Another MTD 31cc Build

soq

New Member
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1:07 PM
Joined
Oct 8, 2023
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6
Location
British Columbia
Hey there fellow enthusiasts,

I'm building my second friction-drive contraption with the well-known Ryobi/MTD/Craftsman 31cc 2-stroke. The first was a real redneck-class bodged-together mount with a used but reasonably good motor. The only modification was to retard the timing a few degrees. With 1.5"dia. peg it was pretty fast (50km/h or so) but not so good on the hills. Lasted quite a while, probably on the order of 10k km, but compression was slowly lost due to overheating, which may have been from a lean mix and other factors.

I've got the new motor all disassembled and cleaned. I've deleted the base gasket with the intention of using a tiny bit of Loctite 609. In this configuration the squish is .0315", transfer is 104 degrees, exhaust is 134.3 degrees. I'm replacing the stock muffler with something much less restrictive, but which should have some back-pressure. I'll be using the stock carb for the time being. I'll probably raise the exhaust port to 150 degrees duration, which translates to 1.88mm of grinding. I understand some people really go nuts and raise the exhaust-port beyond 160-170 degrees on other motors, but I really need more low-end torque versus high RPMs. I doubt I'll widen the exhaust port at the cylinder at all -- I'd rather have a little restriction at the entrance to the port anyhow.

The transfer ports have the slits which I'm advised to fill with high-temp JB-Weld, but I'm looking for other solutions. One that comes to mind is tapping the slit for M2.5 or so and using a screw with no head and a little RTV to fix it in place. Raising the transfer port looks like a job for a milling machine, which I don't have. So i'll content myself with matching the transfer ports between crank and cylinder and widening the bottom a bit to give it a smoother profile. There isn't much material there to begin-with so enlarging the transfer ports is not really possible without major risk of breaking through.

The rings are a little rough with scratches parallel to the stroke, I can feel them with a finger-nail quite easily. I'll probably replace them as new rings are quite cheap ($6.00 each plus shipping. The bore has a nice glaze, and while I can see scratches roughly matching the marks on the rings they can't be felt. Wear on the cylinder is just enough that I can just barely feel the transition beween the stroke area and the skirt where the rings do not touch, so I assume the jug is good. I've not run it yet, but it is producing spark.

The piston looks good; I'll probably polish the face close to a mirror-finish to reduce heat transfer from combustion because it's easy to do. The piston rod is another matter. It's the stamped-type with a triangular hump in the middle. The old motor (which I don't have) had the other type with the turned-out edges, and if my recollection is good, the metal was a lot thicker. I didn't think the old rod was very strong and the new one looks even weaker, so I'm about half-way committed to making a new rod out of steel or aluminum, but I don't yet have any 3/8" stock, which is about what's needed to start with. Doing the holes for the bearings looks to be the trickiest part given that I don't have a lot of specialized tools but I think I can work around that. Any thoughts on this would be appreciated; I've seen one picture of a hand-fabbed rod made of steel so at least it looks possible for a DIY hack.

The aluminum cowling is removable on this motor, which makes it easier to fabricate a new cowling. I'll be using 1/4" plate (1/8 on the outside face) for the motor mount and around the edges in a square box which will have an opening to blow air over the head and ignition coil. Air will be inducted through the hole made for the peg. I did not have this on the last motor, which pretty-much relied exclusively on road speed to get air moving over the head. This was problematic on large hills where pedaling was necessary as you'd get up near the crest of the hill where motor power was nominally sufficient, but by the time you'd get there the motor would be hot, speed would be low, and you still had to get over the crest. That I think I ran it too lean didn't help either. The cowling will double as the attachment point to the hinge mechanism used to hold the peg on the tire.

The crank bearings sound like they are running dry. The seals are pretty tight. I diluted some synthetic grease with naptha and tried to work it into the bearings. It worked a tiny bit and I achieved a significant reduction of friction until the naptha evaporated. I'm guessing there are two bearings in there with a gap between them in the housing, and if so it should be possible to drill and tap a small hole so I can fill it with grease and an air bubble. The air bubble will tend to force the lube into the bearings as temperature rises. If there's on big-ass needle bearing in there some other solution to lubrication will have to be found. Again, thoughts on this would be appreciated.

Once the thing is tested and mounted I'll post some photos.

Edit: One of the things I'm struggling with is locating appropriate fasteners for the build. It's not too difficult to find SS flush or truss heads (in metric) for Allen keys but I'd really like Torx... Most of the ones I've located are intended as replacements for existing products, such as Etsy which has a lot of titanium screws. Amazon has a few here and there but they're difficult to locate. Grainger in Canada has a few useful candidates but they're all in hex, which I can live with but Torx seems to be more durable. Can anyone suggest a supplier that has a decent selection of different head types and lengths?
 
Little motors can pack a punch. Ive got a 2 stroke scooter I swapped an auger motor on it. It had tons of torque, if you goose it it will throw you off the back. It goes about 30mph. I rode it a time or 2 but im probably going to use it for parts because of how dangerous it is
 
Little motors can pack a punch. Ive got a 2 stroke scooter I swapped an auger motor on it. It had tons of torque, if you goose it it will throw you off the back. It goes about 30mph. I rode it a time or 2 but im probably going to use it for parts because of how dangerous it is

30 mph isn't all that fast unless you slam into a wall. Sounds like you could have reduced the gear ratio on that thing. I'm not so interested in getting more top-speed so much as improving the performance so it will climb a steeper hill than the last one.
 
30 mph isn't all that fast unless you slam into a wall. Sounds like you could have reduced the gear ratio on that thing. I'm not so interested in getting more top-speed so much as improving the performance so it will climb a steeper hill than the last one.
It gets squirreley on that little scooter. The going 30mph on 10 inch wheels is not as fun as it sounds. Ive got the scars to proove it 😆
 
It gets squirreley on that little scooter. The going 30mph on 10 inch wheels is not as fun as it sounds. Ive got the scars to proove it 😆
When you said scooter, I didn't think of a scooter you stand-up on. That would be a bit too much; I'd be worried about a death wobble on something that small. How did you mount it?
 
When you said scooter, I didn't think of a scooter you stand-up on. That would be a bit too much; I'd be worried about a death wobble on something that small. How did you mount it?
It was a pre made. It wasn't a death wobble ad much as the front tire blowing from road debris. I luckily walked away wit a bit of road rash, some skinned nuckles, and a few other scrapes. It blew out at 15mph so it wasn't as bad as it could have been.
 
It was a pre made. It wasn't a death wobble ad much as the front tire blowing from road debris. I luckily walked away wit a bit of road rash, some skinned nuckles, and a few other scrapes. It blew out at 15mph so it wasn't as bad as it could have been.
I was riding on a downhill grade in the early AM with sub-0C temperatures and WOT and the pucker factor was pretty serious. I never crashed the bike but at full speed it was plenty fast. I was still and idiot and rode it around without a helmet too often. Fortunately I know how to ride from a youth spent riding a normal bike in city traffic like the proverbial bike courier. The brain develops radar so you always know where everything is around you and surprises are rare.
 
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