Sonic's Workshop

1974 Yamaha MX80 Dirt Bike
by Sonic

We as humans are a funny lot. We have strange habits, believe in a wide assortment of wonderful things, and oddly enough we rarely listen to those who can usually help us.

Firstly though I'll give you the low down on the Honda 400F featured in the last article. As it turns out, after many hours of grief by all involved, it was discovered that the floats on the bike were made when brass floats were no longer the fashion and the new fashion of light plastic floats had not yet come into being. The floats on this machine were made from something still unknown to me. This stuff was probably wonderful for the first 15 years or so, but as with everything, age eventually takes it toll (Yeah, I need glasses now. What? Who are you? ... where am I? Is it medication time yet? - Ed). It appeared that this material was absorbing the fuel in the carbs, not enough to cause gas to fill the float itself but just enough to make all the floats a little bit heavier. This increase in weight would allow more fuel into the carbs, effectively causing them to flood. But only slightly. It wasn't quite enough to stop the engine, but just enough to cause it to produce black smoke from the exhaust (Do these sound like the words of a desperate man? - Ed).

After the floats were changed, in the now million dollar carburetors (on the hundred dollar bike!), it ran much better, yet still exhibited a slight amount of black smoke. I later learned from an OMG reader, that the pilot jets were originally fitted with rubber plugs. Our carb set had these plugs removed at some point, either by accident or by the adrenaline pumped youth of by-gone years. Any how, it seems to be running better, at least for a twenty year old machine with an apparently original power.

Anyway, over to my Virago. For anyone who hasn't experienced the joy of a fork seal leak,

Virago fork seal leak
here's what one looks like. So if you run outside now and discover oily little rings residing on the upper fork tubes of your bike, welcome to the Blown Fork Seal Owners Group. Aren't bikes special?

Enough about my bike, over now to this issues subject. It seems to me that as youths growing up, we always knew better than our parents, and now as adults we still feel like we know more than the person who's trained in what we think we know it's all about. I can take my own tree down ... I can't believe how much a new roof costs! I do my own taxes. Yup, we know much us humans we do. Enter my all knowing pal ...

"Wow Sonic said he couldn't find a good kids bike for under 800 bucks. Couldn't have looked too hard, 'cause I found this great 1974 Yamaha MX80 for $300. Talked him down to $150. I'm better than Sonic, I'm better than Sonic". I'd expect more from my best friend some how, but I guess we're all bargain hunters at heart.

The bike...

I foolishly became involved in this 'project' while chatting over some other business with said pal, and the topic of bikes came up. Can you believe that? He confessed he'd brought, excuse me, 'traded' the equivalent of $150.00 worth of good stuff to buy this MX 80 that was having "a few problems". Walk away, walk away. Don't get involved ...

"What's it doing?" I asked. Big mistake. My ex-best friend began to describe the list: "The back brakes don't work. The front ones barely stop you. The front wheel rocks back and forth between the forks. The clutch cable broke the other day. There's oily little rings on the top part of both forks ... I think it might need new tires and a tune up. Oh, and it runs but it's really loud. What-da-ya think?" SS SSU SSUCKER! Sorry, but sometimes you need tough love.

As far as 23 year old kids bike goes, cosmetically it wasn't bad. It still had its original

Front sprockets:
New (Left), Shark teeth (right)
Yamaha 'bumble-bee' paint scheme along with original black and white seat. It did indeed run and the rear shocks were still good! Okay, that's the end of the good points. Now, appearing for an extended engagement, The Bad Points! Listing from front to back; Front tire bald, cracked, and of fifteen inch diameter size (rare). Front wheel bearings shot along with the front brakes, brake cable, brake lever (half missing), both fork seals (worn so bad that the steel in the seals themselves had been exposed and worn away the chrome finish on the fork tubes), meaning ... fork tubes shot. The list goes on ... Steering head bearings shot, handle bars bent, clutch lever broken, clutch cable chaffed (and now broken), no air filter, left hand engine cover chipped out at the place the clutch cable mounts, breaker points cooked! The front sprocket had a few teeth remaining, while the back sprocket ... sharks teeth mate! The drive chain had been over used and under lubricated, the back brakes were shot, back wheel shot, silencer missing, and the rest of the exhaust pipe had been cut apart and butchered. Well, gotta go. B-bye now.

