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Mini Bike Chain Adjustment Trouble? Answered

I recently bought a bunch of pocket bikes all in good shape but all had some sort of problem.  I took the best parts off of them and made one extremely nice bike.  The one problem I am having with it is that the chain likes to come off.  There is a bolt connected to the back axle that tights and loosens and slcks or tensions the chain.  How do I find the "butter zone" where it isn't too loose to come off, and it is too tight to come off?  The two sprockets are lined up really well, but they are a little off.  Would it be possible that that was the problem?  When I say a little I mean like so small its less then 2-3 mm from drive sprocket to driven sprocket.



Best Answer 8 years ago

If they are not aligned perfectly the chain will have a tendency to come off even if perfectly tighened.

Align the sprockets.

If the sprockets are getting worn out the chain will have a tendency to come off also.

The sprockets are brand new.  The guy never started this bikes engine, and I put a brand new chain on.  They are off by ever so slightly.  Plus I am not sure as to how to align them.  Could I add a washer or two on different sides until it lines up, or bend the frame a bit?  I deal alot with engines but I really just got into motorized 2 wheelers this summer.  Took to them like a fish to water but don't have tons of experience, my knowledge is in the actual engines themselves so guidance is needed.

The sprocket was off by very little and that did it.  But I have a worse problem now.  One side (the side without the sprocket) of the frame was bent a bit, blearily noticeable but it was.  It has started to crack.  It just the part where the rear axle attaches and its just before where a bar goes up for the seat.  Its not a big deal.  Cut it off where its cracked, grind a bit, weld back in the correct location.  Then adjust the chain again probably.

Thanks for all the help!

Some pocket bike frames are bent in on some spots right from the factory!

Not to the point where metal fatigue sets in and cracks form.

Also do you still have the pocket bike? and if you do how do you like it?

I don't have the bikes anymore. I had about 5 and then sold them. I enjoyed them, but I am 6 feet tall and 120 lb, so not extremely tall and skinny, so I didn't have a problem contorting into odd shapes on them. For less flexible people, larger people, and such, probably wouldn't enjoy them. The fifth an final bike I acquired, which I wished I had kept, was a mini dirt bike instead of a street bike the the other 4, now that was comfortable, and I would suggest those to anyone. They have the same engine, but they are taller and have suspension, so no contorting to sit on it and smoother ride. The reason I got more street bikes was because there was a racing league at a local track every Saturday where I lived, and they had different classes, stock, modified, over 50cc...etc. I didn't get into t as much as I hoped though. Still wished I kept the dirt bike one, it was great for cruising the community on.

So as for how I liked them, if you can ride the street bikes comfortably, then 5/5 if you race them at a local track, and 4/5 if you cruise around the neighborhood on them, if you don't find them comfortable, 1 or 2/5. The dirt bike ones, 5/5 for everyone, no matter where you ride them. They are fun little toys though as they can be found on bargain finder type sites for $50 - $100 each.

hmmm yeah I have one of those engines in a eleictric razor mx500 dirtbike ( well I guss it not eleictric any more!)which was turned in to a mini chopper cause of probems ran in to when putting the engine on and hooking the chain up to the rear wheel! That thing is fun to mess around with! So did you ever finsh that custom bike? If you did can I see picture of it?

No unfortunately, the guy that I was getting the frame from took the $50 deposit and stopped returning calls and emails. So after that, I couldn't get air in the sails again. Got a 1963 Ford Fairlane 500 I am fixing up now. Might try and make a few 'Ibles from it on how to do body work, or this and that. I really like this car, but I am still somewhat disappointed that I didn't get to finish the bike, I had some great ideas for it too. Oh well.

Also, that bike of yours sounds interesting. If I had an electric one, I would have converted it to gas as well! Haha!

Ohh that stinks! Those 1963 Ford Fairlane 500s look sweet !Yeah that sounds good to post a few 'ldles on the car. Yeah I'm thinking about trying to put other engine in it so it will be a two engine mini chopper but first I got to look at my wallet lol! Heres a picture of me on the mini chopper I still need to do some work on it.


Looks good! Good size too, not too big, not too small. And wouldn't it be easier to go for a bigger engine rather then 2? There is that bike on the Internet someone made with 50 chainsaw engines...I can only imagine how much a pain in the arse it would be to hook up 50 engines together to run one rear wheel...now that I think about it, 2 would be no problem compared to that! Haha! Good luck, I would love to see it when it's done! :)

How do you think I should hook up the two engines?

Make sure they are identical engines, that would help soo much! That way they rev around the same RPM. Both will need clutches that engaged at roughly the same RPM, then some sort of transmission that couples them without slowing/over revving one down if the other runs faster/slower, but allows power from both to be used. I know some helicopters use 2 engines, spinning opposite directions to balance out. Not sure how to do it exactly...

ohh ok yeah I was not sure if they had to be the same engine or not wasn't sure.Here is a picture from cam2 on pocketbikeplanet on what he did this is what maybe I will do!


