50cc kids' dirt bikes with Morini Franco S5/S6 motors (KTM, Husky, LEM, etc.)

Discussion in '2 smokers' started by TheRadBaron, Sep 27, 2018.

  1. TheRadBaron

    TheRadBaron Been here awhile

    Joined:
    Jul 26, 2010
    Oddometer:
    737
    Location:
    Central Illinois
    There's not a whole lot of good information about these little bikes and their Morini motors out there. I'm hoping that this thread can become sort of a repository for the collective knowledge of these machines. I'm sure there are a lot of dads and uncles out there that know a lot about these bikes and/or motors.

    My 4-year old nephew, Ryker, just started riding this summer and he's already needing an upgrade from the little Suzuki JR50 that he started out on. This kid has natural talent on a bike and I've never seen another kid so driven to learn and ride. He's a machine on that little bike and he can't get enough of it. His dad passed away when he was a baby so I'm stepping in and getting him started with bikes. His mom (my younger sister) is very supportive and I really want to encourage and facilitate his riding. He's not a big kid but he's already outrunning the meager suspension on the JR and he's ready for some more power.
    While looking for his next bike I came across a 2000 or 2001 Husqvarna "Husky Boy" 50 for sale for cheap. Pretty complete but had very low compression and was kinda ratty. I didn't know anything about these bikes but the seller assured me that KTM and lots of other companies used the same motor and parts availability was good. I'm not afraid to work on my bikes. In fact, I prefer to buy them non-running and fix them up. So the price was right, it was the perfect size, it had real suspension, and I assumed it had more power (more on that later), so I bought it. It's a 50cc 2-stroke with a single speed gearbox and a centrifugal clutch. Same as the JR50.
    I figured I'd just slap a set of rings in it, maybe a piston, and for maybe another $50-$100 it would be ready to go. Well, you know how that goes.
    I didn't take any pictures of my bike before I tore it all apart but here's a brochure picture of it and a picture of a used one that I found that looks just like mine.
    huskyboy.jpg huskyboy2.jpg
    IMG_0878.JPG
    Once I got the bike home I started doing some research and found that Husky offered three different models of this bike in these years. The Husky Boy "Boy", the Husky Boy Junior, and the Husky Boy Senior. The Jr and Sr have better suspension and a more powerful motor, and are physically taller. I think the "Boy" model that I have is the best one at this point because I think the other models would be too tall for Ryker, but I was concerned about the lower-performance motor. I was hoping for a tangible upgrade in performance over the JR50 but everything I found suggested that the motor I have is likely very similar in power. So I began researching the motor.
    Turns out that it's an Italian-made Morini Franco S5-series motor. There's some good info at www.morinifrancousa.com about the different motors they made and the different companies that used their motors. I quickly determined that what I had was an S5-series motor. Through that website and a few other I was able to learn the different S5 models.

    S5-E Base model. Iron cylinder with small fins. No reeds.
    S5-N Similar to E model but with aluminum cylinder (plated bore) with small fins. No reeds.
    S5-T Aluminum cylinder, large fins, small reed block with 2 bolts (pretty rare I think)
    S5-GS Aluminum cylinder, large fins, larger reed block with 4 bolts

    My Husky Boy is fitted with the S5-E motor. This kind of bummed me out when I learned it since it's the lowest in terms of performance. Another thing that bums me out is that the parts availability for the motors isn't as good as I had hoped. The morinifrancousa site has a lot of stuff for sale but a lot of stuff is out of stock and other parts that should be available aren't. For example, you can't just buy a ring set for the S5-E. You have to buy the whole piston kit. And they're out of stock on oversized pistons. I did find a company in the UK on eBay that's selling oversized pistons for the E model and I might end up going that route in the future. Wiseco sells a piston kit for the reed valve motors (probably the GS rather than the T) but it won't work for the E model. The reed motors have a window in the piston skirt while the E doesn't.

    A lot of different companies used these motors in their little dirt bikes and quads so that gives you some options for parts searching. Some of the more common ones are KTM, Husky, LEM, and Malaguti. I guess KTM used the Morini Franco motors from 1995-2001 (or so) and they're probably the most common in the USA. It seems that these motors are more common in Europe and I guess that makes sense. Early on in my searching I found a KTM model that used the S5-E motor. It was a 2000 Mini Adventure and from what I can tell it's very similar to my Husky. The cylinder and piston kit was shown as still available and cost around $200. Even though I didn't originally intend to spend that much money I like the idea of a truly new top end so I ordered up the kit from Babbitts online. Bad idea. It took a few weeks to arrive and when it finally showed up it was a mess. It was packaged really badly with parts just rattling around. One of the piston rings was broken in half and the piston wasn't even correct for the bike. It had a round window in the skirt and appeared to be the piston for an S5-T with the small reed block. I contacted Babbitts about it asking if I could keep the cylinder and return the piston kit for a partial refund but that went nowhere. "We can't do that" was a recurring theme. While the customer service wasn't really BAD, that kind of rigid refusal to work outside the box to find a solution is pretty weak. Whatever. So that whole kit is being returned.
    There's a Husky dealer out in Oregon called Bill's Motorcycles Plus that I read had a lot of Husky Boy parts and knowledge about them. I called them up and I guess the wrong piston problem isn't an isolated incident. I guess all the cylinder/piston kits come that way now, from KTM and Husky. Not sure what the heck is going on there.
    #1
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  2. Dao1

    Dao1 Long timer

    Joined:
    Mar 17, 2014
    Oddometer:
    1,999
    Location:
    Austin TX
    In addition to the Mini Adventure , The Morini air cooled motor was also used on the KTM PRO SR through 2000 & KTM PRO JR (shown below) through 2001.

    this pro jr. was my son's 7 yr old birthday present.
    [​IMG]
    #2
  3. TheRadBaron

    TheRadBaron Been here awhile

    Joined:
    Jul 26, 2010
    Oddometer:
    737
    Location:
    Central Illinois
    Good deal. That's a great looking little bike. I never updated my progress here but I have the Husky Boy running and riding. I put it together with a used top end that I got from eBay. It runs pretty good. It has a bit more power than the JR50 it's replacing but the chassis and suspension are vastly superior. Ryker's been riding it around and doing well but he's going to need more power soon.
    I've been collecting parts from the LEM CR2 model that seems to use the S5-N motor which is the non-reed version like the S5-E but with an aluminum cylinder with plated bore. The exhaust is also quite a bit more aggressive looking. Much more like a proper expansion chamber than the little Husky Boy pipe. The piston also has a different skirt profile that I'm thinking will give a bit more high RPM power.
    I've managed to get a CR2 aluminum cylinder, new S5-N piston and rings, CR2 expansion chamber exhaust, and a cylinder head from an S5-GS motor that has a combustion chamber shape that should work better at higher RPMs. I also have some clutch springs that engage at a higher RPM that should help.
    I'll probably be experimenting with the bike and making incremental changes through the fall and winter.
    #3