3.5HP to 5.0HP Mercury OB carb upgrade

[carlosproa]

Originally Posted by Craig Cape Town

I also have had to change the carb. on my Merc 3.5. (with a Tohatsu carb and a fraction of the price of the Merc carb)


Just be sure to run the carb dry by shutting of the fuel valve, if the motor will be left standing. I also use seafoam in my fuel to keep it clean. Seems to work well.

Thanks
Do you have the long leg like the picture? Have you change the impeller? My model seems to be seized at the threaded rod at the lgear house. am I assuming right that once the nut is released, the lower unit should come out?

[carlosproa]

OK for what is worth, I finally was able to unlock the lower unit.
Since this is an extra long shaft and it has the 6 inch extender, I thought I document in this thread how I finally took it of as is different than a normal outboard

First thing, they replace one of the bolts with a long rod secured with a washer and nut;and there are metal pins to secure the extension to the lower unit

Since this engine has FWD and neutral, make sure to place the engine in neutrals day look for the soft rubber cap on the leg to expose the linkage. There is a 10 mm bolt, take a picture of it and note the exact location with a ruler


Next take upper and lower bolt securing the electrical xtension and lower unit as picture


Then take out the nut and washer and if needed replace the zinc



Theses engines may build a lot of corrosion so using a soft hammer (yellow and red plastic ends) to tap around until you can pull down the unit. This step may be fast or take forever ( 4 hours between two days and tons of WD40)
I had to apply lots of wD40 to the end of the rod, bang a lot - careful on the fins as they are thin and brittle - I broke one so will see if I can epoxy later

So if you are successful then the lower unit will look like this

Then very gentle take the extension out by removing the bolts as shown and carefully tap the extension out



Ok now you see the water pump housing. Mercury recommends replacing the whole thing, I am not doing that
So basically 4 bolts to remove the cap of the water pump and expose the impeller


Take out the impeller and be careful not to lose the key. See how bad this impeller is, and the previous owner did not install a gasket or made a liquid gasket




That is for now, I need to get now the impeller and gasket, grease, etc

[mlydon]

We put a 20hp Chinese eBay carb on a yami 15, and have a stock yami 15, on two identical boats (16ft work skiffs). The “20” will plane the boat and get moving nicely, the 15njust hogs along and won’t plane....same engines, just a different carb.

Matt

[NorthernMac]

I say do it, I see zero reason not to.

[carlosproa]

Originally Posted by NorthernMac

I say do it, I see zero reason not to.

Will have to change prop to metal with a different pitch as rpm will vary

[Letterkenny]

Circling back here - has anyone done this and if so, how is it performing? I’m thinking of trying this and more so just hoping it has better jets so it doesn’t clog as easily as this one.

[kerrydeare]

I suspect that the fuel system is responsible for the majority of issues with these small outboards. I was fascinated to learn about the 3.5 to 5 upgrade available for my Tohatsu 3.5, and several videos showed some of the advantages. The video authors seemed quite knowledgeable and quite pleased with the upgrade, and almost had me convinced to try it.

At present I am sticking with the factory setup, but I did finally manage to fit a second (!) inline fuel filter under the cowling. The first inline filter, which was a complete surprise to me after 13 years of ownership, is located in the thicker hose that runs from the fuel tank to the shutoff. You'd never know it was there but it shows up in some of the diagrams in the service manual. Once you know about it, it makes complete sense. It also reminded me of the combination fuel shutoff/fuel filter that attached directly to the underside of the fuel tank in the old 2 stroke Tohatsu (or Nissan) 3.5 HP motors.

Getting the second filter in there took imagination, but it's indeed possible. It's a matter of figuring out the installation SEQUENCE, and every millimeter of space is important. See photos.

[two-rocks]

I’ve done similar to almost all my small outboards.

Just this week - bought merc 4hp a put the 6 carburetor on it (TOHATSU). Everything else is identical (except the prop pitch). In my case I wanted the 4hp prop, but with more power. Also, these are never just jets (in my experience). The 6 is a significantly larger throat in the carb. Don’t skimp on the carb, avoid the knock offs.

These tiny engines need good fresh fuel, always.

You will notice the difference, but cost/benefit and your goals need to be considered.

[Letterkenny]

Originally Posted by two-rocks

I’ve done similar to almost all my small outboards.

Just this week - bought merc 4hp a put the 6 carburetor on it (TOHATSU). Everything else is identical (except the prop pitch). In my case I wanted the 4hp prop, but with more power. Also, these are never just jets (in my experience). The 6 is a significantly larger throat in the carb. Don’t skimp on the carb, avoid the knock offs.

These tiny engines need good fresh fuel, always.

You will notice the difference, but cost/benefit and your goals need to be considered.

Not sure about avoiding the knock off carbs. I bought a cheapo on Amazon since I kept having issues with my stock carb and seems better built and is running great. Those jets are just too small on the thing. I am likely just going to stock up on several of these cheap ones and almost treat them as disposable for now. Will eventually switch to electric unless they come out with EFI in a lighter weight.