The only easy way to go about this, (Maybe), would be to remove the carbs and install a pipe that connects both intake manifolds and this pipe, at its mid point between the two engines you use one larger carb. This avoids problems with having to try and sync up both carbs. Next, remove the clutches, and have the exact same size gear on both engines and a chain to connect them, that way when one starts, so does the other. This would allow you to have them offset, one at bottom dead center while the other is at top dead center, which would reduce vibrations considerably. You would have to have a clutch on one of them to drive the wheel, but only on one, and it would be independent of the chain that connects the 2 engines, so both engines would stay timed together. Also, use one engine as a "Master" the other as a "Slave". The master would control when it, and the other one (Slave) fires, preferably 1 stroke apart, like before to reduce vibrations. Something like this should allow both engines to run perfectly synced, if they are identical engines to begin with.

So to sum up, you would have to make a pipe that connects the 2 intake manifolds and in its center, as to not favor one engine over another, mount a larger carb. Next you would have to weld on/bolt on a gear on each shaft of the same size and link the two engines with a chain. Time both of them 1 stroke apart. Mount a second ignition coil, exactly 180 degrees from the ignition coil on one of the engines, use the coil from the slave engine. If done very accurately this will fire both engines 180 degrees apart and keep everything synced. Lastly, off of one of the shaft mount the clutch, independent of the chain drive linking the engines, and this will what drives the wheel.

Does this make sense? The way it is set up in that picture, you are trying to sync 2 carbs to give engines the amount of gas they need to run at the same speed, impossible (Almost) to do. Because they both have clutches then linked together, when engaged they could engage firing at the same time, which would make the whole bike shake. Also, without having a feedback system of sorts that can adjust each engine with the other, once both engage, the faster of the 2 engines is driving the wheel and driving the other engine faster and therefore will get less power then a single engine. Its surprising, but if one engine but in the setup in the picture, if one engine runs slower, it will actually cause resistance to the other engine, reducing power.

Could you do a quick drawing of it? I get most of your saying. The drawing does not have to be great just something smiple.

Will post something for you by Wednesday night. Been really busy, plus I want to make sure its a decent diagram. :)

So first, weld 2 identical sprockets right to the crankshaft of each engine so they can be linked with a chain to keep both in time together. Add a clutch to one so that the power can be transmitted to the back wheel. The clutch is independent of the 2 sprockets you welded on to time the engines together. Use one engine to create spark for both (Make sure the spark is timed properly for BOTH engines otherwise one could backfire etc...). Lastly, upgrade the carbs to a larger carburetor and link the 2 engines to that via a pipe. Put that large carb in the middle of the pipe so that it should feed both equally.

When doing the spark, you might be able to use the magneto from each engine for them selves, rather then use one to drive both spark plugs. And when you link the 2 engines with the chain, make sure one has its piston as high as it will go, and the other has the piston as low as it will go. This will greatly reduce vibrations.

Good Luck! I want to see it when its done! :D


Hey Thanks! I did not think the pictures and stuff would look like that! Thanks!

That won't work, that will actually give you less power then just one engine. The reason you can't do it this way is no matter how hard you try, one engine is always going to run faster and its going to be driving the wheel as well as trying to turn the second engine faster then it wants to go, so you actually get less power. The only way to do it is to have a transmission unit that couples both engines to one shaft independently of the other.

yeah it would be easyer! But I think I would rather learn how to do a two engine on bike! Yeah I've seen that chainsaw bike but I don't think it moves because I have never seen a video of it moveing!

Yeah that does not sound like that was factory done! lol

Ya! Haha! Factories usually avoid making defective products! :P

If there are bolts on both sides of the rear axle you can align the rear sprocket from there.  Loosen or tighten the one on the opposite side to align the pulley after you have got the right tension on the chain.


8 years ago

I think its the sprockets being out of alignment causing the chain to jump as well... It spins really fast when your at speed. Depending on how the sprockets attatch (does the back one bolt to a hub?) you should be able to shim it... Moving the front sprocket would be a little trickier as on most full size motorcycles it slides onto a splined shaft. Hope you get it sorted out cause that is one sweet looking mini!

You want the chain to move about an inch on a full scale bike, so I'd guess this chain is half the length and you want half an inch of movement (up/down between the sprockets)?
Make sure the rear wheel is aligned correctly



8 years ago

I think Re-design is right, the sprockets aren't aligned. I rode bikes for many years and have dealt with similar problems several times.
Adjust the rear wheel to take out all the slack in the chain across the top of the sprockets. Remove the front sprocket cover so you have access to it.
Using a straight edge, (a 2 foot level should do it on a pocket rocket)  hold it against the face of the chain on the top of the sprockets, down the full length of the chain, from the back of the rear sprocket to the front of the front sprocket.  The chain should touch the straight edge for its its full length. If it doesn't, the sprockets aren't properly aligned.
I don't know on your particular bike if it uses shims or an adjuster nut to correct alignment but whichever it is, correct the alignment so the chain is straight, re-tension it and you should have the problem fixed.