that weighs 26kg!

**microbream** · 06-01-2011

Come on guys sharing is caring.
Please give me some tips before i butcher a perfectly good merc.
Does any one know what the stock timing is?
and is it fixed or does it advance with RPM?
are programable CDI's an option?
do they run rich or lean stock?
will i be able to pull start 155 psi compression?
are there carbon reeds available?
can i run more than 6000 rpm (it is undersquare 55 x 43)?
can i spin a 12" pitch prop?
what is the meaning of life? (ahh forget this one)

cheers
microbream (microbass was taken!)

**Lil' Blue Rude** · 06-01-2011

I would think you would be able to pull start it with 155psi pretty easy, as for the reeds check with Chris Carson at chriscarsonemarine . com He claims he'll make reeds for anything, great guy to deal with.
That's my 2 cents. Good luck with the build.

**Mark75H** · 06-02-2011

Originally Posted by microbream

Come on guys sharing is caring.
Please give me some tips before i butcher a perfectly good merc.

Might as well start chopping ... this is not a motor that is raced nor hopped up

You are pretty much on your own

**Seagull 170** · 06-02-2011

25 is pretty good for a 5 horse motor is this the Tohatsu one, what size/ make carb, what lower unit gear ratio, through the hub or over the prop exhaust, is it a liner powerhead & can you still buy cranks for it?

**microbream** · 06-02-2011

Originally Posted by Seagull 170

25 is pretty good for a 5 horse motor is this the Tohatsu one, what size/ make carb, what lower unit gear ratio, through the hub or over the prop exhaust, is it a liner powerhead & can you still buy cranks for it?

yes it is a tohatsu (nissan), carb is a 30mm Kiehin. Gear Ratio is 2.15:1.
has got a though prop exhaust. It is linear and you can definitly buy cranks.
it is still being manufactured today.

**microbream** · 06-02-2011

regarding timing spec says 5 @ idle up to 30deg@6000rpm.
i have read that retarding the timing at high rpm is prefered for motoX, however is this a tuner pipe 'powerband' aid?
I will have a play with shifting the curve up or down but 30 deg sounds pretty good to start with.

**Seagull 170** · 06-03-2011

30mm is a lot of carb for 100cc @ only 6k, is it the standard one?
See if you can borrow a slightly smaller carb 28 or 25 the increased gas velocity might have a positive effect on crankcase filling at up to 7k.
What's the piston to squish band clearance at TDC?

The way to go with the prop with such a light load could be reduce the diameter to increase the revs, introduce some cupping & raise the motor to just before it won't hold on.