I recently bought a 1980 model Honda Express NC50. I got it cheap because the starter spring was broken. After a little tinkering this morning I have been riding it around all day. Here is the scoop:
The bad news. You can not fix the spring itself.
The fix. Convert it to a kick start using stuff that is in the case.
Disclaimer. The spring is a wicked piece of wound up metal just waiting to hurt you. Respect it. Attempt this repair at your own risk. If you do not have the knowledge or ability to safely attempt this repair don't. This a description of how I modified my bike. It may not be suitable for every situation or every individual. I take no responsibility for your actions. If you do this you do it at your own risk. You have been warned.
Step 1) Remove the spark plug wire. Clean off the transmission case so dirt will not fall inside when you open it up. Drain the oil out of the transmission case. The third bolt from the back of the case on the underside is the drain bolt. It has a Phillips head as well os a hex head. Open the case fill plug on the front of the case. Remove the starter kick lever.
Step 2) Remove the transmission case cover. Use care and
try to preserve the gasket in one piece. Note which length bolts go in which holes. They are not all the same length.
Step 3) Take the cover to a work bench. Inspect the spring to verify it IS broken. The spring looks like a giant kids toy wind-up spring. If it has snapped you should be able to see both broken ends. If the spring is not broken then STOP. The problem is somewhere else. Leave the spring alone.
If you can see the spring has broken perform this test. Turn the gear that the spring engages by hand. Can you see the broken section of spring turning with the gear without building up any tension? Can you wiggle the gear without there being any tension? If both of these are true then proceed. If the spring is winding up, or if the gear is under a lot of force STOP now.
Step 4) Once you have verified the spring is loose you can start the conversion. Remove the bumper/stop assembly from the case. It is held in by two hex nuts. If one of the studs starts to back out instead of the nut turn you will have to hold the stud with a skinny wrench while you loosen the nuts. After the bumper/stop assembly has been removed you can remove the two studs it mounts to.
Step 5) Check again that the smaller gear the spring turns is free to move and not under any tension. Check to see if the larger part of the spring is fully seated in the case and not popping up anywhere. loosen, but DO NOT remove the three screws holding the gear plate to the case. You will need a T30 security Torx bit to do this. Use a good quality bit. The screws are tight and you will strip out the screws or the bit if you use a cheap bit. Back off each screw 1/16". With a long screw driver pry up on the plate a little. It should come up and be able to wiggle. If it moves by itself, or is under tension then STOP this repair and tighten the screws back down. If all is well remove the screws. Lift the plate, both gears, and the chain out of the case. The larger gear may require a push on the shaft outside of the case. The larger part of the broken spring MUST remain in the case. DO NOT pull up on it for any reason.
Step 6) Take the chain off of the gears. It should be loose and does not need to be broken apart. Pull the broken spring end out of the smaller gear. The larger gear has a ratchet mechanism that allows the gear to be driven in one direction only. We do not want this. Take the snap ring off the gear and remove the spring, washer, and ratchet half. Remove the second snapring, washer, and the gear off of the shaft. Remove the return spring. Re-assemble the gear on to the shaft. Put the washer and snapring back. Place the ratchet half back on the shaft. With a stack of washers, or a piece of metal tube cut to length, take up the distance between the ratchet half and the snap ring groove. This will replace the spring that was there. Replace the snapring. The ratchet should now be locked solid with little play. If you need too add shim washers under the snapring until there is no play.
Step 7) Re-assembly. Put the gear plate, both gears, and the chain back to gether. You will not need the return spring. Set this assembly back into the case. Install and tighten the three screws that secure the plate. Verify turning the kick starter shaft will turn the chain and gears.
Step 8) Back at the bike remove the release lever, spring, and support shaft. It all should just pull out. You are not going to need this anymore. Check for, and clean out any dirt that may have found it's way into the transmission case.
Check to make certain the thrust washer is in place in the final drive pinion, and one on the bigger starter gear shaft.
Place the transmission cover back in place. You may need to wiggle it a bit to get everything in place. Put a couple of screws in the cover to hold it in place. temporarily put the kick start lever on the starter shaft. Turn the lever. If all is right it will ratchet in one direction and turn the engine over in the other. make sure everything turns over smoothly and doesn't rub. Put the rest of the screws in the case cover and tighten. remember to put the short screws into the right holes and the drain screw into the proper hole. Refill the case with fresh oil.
Step 9) Nearly done. Put the spark plug lead back onto the plug. With your foot raise the kick start lever to a comfortable hight. Turn on the ignition and give it a kick. Without stepping to vigorously you can get the engine turn over three times or so. The kick start will disengage as soon as the engine starts. With the engine running the lever will drop down. You can either pull the lever off of the shaft and put it on the bike someplace with a bracket or a strap, or you can tighten up the bolt and leave it on the shaft. If you choose to do this use a strap or make a bracket to hold up and out of the way.
It ain't perfect but it does work and it is a close to free way to get your bike running again.
The next step will be to add an electric starter to the bike. I will write that one up when I get it to work. :o)