1974 Yamaha MX 100...No Spark!

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5/17/2012 7:13 PM

So when I bought this little bastard it didn't run and the guy said it was running 6 months previous to that....blah blah blah we've all heard that bullsh*t!!

So far I've soldered in a new condenser, replaced the points and gapped them, cleaned any rust on the flywheel and I've had good ohm readings on the coil and everything seems to be in working order. With the plug in the cap grounded to the cylinder head, I've kicked it over a zillion times and I get nothing.

The recommended plug is a B8EV, would a B8ES make a difference?

Would should I look for? I've eliminated the old kill switch to simplify my troubleshooting. I also order a new condenser as I'm assuming the one I soldered in might be bad...it came in a NOS "ND" brand tune up kit.

Thanks guys!

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Do It In The Dirt!

5/17/2012 10:12 PM

Plug cap and coil wire have a good connection? Try another cap. Are the points breaking at the right time?

One of the best things you can do is to keep your eye out for a late model DT100MX - RT100 CDI system. They bolt right on .

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5/17/2012 10:17 PM

I used a 1980 DT100MX CDI system on this bike. 73 LT-3 MX with a 74 MX100 engine. Worked like a champ.

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5/18/2012 5:59 AM

If all else fails, give me a shout.

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5/18/2012 8:38 AM

Project Racer is the ignition Guru and what he says goes but, finding working ignition parts that last has always been a problem for me. Buy that PVL from Newmann and you can forget about ignition problems. Plus, he has the best prices in the World.

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5/18/2012 12:40 PM

Like what everyone else has said but a couple of things to check is disconnect would be the kill button, then remove the plug cap and check spark at the coil wire. Also you can remove the plugwire from the coil and check spark at the coil. You said you removed the rust from the flywheel? What did you use after the steelwool or sandpaper to remove the rust? I would reccond spraying it with either disc brake cleaner or contact cleaner. Check the coil where it is grounded to the frame also.

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5/23/2012 5:00 AM

That internal flywheel ignition Joe's offering will make that thing so much more fun to ride.....

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MOΛΩN ΛABÉ

5/24/2012 8:17 PM

I just got another stator coil for kicks and bolted / soldered it in and no luck. Everything seems to check out fine with my OHM meter but I've still got no spark. I should just bite the bullet and buy that PVL, but it's hard to justify spending that much when it's almost twice what I originally spent on the bike. I'm only racing the bike once next month before I put it aside until I have the money to tear it down and restore it. Decisions...decisions....

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Do It In The Dirt!

5/24/2012 9:32 PM

There is a source coil,condenser, points,and the coil under the tank. That is the whole system on the MX100.

First thing is to check the lobe that breaks the points for wear. If the points open and close as you rotate the flywheel, you should be ok in that area.

Is the flywheel marked with a "T" and an "F" mark that aligns with the case mark? It should be. You want to make sure the points are breaking as the T mark aligns with the case marks. The F mark is too late. Use a piece of celophane cut into a strip with a bit of duct tape on one end for a handle, slid between the points while closed, as the T mark on the flywheel starts to align with the case mark , you will feel the celophane release as the points begin to open. Snug the point set screw down.

This should get it close enough to make fire, with good components. A new source coil can probably be had for relatively little money on Evil Bay. With a new condenser,points, and ignition coil, probaly have $100 in total parts, that thing should make spark.

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5/25/2012 2:44 PM

I've got one of the 100 enduro models....maybe a 76 or 77 out here in the shop. Is that the same ignition? I'll pull the parts and you can use them for your race if you want. Pay the shipping and they're yours. This thing ran when I parked it a couple of years ago.

It's just a future resto waiting to happen, so I'm happy to let you use the stuff.

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MOΛΩN ΛABÉ

5/25/2012 3:38 PM

You guys are a good bunch!! Let me see what I can figure out here in the next few days, maybe I'll buy / try another spark plug coil or I'll take up the DR. on his offer or maybe I'll bite the bullet and take up Newmann on one of those PVL's?!

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Do It In The Dirt!

5/29/2012 5:05 PM

Question #76,238

So I have two spark plug coils I've been using while trying to get spark. The first is the original that came with the MX100. The second is a newer spare I bought for my MX360.

While looking up spark plug coil part numbers, I noticed the MX100 part number is different than the part number for an MX360 spark plug coil.

Would this make any difference? I'm wondering if the original MX100 spark plug coil I have could possibly be junk and the new MX360 spark plug coil might not work for an MX100 for some reason?

Am I onto something? Haha

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Do It In The Dirt!

5/29/2012 7:24 PM

At least I have a box that contains about 50 coils, stators and cdi boxes all for the 74-78 CR125's to choose from. Of course, when one doesn't work I throw it back in the box. Maybe that's why it never getts easier??

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6/5/2012 7:26 PM

WE HAVE SPARK!!

Not sure if any of the things I replaced possibly helped, but one of the problems was the wire that solders to the condenser and makes contact with the return spring of the points was accidentally grounded to the backing plate of the points. OPPS!!

