Sprockets Sprocket Alignment Problem 2-stroke

Discussion in 'Transmission / Drivetrain' started by guadalupe, Aug 28, 2009.

  1. guadalupe

    guadalupe New Member

    Hello everyone! I am posting here because I am extremely frustrated. A few months ago I bought a "Jet 66cc/80cc" 2-stroke engine from "gasbike.net". I also bought a 200 dollar mountain bike for which to mount it on. I worked endlessly trying to get the engine to work on the mountain bike but came to a problem with the engine cogs and the rear wheel sprocket not being aligned enough.

    This would cause the chain to come off the engine cogs after 15 or so full rotations. It seems that the the engine cogs are angled slightly so that the rear sprocket would have to be closer to the center of the bike (I am not using the outside rubber spacer and have placed the rear sprocket flesh with the back wheel and have flipped it so the teeth are as close to the center-line of the bike). It seems the rear sprocket needs moved further to the center which is impossible? I have tried mounting this engine on 3 bikes and still I have the same problem, everything else works on the engine.

    bike.jpg

    Sorry I would have posted pics of the bike but I am not home right now. I will post pics asap. It would seem this is the end of the road for this project. :icon_cry: I have taken the engine to multiple places and have been turned down by everyone because of liability they refuse to work on it. Any help would be great.
     

  2. Nitropeewee

    Nitropeewee Member

    with out a good pic its hard to tell whats going on. but sounds like u need to flip the back sprocket around so the dished side is faceing out. try that good luck
     
  3. guadalupe

    guadalupe New Member

    I have tried that. That would only make the chain further away from the center-line of the bike frame when the chain wants to be closer to the centerline. Sorry if i'm not being articulate enough. I wish I had pics. This diagram is the best I can do right now.

    mybikke.jpg
     
  4. Al.Fisherman

    Al.Fisherman Active Member

    Try this

    Try this to see if the sprocket or something is bent. Take the spark plug out, remove the chain and using a good drill attach a socket on the sprocket nut and spin. It's just my $.02 A impact wrench pneumatic or electric would work also. Other then that it's hard to say not looking at it.
     
  5. Ode

    Ode New Member

    I'm thinking that the solution is in the rear wheel.
    You say you bought a new, $200 mountain bike, which may have a boat-load of gears in back, which may mean that rear wheel hub is, over-all, wider than it should be for your application.
    I too, have a Gasbike 66cc motor, mounted on what was originally, I think, a 10 speed Murray bike. (It's been disassembled so many years I don't know what the original configuration was.) I did have to flip the rear sprocket so the teeth were as close to the center of the wheel as possible. I still have the original (?) 5 cog freewheel on the right on a hub I laced onto a rim myself with the necessary offset to keep the rim centered (I needed to upgrade to a 2mm spoke) and have had zero problems with the chain coming off, though I've fussed with the idler pulley to get the chain to feed onto the rear sprocket without any bias or wear. I think the position of that idler pulley plays a big part in keeping the chain centered and happy.
    The fewer gears in the back, the narrower the assembly can be, all the way to a single sprocket on the wheel hub.
    Here in Chicago, with 2 chain rings in front, I'm always only in about 8th gear, with the front, largest chain-ring employed, and have no need to shift to anything higher or lower, so really, I don't even need the 10-speed pedal-driven capability of this rig. I need only one speed to get me going. If I just had the 1 sprocket in back, to get the thing moving away from stop lights, I could minimize my rear wheel hub width dramatically with a single speed hub. The idea of having as many as 18, 21 or more, pedaling speeds on a motorized bicycle is just "teats on a boar hog", as we used to say in Indiana.

    Ode
     
    Last edited: Aug 30, 2009
  6. Al.Fisherman

    Al.Fisherman Active Member

    I did away with my front gear derailleur..changed from a 15 speed to a 5 speed, eliminates extra on handle bars, also don't need that many gears and NO chain problem. If I break down, I'm not about to peddle more then a mile...I have a GOOD cell phone. Maybe one of your derailleur is causing your problem.
     
  7. HoughMade

    HoughMade Guest

    What about the tensioner? Are you using it? It can...and should be bent to feed the chain to the rear sprocket and help deal with slight alignment problems.
     
