My first motorized bike build

Discussion in 'Photos & Bicycle Builds' started by Timbone, Apr 27, 2014.

  1. Timbone

    Timbone Active Member

    Deep down, I would like a motorcycle but I can't justify it. Payments, insurance, etc. forget about it. I am a practical transportation kind of guy. You've got a 20K motorcycle that gets 30 mpg? No thanks, not for me.

    I got the idea from various cafe racer motos I've seen. In particular, I was in a bike race and the official race vehicle was some kind of 2 stroke motorcycle/bicycle thing and it's been on my mind for months. I have good mechanical skills so I figured I should just do my own thing.

    I picked up an old fixie from Craigslist for $70. 700c wheels, road bike geometry.I thought this would be the perfect steel frame to work with. Wrong!

    The bike had two heavy steel wheels but high flanges and 40 spokes. The rag joint sprocket combo was not going to work.

    I had to search high and low for a 700c 36 spoke rear wheel with acceptable flange and freewheel. That cost me $90.

    Bought my China Girl kit from for $140 and I am am very happy with what I got. The 44 tooth sprocket was pristine and absolutely true.

    Engine mounting was easy but, unfortunately, I couldn't get the carb on the intake. So I had to remove the engine THEN mount the motor.No clearance issues.
    My second problem arose after I had built the rag joint: the chain would be right into the chain stay. So I had to break it down, reverse the dish,and rebuild and retrue it. Both times the sprocket came out perfectly true and round. Still,the chain was dangerously close to the chainstay. More on this later.

    I may have left one link too many on the chain. More on this later.

    The next major problem involved the clutch lever. It was too tight to fit onto the handlebar so I tried to pry the enclosure just a bit and the damned thing broke! I figured it would be an easy fix with JB Weld but, for whatever reason, the seal did not take. So I tried something else: I screwed the clutch lever into place on the handlebar and it felt good, broken or not. Then I added a bead of JB Weld into the broken part and, 24 hours later, it was a solid hold.

    The clutch itself was difficult because I had no idea what proper operation would look and feel like. I finally figured out from watching a youtube video that the clutch was locked up. So I removed the clutch plate, removed the flowernut and loosened up the mechanism inside. The final flowernut setting was just shy of handtight, leaving about 1/8" of play when the clutch is operated. Clutch works great now!

    The carb was fairly easy.I left the c-clip in the middle notch and mixture seems good as I have a nice chocolate color to my sparkplug. My only problem is that, with the small handlebars I am using (remember cafe racer theme) I had much too much throttle cable. I thought about cutting the cable and soldering a new butt to the cable but I chose an easier way:I just wrapped the cable once around the seatpost. Excellent throttle action and response to input.

    First attempts to start were confusing but I knew immediately that I had a good motor. I had spark and it was breathing nicely. It wanted to GO! First start requires choke but after that this China Girl wants to run!

    The weak spot in the entire system is the chain tensioner. To get a full half inch clearance for the chain,I had to create a big spacer from a stainless steel sleeve. That skewed the rear triangle and the rear wheel doesn't sit perfectly centered into the frame.But I have good frame clearance at all points. But I think I lost a bit of chain length due to these adaptations and I need to take up a good amount of slack. The chain tensioner does a great job of keeping the chain on the rear sprocket but after some use, it yields a bit of tautness and I have later chainwhip into the chainstay. I've lost paint and a bit of metal on the chainstay but there's no noticeable damage to the chain. In fact, this thing is running great. Very smooth! And with an 18tooth cog on the pedal side, this operates very well as a bicycle.I won't be using it for any Cat 3 races this year but I can lock down the clutch and cruise 10 mph very easily.

    Overall,I am thrilled with my motorbike! What fun! I plan to do many commutes during the periods of warm weather.I am a very strong cyclist but practical trips are very difficult on my carbon fiber racing bike. This is exactly what I wanted. I've burned just a little less than a gallon of gas and I have covered probably more than 100 miles. Very pleased! I figure the motor is pretty much broken in now. Oh yeah: at first I had the idle screw basically all the way out. I liked the motor stopping when the bike stopped. But I have since set the idle to PERFECT: idle screw to full close THEN 4 full turns out. PERFECT! I fine by hand at stops.

    My first big problem is to monitor the lateral chainwhip and ensure that no real damage ensues.

    If I were to do it all over, I would have bent the chainstays in advance to ensure a good rear triangle and adequate chain clearance.

    I have a brake, clutch lever combo on the left side, requiring two fingers for each. The clutch requires a good deal of strength so I am thinking of ways to improve leverage.

    My rear wheel is a concern as well. I will build up a new rearwheel/ 44t sprocket combination and replace what I have now. I am thinking 27 inch wheel by 1 1/4", that way I will have a little more rollout (and a bit more speed) and the ability to buy cheap rear tires with plenty of rubber.

    I will also upgrade the CDI soon to Jaguar. I want this engine to last. I will get a good NGK spark plug, too.

    People love this thing! Several guys have asked me where I bought it. I tell them you can't buy it; I built it myself. Then they ask me for my business card. I may be onto something!

    Thanksto all the members of this forum who helped me with my build. great resource!

    Ill try to upload pics from my phone.


  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    Well congratulations, Timbone.

    It sounds like the typical puzzles that the newbie faces didn't slow you down much at all.

    These bikes are a blast. Given the chance, I tend to advise people to keep the speed down and not think of these as motorcycles. I only threw that in, though, because I think it's a good idea to repeat it from time to time. It kinda sounds like you're already thinking that way.

    Have fun with it.
  3. Timbone

    Timbone Active Member

    Thank you, sir! I'll take that as a very high compliment!

    This is a natural for me. I am a bicycle rider and I have excellent handling skills. And I am quite mechanical and creative. It really is special to have a one of a kind item that no one else has. I am gonna keep working and tweaking on this thing and try to turn it into a very reliable transportation device.

    My focus now is the chain tensioner. Of course, I see several different types on this site, and I think using a cog and a strong spring is the way to go.

    I have a short 16 gauge metal piece, about 6" x 2 1/2" to which I plan to attach a idler pulley from an old Shimano Ultegra 9 speed rear derailleur. First tests show this thing to be very strong. I'll drill a hole for the pivot at one end, attach the cog in the other end of the piece, offset the thing about 1/4" to 3/8",then attach the pivot to the chainstay. Then I'll find a strong spring and have it pull the piece upwards from the chainstay.

    Tried to recycle an old mountainbike derailleur but it was too complicated. Keep it simple,but keep it cool!

    No, this is not a motorcycle; I have no desire to go 50+ unless it is on a great downhill on my road bike (bicycle)!

    Thanks again,