Now I'm trying to get this thing to kick over and I'm wondering if any of you MX 100 specialist have any recommendations? "Project Racer"?!! The carb is the stock mukuni, I've gone through and cleaned it very meticulously, the main and pilot jets are stock and clip on the needle is on the third slot from the top. I've also got the slide height adjusted so that when I turn the adjusting screw in, the slide just starts to rise up.

My MX360 fires on the first or second kick cold with the choke on. I'm trying to do the same with this 100, but it seems like I'm fouling plugs...maybe no choke?

I've also replaced the stock reeds with boyesen, the cylinder was bored and a new piston, rings and etc were installed.

Wish this damn thing would just fire right up!!

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Do It In The Dirt!

6/5/2012 8:19 PM

Zeke27G wrote:

WE HAVE SPARK!!

Not sure if any of the things I replaced possibly helped, but one of the problems was the wire that solders to the condenser and makes contact with the return spring of the points was accidentally grounded to the backing plate of the points. OPPS!!

Now I'm trying to get this thing to kick over and I'm wondering if any of you MX 100 specialist have any recommendations? "Project Racer"?!! The carb is the stock mukuni, I've gone through and cleaned it very meticulously, the main and pilot jets are stock and clip on the needle is on the third slot from the top. I've also got the slide height adjusted so that when I turn the adjusting screw in, the slide just starts to rise up.

My MX360 fires on the first or second kick cold with the choke on. I'm trying to do the same with this 100, but it seems like I'm fouling plugs...maybe no choke?

I've also replaced the stock reeds with boyesen, the cylinder was bored and a new piston, rings and etc were installed.

Wish this damn thing would just fire right up!!

You might take some of your pre-mix and put a small amount into the cylinder thru the spark plug hole, if you have compression and spark it should at least try to fire.

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6/5/2012 10:20 PM

IT'S ALIVE!!!!!!!!!!!!!!!!!!!!!

I played around with the carb for awhile and though I'd pull the tank to connect the kill switch into the connecter near the ignition coil. I had disconnected it while shaking down the stator problems I was having. I thought I'd also bolt back in the original ignition coil just to see if I still had spark with the original coil. I kicked the bike over with the plug up against the cylinder and I realized I had a lot more spark than the re-pop china ignition coil I had bought off of evilbay. I threw the tank and seat back on, said a little prayer to the gods of motorcycles and on the second kick, the little bastard fired right up!! A quick carb adjustment and I went for a cruise down the street at 9:30PM! Haha! I'm sure my neighbors hate me even more, that 100 is louder than my 360!! I might have to throw an one of those DG generic retro silencers on it. Or not!! Haha!

I'd also like to say THANK YOU for all the help guys!!

Cheers,

Zeke

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Do It In The Dirt!

6/10/2012 3:08 PM

So I've been doing some test rides on the 100 and it's sort of a turd out of the hole!

I'm sure the poor thing could probably use a new clutch, but what other things would you guys try changing? The gearing and jetting are stock so maybe I could make a change there to wake the thing up?

I really have to rev it out to get it to accelerate decently.

Any ideas?

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Do It In The Dirt!

6/10/2012 3:15 PM

Zeke27G wrote:

So I've been doing some test rides on the 100 and it's sort of a turd out of the hole!

I'm sure the poor thing could probably use a new clutch, but what other things would you guys try changing? The gearing and jetting are stock so maybe I could make a change there to wake the thing up?

I really have to rev it out to get it to accelerate decently.

Any ideas?

Add a couple of teeth to the rear sprocket, or a cheaper way is go 1 tooth smaller on the counter sprocket. That was pretty standard on most of the Yamaha's back then.

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6/10/2012 4:25 PM
Edited Date/Time: 6/10/2012 4:26 PM

That motor makes hp from about 9000-11000. What power band were you expecting?

Yes, it's going to sound like your wringing it's neck to make it go, but that's what a 100cc motor does....just do it and hang on!

Oh, and one tooth smaller CS is a must....

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MOΛΩN ΛABÉ

6/10/2012 5:24 PM
Edited Date/Time: 6/10/2012 5:27 PM

I'll get a smaller countershaft sprocket and wring it's neck!! Haha

Thanks guys!!

Also...sometimes when I start it, (it likes to start without the choke) the idle runs up and I have to adjust it down using the slide height screw. After a few turns of the throttle the idle settles down too low and I have to adjust the screw back in. I'll then do a few laps around the block and when I'm ready to slow down and stop, the idle is so low, the bike wants to die.

Any ideas?

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Do It In The Dirt!

6/12/2012 10:21 PM

The little yellow beast is scootin' now!!

I dropped the front sprocket one tooth and replaced the pilot jet. The bike starts up and idles fine and it really accelerates well now!!

I'm looking forward to restoring this little beast and beefing up the front end.

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Do It In The Dirt!

5/22/2014 4:10 AM

Hey zeke what are you running as far as your jetting? I've played with mine and I can't get it right for the life if me. I have the oil injector removed.

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