  8. guadalupe

    guadalupe New Member

    Pics

    full_type.jpg
    full_pic.jpg
    DSC04020.jpg
    DSC04024-1.jpg

    Here are some pics! I had my friend over who has been doing bike-related stuff for years and he said it might be the rear sprocket. The rear sprocket is a little uneven so that might be the problem. But still the engine cog seems to want to send the chain into the middle of the bike instead of along along the side of the bike...
     
  9. Scotchmo

    Scotchmo Member

    There are a couple of things that you can do to solve your problem. Move the chain guide back some so that it is closer to the rear sprocket. With the pulley removed, use a large crescent wrench to twist the bracket some so that the pulley is running close to parallel with the rear sprocket. Install the pulley and recheck the alignment again. Loosen the bracket and rotate it in or out to adjust. Then secure it tight. That should allow your chain to feed correctly onto the rear sprocket.

    The chain tensioner is also a chain guide. Some bikes have rear forks that are fairly straight. Yours spreads out at an angle. That is why you need to put a twist in your chain guide. The chain guide keeps the chain on even when the sprockets are somewhat misaligned.

    In the last picture, you have the sprocket bolts pointed out. If you have enough clearance, it should be OK but most people install them in the other direction.
     
    Last edited: Sep 13, 2009
  10. guadalupe

    guadalupe New Member

    "There are a couple of things that you can do to solve your problem. Move the chain guide back some so that it is closer to the rear sprocket. With the pulley removed, use a large crescent wrench to twist the bracket some so that the pulley is running close to parallel with the rear sprocket. Install the pulley and recheck the alignment again. Loosen the bracket and rotate it in or out to adjust. Then secure it tight. That should allow your chain to feed correctly onto the rear sprocket.

    The chain tensioner is also a chain guide. Some bikes have rear forks that are fairly straight. Yours spreads out at an angle. That is why you need to put a twist in your chain guide. The chain guide keeps the chain on even when the sprockets are somewhat misaligned.

    In the last picture, you have the sprocket bolts pointed out. If you have enough clearance, it should be OK but most people install them in the other direction."

    Hey thanks for responding. I think that your comment has been the most informative yet! I originally had the sprocket bolts pointed inwards but it became a HUGE hassle to tighten untighten them with how much i was trying to readjust that rear sprocket.
    I'm not sure what you mean when you say "rear fork". The chain guide (not currently on in the pics I took) seems to do an excellent job feeding the chain to the rear sprocket but since the chain feeds into the engine sprocket from the top this doesn't seem to help much? Also I fear my rear sprocket may be uneven, would this create a big enough problem to be throwing my chain off everytime? Thanks!
     
  11. Dilly Bar Rob

    Dilly Bar Rob Member

    guadalupe, what do you mean by your rear sprocket being uneven? Does it "wobble" from side to side (bad) because of the rag joint, or is it slightly off center (chain tightens & loosens a bit, not as bad) ? Just get it as good as you can (looks like you have), I would think that most members here with HT's (without shift kits) have slight sprocket "off-centerness" with no ill effects.

    The sprocket that came with my last kit seemed to be machined a hair "off", the teeth actually seemed to be a tiny bit higher on one side then they were on the other. It wasn't enough to cause any trouble, at least in my case.

    You can try to touch up the teeth on the sprocket with a file or small angle grinder, helps the chain to sit on it better (too much of that bling-bling chrome from the factory :whistling: ). I didn't have problems with the chain coming off but filing the teeth definitely made the chain run smoother and more quiet.

    One more thing I can think of - you say that the engine sprocket seems to want to "feed" the chain into the rear wheel. Does the engine look like it is centered on the frame properly (looking from the top)? Perhaps one of the mounts in the engine casting could be a hair off from factory causing the engine to sit at a slight angle to the frame....
     
  12. Scotchmo

    Scotchmo Member

    The rear fork is the lower frame tubes that extend to the rear wheel. That is where the chain guide is mounted.

    I see the chain guide in your picture so what do call the chain guide?

    Is the chain jumping the front sprocket (doubtful) or is it jumping the rear sprocket? Move and adjust the chain guide as I suggested and it should be OK. If your sprocket is wobbling a noticeable amount, adjust it too.

    The correctly adjusted chain guide can compensate for considerable front to back misalignment.
     
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