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About two weeks ago, I picked up an old MTA2 from a friend of a friend. It was all stock and had been sitting on a bench in his garage for at least 10 years. I rode it around his yard but couldn't really open it up and he lives on a dirt road so I couldn't open it up there either. So we loaded it up and took it to our high school parking lot for some speed runs. I was kind of disappointed that it only went 26 Mph, I thought it would do at least 30. When I got home, I pulled out my new 47cc engine and big bore kit that I had purchased years ago for just such a project. 2 hours later, I had the engine all done and 2 hours after that, I had it installed and my mess cleaned up. The next day I cleaned out the tank and bought some fuel line, filter and a can of VP racing 40:1/50:1 pre-mix for it. After slowly pulling it over about 4 times and flipping the choke on, it fired right up. I then began the break-in process by letting it warm up and cooling down several times before starting the rev-ups. After about 4 hours of the warm up and cool down process, I began the rev-ups. I did the rev-ups for about 5 minute at a time then shutting it down to cool off before another session for a total of 2 hours. The day after that, I started to break it in under load. I buzzed down the road at about 10-15 Mph until it was warmed up then brought it back to cool down, I did this for about another 2 hours and my tank was just about empty.

The other day, I decided to take it over to my Dad's house and do some speed runs. I was able to get 28 Mph with the new engine on the stock gearing of 11.3:1 ( 6t front and 68t rear). The stock engine pulled all 5'-9" 165 lbs. of me around okay but the new engine with big bore kit really pulled, the difference was like night and day. On my last run, the pull cord and exhaust broke so I called it a day and went home. I went back to my Dad's house today to patch things up and since the engine had plenty of power and got up to top speed so quickly, I decided to drop in a 60t rear sprocket for a 10:1 ratio. When I pulled the wheel off to install the sprocket, I noticed that my upper engine mount was broken. After installing the 60t sprocket, the chain was 3 links too long and it didn't have a master link so I wasn't able to make any runs.

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Damn, bike is a fighter. I guess that's what happens when it sits for 10 years. That exhaust pipe looks pretty decent but add 2 to 3 inches in the belly. The new pipes that can be bought have chambers that are a quarter of that size. I bet the low end pulls really hard. Lookin good!
Keep us posted!
 

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I run 7/66 gears, the 6 tooth #25 is to tight of a corner for that chain. 7/66 gears has a 9.42 to 1 ratio, decent start and more top speed. On some of my hot rod engines I run 7/60 gears, 8.57 to 1 cause it has the power to pull it.
 

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Discussion Starter #5
I run 7/66 gears, the 6 tooth #25 is to tight of a corner for that chain. 7/66 gears has a 9.42 to 1 ratio, decent start and more top speed. On some of my hot rod engines I run 7/60 gears, 8.57 to 1 cause it has the power to pull it.
I ordered some 7t front sprockets, I agree 6t is quite small. I'll leave the 68t rear on, maybe I won't have to cut the chain?. 7t front, 68t rear gives a 9.71:1, should be a good start. I have a new pipe on the way too as well as a new carb with jet assortment, red coil, boost bottle, lightened flywheel and I needed a new kill switch. The guy I got it from had 2 bikes initially but one was toast so he took all the good stuff off and tossed the rest. The box of good stuff came with the bike which included another engine but it must have been sitting outside for a while and got some water in the crankcase. The bearings were rusted and wouldn't budge, even the springs in the seals were rusted so bad they broke. I sand blasted the block and flywheel to experiment with. The only bad thing about these being so cheap is, you can get a whole new engine for $20-$35 more than just the bearings and seals. I used my digital calipers to find out the thickness of the stock base gasket which came in at .018". I'm going to order some .015 copper base gaskets as I'm not looking to up the compression, I just don't want the paper one blowing out. I'm also going to play around with the reeds, maybe make my own baseplate and shave a millimeter off the stop block, should be a good start.
 

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Nice, I like it. Yeah I know you buy a good set of bearings and seals and for sure can get another engine for not much more. Considering the one I bought was a $45 engine. I like where you're headed, definitely want to see the results.
 

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I like you already, way to go with the parts. Amazon has 23,000 rpm C3 bearings, a case of 10 cost around $14, that fit the Cag engine. I am also using blue Sthil chainsaw seals, both you need the numbers of the old parts.
Also, for around $20 you can get a 7 tooth 3rd bearing clutch housing, Its $8 for the pinion gear, plus shipping, for a few bucks more you get an whole thing. It takes an act of God to get an old pinion off the bell. I have a wrench for 6 to 10 pinions but you have to top the bell from turning and its rusted after all this time. There is no compression on the base gasket, only case pressure, maybe 20lbs. Once the pistons hit the top of the exhaust port compression starts. You can leave the base gasket of and use a sealer, like Yamabond, Honda and many other brands.
Lets work on that old engine, and make a beast.
 

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Discussion Starter #8 (Edited)
I made it over to my Dad's today to work on lightening my stock flywheel. The aftermarket ones have half of the fins milled off and then CNC machined in-between to reduce the weight more. I've been building engines for quite some time and the only real way that I know of to get the best RPM gain is to reduce the diameter of the flywheel as well as reducing the rotational drag. With the aftermarket flywheels using the same size fins, just less of them, really isn't reducing the rotational drag, it's just reducing the burden of turning twice as many of them. Think of it this way, if you were a passenger in a car driving down the road and put two folders of the same size, one in front of the other, out the window, it would have the same drag as if there were only one. Now reduce the size by half, it wouldn't matter how many you had, the drag would be less. So with this in mind, I turned the outer lip off my stock flywheel and shaved the fins down to the same height as the starter cog. I still have to true it and balance it but you get the idea, as long as my cylinder head temps are below 200, I'll be happy.

