Vintage Dirt Bike Q & A

I have lost my spark. I think the plug wire broke inside the jacket. I noticed a nick in the wire right above the plug boot, as I was pulling the plug to check the color of the spark. I also notice that about a 1/2 inch above the boot the wire feels very limp. IF this is the case will coil off of a MT 125 work? how about other coils?

Well I guess I have good news now. It would appear that somebodyhad te kill switch in the off position . So we do have spark after all

I took the flywheel cover off. There is a bolt in the middle of it. I figured that it is reverse threaded because the flywheel turns counter clockwise when you kick the motor. Now i have two small delimas. How do you hold it still to remove the nut, and did I mess up the timing when I turned the flywheel withthe wrech?

your bike has points behind the flywheel. They run off a lobe or cam build onto the inside of the flywheel. You won't hurt the timing unless you remove the points or adjust the backing plate. To hold the flywheel, use a double side open end wrench. Place one side on a web cut into the flywheel and prop the other end against the shifter area. The nut might no be that tight. You could also put the bike in gear and have some one sit on it and hold the rear brake. while they do this attempt to loosen the nut. This is easier than the first way. DO NOT PLACE A SCREWDRIVER OR ANYTHING ESLE INSIDE OF THE FLYWHEEL ASSEMBLY TO HOLD IT. I believe you loosen the nut like any other, it is not a right handed nut. Most of the time you will need a puller to get the flywheel off. You can try to tap it lightly with a hammer an pull it off by hand, but this doesn't work very often. You can buy a puller at sears (claw puller). Also, once you begin to tighten the nut on the puller assembly tap it with a hammer once every few turns if the flywheel is not coming off.

Take your time

thanks

Scott,
What ever you do? Do not use a hammer or a claw puller on the flywheel or you will truly have very bad news . First the flywheel has magnets in it. What happens when you strike a magnet? It loses polarity, not a magnet anymore. As far a claw puller goes, don't use it it will ruin your flywheel. Anyone who has used it was lucky. A previous poster told you about the right tool to purchase on ebay. I will cost you about $15.00. The right tool for the right job will save you time and money. Do your self a favor and be patient wait for the carb kit. I guarentee that is your problem 100%. But if you continue to experiment, you will indeed be parting out this bike. If after you put in the carb kit you are still having any problems. Just post a question for me and I will give you the correct answer. I know how to correctly do all the work you need to on this bike. I know it will run and run good after you rebuild the carb but not until. You are flooding the bike it just will not run right now. In the meantime visit this site http://www.dansmc.com/MC_repaircourse.htm and read. I look forward to your post in about a week saying in is running great. It will be the fuel problem. Be patient. It will run.

First off, I have been following these posts for a fews days and they are getting a little ridiculous. This magnet polarity thing? That has got to be the most stupid thing I have ever heard. From reading all the posts, anyone knows that when a hammer is used on a flywheel it is not to be metal. I have never heard in all my years of a magnet shattering when a flywheel takes a few taps with any hammer. This may be because there is at least an .125 to .25 inches thick steel that the magnets are behind on the inside of the flywheel. Maybe the genious from the last post can explain how the magnets send current to the plug. I personally would love to hear this. Anyway, I don't know how many engines you people have worked on, but the way to solve a problem is to start from the beginning and exam everything and then make a decision what is wrong. The flywheel needs to come off first and then check the entire electrical system. After this check the fuel system completely and make a decision. These engines are not complex at all!!!!

Mike,
No one said the magnets would shatter when hit with a hammer but they will lose their polarity. The following explaintion is for you, Mike. As the magnet in the flywheel spins around the ignition stator coils electricity is built up in the ignition coil. When the points open, it disrupts this electric field, causing the spark. Does that explain it to you. The statement in my last post was for Scott, It appears that Scott needs help; has little knowledge of motorcycle mechanics and may not know any better than to use a metal hammer. Since he stated that he had no spark and realized that his engine stop switch was turned off. I am trying to keep it simple. Since a flywheel for a 30 year old bike, that was in production for 3 years, is diffcult to locate and could be expensive. Wouldn't you agree that it would be wise to use the correct tools? It is obvious from the posts that the problem is flooding and your right in your assessment, spark, fuel and compression. Since Scott already has spark currently there is no need to remove the flywheel. On the MR175 the stator plate is fixed. The timing is set by the point gap. Because of this, the timing will be close enough to start the bike and the point gap can be set without removing the flywheel. Since it is a magneto ignition the voltage is too low to pit the points. On these motors the heels of the points wear before the point surface. So why would you suggest to remove the flywheel, Mike? Now, on to the fuel. Scott stated that when he turn on the fuel petcock, fuel poured out the overflow tube. Do you think it is something other than the float needle and seat? No it is the needle and seat or rust from the tank obstructing the needle from closing. So change the part and put in a inline fuel filter. Now compression, Scott stated that it was a beast to kick. So compression is good. So Mike, your posting was the stupidest thing I've ever heard genious. Scott, Mike is right about some of the posts. But everyone is just ttying to help you. Just change the needle & seat first and we'll take it from there.