Exhaust adaption
Fortunately his wife wouldn't let him put their daughters on it until it was safe. I think they'll be 32 by then. I somehow got the idea that this would make a good story, especially right around Christmas, when all of us know it's all running around with a big heart, good intentions, and clouded judgment (Bah humbug. Nothing like a bottle of rye and a lonely dark room ... sniff - Ed). A bike like the one I've just described is great if you have the time and knowledge to do it yourself, or if you're looking to spend some quality time with your son or daughter building up a piece of history. Unfortunately in today's Nintendo world of click-a-button-and-it's-done, most kids expect it to run perfect in the first five minutes and may not have the patience to stick it out with you. My own kid loves his machine and takes good care of it, but I know it's tough when we're doing routine maintenance on it and his friend comes to play. "Finish what you started, then you can go", is not what he wants to hear.

I was really quite surprised at the fact that most of the parts we required to rebuild this little bike were still available from Yamaha. All the cables, bearings, assorted bits and unique bites were in stock (and in Canada) and were reasonably priced.

I started with the motor, draining the old transmission oil and removing the flooded gasoline from the crankcase. The breaker point was replaced, which is incidentally located behind the rotor - requiring a Yamaha rotor puller (why put something that needs to checked and changed regularly behind something that is a sod to remove at the best of times?). The L.H.S. engine cover (the one that supports the clutch return spring) was inspected to see if it could be saved. Not! Welded then. Nope! It's made from magnesium so it just melts. How do I know it melts?... I was able to weld up the oil injector mount on the intake, saving us some cash, and realizing that if that had a bit bigger crack in it the motor would likely have seized. With the motor back in I rebuilt the carb, pretty good at it now after the 400F thing. Brass floats - good. Not leaking - even better. The carb was replaced on the intake and the bike was fired up.

HOLY COW IS THAT EVER LOUD!! Ears still rigging I pulled the front end off and disassembled the forks. Yuk, the springs actually looked like solid tubes, nothing like a spring at all, The gaps

Sad and abused...
in the spring were completely packed with oil, dirt, and sludge to the point that it was a solid mass. Seriously, the stuff would not even wash out. I had to use a flat blade screwdriver to pick the stuff out and then soak the springs in solvent. Once thoroughly cleaned the tubes were checked for straightness and then sent out to the metal platers for re-chroming, which was cheaper than buying new tubes. The broken front fender was replaced with a saw-modified 81 Honda XR80 jobbie, which I painted yellow to better match the bike. The front brakes and wheel bearings were replaced with Yamaha factory parts, but the tire was proving to be a bit of a bother so it was left for a while until one could be located. The rear brakes were done at the same time as the chain and sprockets. A new tube and rear tire were installed also. A new stock Yamaha exhaust pipe with a slip on after market silencer was installed to tame the noise problem, and the bike was treated to new levers and cables all round. The L.H.S. engine cover had to be replaced due to years of rough use .... and some recent melting. The front tire was finally located through Dunlop's office and was sent to our local tire place. The old tubes between them had 11 patches on their war torn skins - unbelievable! A tube for that bikes wheel size was $7.90. Nothing compared to the safety of your kid.

Cracked front tire
The project took from July 2 until November 19, on and off, and I must say looks pretty good, although I'm not so sure about my friend. Parts old or new cost money, usually lots of it. I was really quite surprised at how reasonable most of the stuff actually was for this little antique. Everything including oils, grease, taxes and parts was $523.18. Amazing, since I figured the stuff would be discontinued. The free labor we're going to discuss later.

For an old bike that had sure seen better days I'm pleased with it's performance. The motor still has plenty of kick, the transmission is fairly smooth and most of all doesn't slip out of gear. Yes, we humans are a funny lot. I guess if I'd looked harder I could have found myself another project much sooner, but I didn't listen to those folks telling me about the bike that's in a neighbours garage that once belonged to this guy ... except he moved away and these other kids had it a while but dumped it into the creek ... and then their older brother fished it out and kept it for a summer, or was it two?... I didn't have to listen - I have 'friends'.


© 1997 Canadian Motorcycle Guide