These engines are just clones of the Subaru/Robin NB411 and were intended for use on brush cutters and such. The fins were designed to keep the engine cool for long periods of time. I'm not going to use my bike for circuit racing or long runs, just the occasional run up and down the road and a pit bike if I ever make it back to the track. Modifying engines isn't just about racing, it's about making something that works better for a particular purpose and learning what works and what doesn't, if you're not breaking things, you're doing it wrong!.

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Great job, that's the best way, take it down to the magnet ring. I used to break the fins I wanted off with pliers then grind them down with my Dremel and carbon cutter, lol. Yours is easier to set the .020" magnet/coil gap.
 

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Discussion Starter #10
I like you already, way to go with the parts. Amazon has 23,000 rpm C3 bearings, a case of 10 cost around $14, that fit the Cag engine. I am also using blue Sthil chainsaw seals, both you need the numbers of the old parts.
Also, for around $20 you can get a 7 tooth 3rd bearing clutch housing, Its $8 for the pinion gear, plus shipping, for a few bucks more you get an whole thing. It takes an act of God to get an old pinion off the bell. I have a wrench for 6 to 10 pinions but you have to top the bell from turning and its rusted after all this time. There is no compression on the base gasket, only case pressure, maybe 20lbs. Once the pistons hit the top of the exhaust port compression starts. You can leave the base gasket of and use a sealer, like Yamabond, Honda and many other brands.
Lets work on that old engine, and make a beast.
Just ordered some NTN 6202 C3 bearings and some Stihl blue seals. The seals are pretty generic throughout the Stihl line it seams, part# 9638 003 1581 15x25x5mm. Thanks for the tip, if anything, they'll be easier to see when I grenade the engine and they go flying by. As for the base gasket, I'll have to measure my piston deck height. Seeing how the head keeps the cylinder down and it's a 2-piece head, measuring deck height accurately may require cutting the center out of the head in my other kit to torque it down. The sellers of the big bore kit claim 15:1 compression ratio which I doubt, 15:1 is border line diesel compression and if it were that high, I'm sure I would be getting pre-ignition from the 94 octane I've been using. It does have a ton of compression but realistically, it's probably 10:1-10.5:1.
 

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Very nice, loving this thread. I do remember reading threads from boosted306, he used to do something similar with his flywheels, I'd have to look back for the specifics but I'm pretty sure he milled the outside like you did.
I definitely wouldn't mind having one or two milled like that.
 

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I have used the stihl seals in the past they seem to work okay as long as you glue them in with something there is no ledge for them to sit up against like on a stihl engine and they can just push right through being a bit thinner than OEM seals they don't fit as tight in the crankcase but still a good alternative since whoever is selling the seals on eBay right now is price gouging the shipping...
 

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Very nice, loving this thread. I do remember reading threads from boosted306, he used to do something similar with his flywheels, I'd have to look back for the specifics but I'm pretty sure he milled the outside like you did.
I definitely wouldn't mind having one or two milled like that.
I still have to true it in a lathe and balance it, I used a table sander like the one pictured to get it this far. I tried grinding it on a grinder and it was taking forever, it took about 30 minutes on the table sander.

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Well, damn, I have a table sander. (Head gears turning)
I dont have access to a lathe, yet. Maybe I will take a used one I have and go for it. Worry about turing it in later.
 

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Dude, I have one of those, H/F Tools, use it all the time. You can cut a stock cyld with a hacksaw, just below the fifth fin, then break off the 6th fin and grind that part to a circle, then the height of the cyld. I have a small alum surface plate, I put wet/dry sand paper on it with double sided tape. To finish where the head gasket sits so it flat. A piece of glass can be used also put the sandpaper on it. Don't need a lathe to build a killer engine.
How do you balance a flywheel, it has a heavy magnet and 180 degrees to the counter weight?? Some of my hot rod motors I had to add weight to the flywheel. To help pull start it and once its spinning that weight carries pushing power. Light ones are more for a stock or lightly modified ones.
Here are 2 newer alum pull start housings, the second being the best, I have used both. #2 has a Polini style flywheel w/ levers, you get so much more leverage on the pull.
 

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Well, damn, I have a table sander. (Head gears turning)
I dont have access to a lathe, yet. Maybe I will take a used one I have and go for it. Worry about turing it in later.
I did cheat a little, I cut about a 1/4" off the fins with a cutoff wheel prior to the sander. I was really surprised at how fast it took it down though. The flywheel did get pretty warm so you'll need some gloves. I haven't balanced a flywheel before but I think I might be able to use a model airplane propeller balancer or a lawn mower blade balancer. I was poking around the shop today and found a piece of 1/4" thick aluminum plate that'll work for my reed plate, now I just need to get my intake so I can match it to the one I'm going to use. I keep forgetting to bring the reed stop for modifying, I'll put it by the door tonight so I don't forget it... again. I'll post some shots of the reed stop once it's done.
 

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I picked this photo from somewhere, See the stock and modded.
It just dawned on me that I don't have to wait for my intake, the reed plates are all the same no matter what intake is used... duh!. I traced out the old reed plate and while doing so, I noticed they're only using about 3/4 of the useable area for the opening. My reed stop idea is about the same, I'm using the same contour, just bringing it up to where it's almost to a point. I'm going to use fiberglass reeds, I like them because I can see through them and know there's an even sealing area all around.

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Way to go, why do you need the 1/4" spacer it only adds more space inside the cases, a no no for hp engine. Your going to relieve pressure in stead of build more. That's part of the reason for the full circle crank, it fills up space making the pressure more. That pressure feeds your fuel flow.
 
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