If you want to see is the needle valve is bad spray a small amount of starting fluid into your airbox. If the bike runs, not just trys to start then you know that you have a fuel problem. If the bike kicks, backfires, or doesn't start, although you have spark, then the flywheel has to come off as Robert stated earlier because a kewway is probably busted.

I apologize for the polarity comment, I found this while browsing:

3) Via shock: this really only applies to older materials such as magnetic steels and Alnico materials; the mechanism that creates coercivity means that they are susceptible to being demagnetized if enough energy is transmitted through the material via a shock, such as being dropped or hit with a hammer. Modern materials do not suffer this type of problem.

Magnetic Carbon Steel:
These were developed in the eighteenth century. These steels are normally alloyed with tungsten and/or chromium to form carbide precipitates under appropriate heat treatments, which are effective in obstructing domain wall movement. These magnets have a high magnetic saturation, far superior to lodestone, however, they are prone to demagnetisation necessitating the use of long shapes to minimise demagnetisation fields.

Alnico Magnets: (alloys based on Al, Co, and Ni)
This group of magnets developed in the 1930's were the first modern permanent magnets offering considerable in magnetic hardness over the magnetic steels previously available. Their properties rely on the shape anisotropy associated with the two phase nanostructure comprising of ferromagnetic Fe-Co needles in a matrix of non-magnetic Al-Ni. Due to their high Curie temperature, ~850?C, they are still used for certain applications today.

I am not sure that these bikes used these materials since they were used for magnets a long time ago but the site says that some are still used today. Really the only scientific way yo figure out what a flywheel magnet was made of was to have it analyzed.

check out this site

http://www.aacg.bham.ac.uk/magnetic_mat ... istory.htm

I also believe the best mechanic is nothing without the right tools. Some bikes had special pullers that bolted onto the flywheel in order to remove the flywheel, most do not. I think what Robert meant was that you have to use a gear puller (claw) to remove the flywheel. These have wide flat grips that do not damage a flywheel. You have to use some sort of hammer or heavy tool for any type of puller. If you tighten the puller and the flywheel doesn't break loose, the worst thing you can do is keep tightening. A large amount of energy is built up in this process. I have seen guys shoot flywheels 2-3 feet when they don't tap the end of the puller in the early stages of flywheel removal.

The MR and early CR's are notorious for kicking back while trying to start. It is because of the advanced spark. This bike does not have any spark advancement device. So, I am almost postive the keyway is just fine. The claw puller just doesn't work on the bike. I have tried. You can try if your like. I am just trying to save this fellow some time. And yes you are right about tapping the puller lightly to remove flywheel. Deadblow hammer works the best. So now that we know have to remove the flywheel. Scott, I believe that after replace the needle and seat; you will just need to gap the points and that can be done without removing the flywheel. And in answer to your question on value is about $1000.00 restored give or take. These bikes just aren't as popular as the early CR's.

Well the there is no way anyone can claim that there is a lack of passion for these motorcycles on this board

My main concern is that I don't want to spend a ton of money just to find out that one of the crank seal is leaking air. I will buy the carb kit and check the points gap. if that does not get her running I will more than likely put another motor on her until I have the money to rebuild this motor completely. I really feel like it is just flooding out though.

I am also glad to hear that these bikes do not hold a great value. I really want to paint this one red and just use it to ride with my son. Did I mention that this bike was a freebie? the guy just wanted out of his barn. I still have to get a chain for it and that is about it to have it ride ready. She is in pretty good shape i just don't have ton of money to put in the engine. If I can get a running 125 engine for a couple of hundred bucks that would be better for me right now. If I can get this motor running for less, that would be awesome.

Scott, If the bike had oil in the trans case. You should be alright on the crank seal. Trans fluid in place of oil and riding it will help if the seal has shrunk. Also, the color scheme for these bikes are. 75 Gas tank green, white fenders, 76 Red tank, white fender, 77 all red.

It did have oil in the gear box. I drained that out and put in some ATF. I also found out tonight that I am not getting spark on every kick. It does not spark on the 4th kick. Also sometimes the spark is brighter than others although it is always blue. a couple of times it looked like it sparked a couple of times in a row on one kick. What could this be?

I bet y'all are getting sick of all my questions by now

If you are getting blue spark that is good, very good. Don't mess with it. Don't worry about the spark until you change float needle. It could be a number of reasons. If you read the post about how a magneto ignition works, it may become clearer. Your kick starting does not simulate a running engine. It is just to get spark to start the engine. If you have ordered your carb kit you will have by next weekend. Just use the float needle and seat from the kit. Don't change any jets. Your jetting be may fine. The kit is $20.00 including shipping. Honda charges $35.00 for the seat and needle. If you are compelled to do some work prior to fixing the carb. 9 out of 10 times the mufflers will be clogged up. So remove the pipe and take it to a radiator shop; have them put it in there caustic tank ( cost about $10-15) and repaint it. That should get you by for a few days.
Scott, it will not run flooded. You my get it to kick over but a second or two but thats it. Your spark is excellent if it is blue and your compression is good. No matter what you do right now it will not run without the kit. So work on the muffler and it will run very good when your done.