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Truenotch's Tiny Tracker - Yamaha MX100


Truenotch

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I've been chasing the cause of the seize since last weekend and have done a couple of things. The bike acts lean on the overrun, which is usually caused by a vacuum leak or a lean pilot circuit in the carb, so I've been through a few things in search of the lean hang. 

  • Replaced both crank seals - no change 
  • Set float level in the carb - no change
  • Changed the reeds over (they weren't seating completely flat on the block)
  • Changed 17.5 pilot jet to a 30 from another carb - it's now very rich up to 1/4 throttle, but still hangs and pops on the overrun
  • Refitted air box properly (it was loose when it seized and could have been leaning the mixture) - still hangs
  • Disconnected the oil pump and changed to premix

I highly recommend Blackwood Yamaha in Cambridge for Yamaha parts. They were super quick to respond to my queries and I had my crank seals on the same day. Their prices are reasonable too. 

I took it for a  hoon yesterday after work and it didn't seize, so that's a bonus. I'm using the old rattly cylinder for testing so I don't have to care if it seizes again. It went pretty well and cruised happily enough at 70-75k, but felt like I was wringing its neck at 80+. When it seized last weekend, I was doing 85kph, so I must have been pulling a few revs! 

It's hard to find information about the MX100, but the specs I've found online for the older DT100s  say that the max power RPM is 7500 (pre 1980 DT100s have the same engine but with points instead of CDI). I did a few calculations to see what revs I was pulling at 85kph: 

551700441_ScreenShot2019-11-16at1_36_35PM.png.13dc48250c5076ca1b61c99e23c0db42.png

(Black is standard gearing, red is my new planned sprocket size)

At 85kph I would have been doing around 8300rpm! This is probably a bit hard on an old engine that's only designed for 7500-8000 max. The red numbers in the table show the RPM for the rear sprocket I want to fit - this will sit me at about 6700 at 85kph which is a much more manageable number! It will also allow me to wind it out to higher speeds when I need to. 

Here's a link to the wheel speed/RPM calculator I used: http://woodsware.aciwebs.com/gears/

I'm not convinced that the problem is fixed yet, but it's looking promising. I'll head to @Geophy's place on Sunday to use his ultrasonic cleaner to make sure the pilot circuit is nice and clean. 

Here's my list of what might have been the problem: 

  • Lack of lubrication resulting in high cylinder temps (trusting the original oil pump) 
  • Too many RPMs 
  • Air box boot not fitted properly - LEAN
  • Partially blocked pilot circuit - LEAN
  • Piston to cylinder clearance too tight on new cylinder
  • Ring gap not set correctly
  • All of the above 

I'll get the new cylinder back onto it and will go for another hoon and see what happens. It's under 2 weeks until the big ride, so the pressure is on! 

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@peteretep - it’s a little pump that runs off the primary reduction gear / main clutch gear and it’s operated by the throttle via a split cable. As you wind the throttle on it opens the pump more and you can set the min/max position of the pump.

The pumps can wear our and get weak, which I suspect has happened here. I’ll probably rebuild it later on for ease of use.

I tested mine by pulling the hose off and idling the bike with the pump turned to max and it gave a dribble so I figured it was good. The proper way is to set the RPM and measure how much comes out (which I didn’t do because no tacho).

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Gave the carb a good clean with @Geophy's ultrasonic cleaner today. Blew out all the jets and passages and re-assembled it with the standard 17.5 pilot jet. Whacked it all back together and the hang + pipe bangs were exactly the same. 

Then I wound the idle screw out as far as I could so it wouldn't idle and it sounds good: 

Maybe the pipe bangs are a normal characteristic for these motors when the idle is cranked up a bit? 

Took it for a hoon and adjusted the mixture screw a bit. It goes really well! I might have been chasing a red herring by looking for a lean issue with the carb, but at least now I know it has a very clean carb, new crank seals, correct oiling (premix). 

Next job is to gear it up :)

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New sprockets are fitted and I’ve had some successful testing :) . 

Glad I didn’t gear it up any further because it’s only just able to pull 5th gear at the moment. You’ve got to hang on to 4th for quite a while and then bang into 5th at 80k or so. It’ll cruise on the flat at 85-90 and I’ve seen 96kph on the gps with a bit of a downhill run. 

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Popped the “good” cylinder back on and took it for a hoon out to @Archetype‘s place for a few laps of his paddock. It’s fun on the gravel and grass!

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It died on the way home and I was shitting myself thinking it had seized. Turns out it had run out of fuel, so I flicked to reserve and powered on. 

Unfortunately I didn’t reset my odo after fueling up, but I think I got around 60km from the 4.5l tank. Hopefully a spare 5L will give me enough range for Te Urewera. 

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  • 3 weeks later...

This thing got a good dose of mud and dust during Te Urewera Undertaking a couple of weekends ago. It performed great and didn't miss a beat over the whole 550km distance! 

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Only issues to report were the rack failing on Day 1 due to too much weight bearing onto the rear guard. It bent rack far enough that the rear wheel caught the numberplate and folded it upwards (folding the guard in half at the same time). The guard pulled straight (enough) and my tools rode in the service wagon for the rest of the event. 

Notice the relocated number plate too. Hah. 

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The other issue was on day 3 when my fuel can (now strapped to the seat) came loose and blocked my exhaust! It happened just after going through a ford, so I was looking for wet spark components. The plug, coil etc were all dry as a bone and it wasn't until a few minutes later that I noticed the fuel can was blocking the pipe. It was super close to burning through and could have turned me into a fireball :D

 

Such a great way to spend a few days and I'm already hanging out for the next one! It's given me some inspiration to start working on the blue MX again, so I'll start chipping away on that over summer. 

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  • 5 months later...
  • 2 weeks later...

Just measure how long they are and then search the measurement and the word shock afterwards on the internet. Try trademe, aliexpress etc. For example if I wanted a 280mm pair of shocks I would search "280mm shocks" on websites I mentioned before. Make sure they have the same mounting points as your old ones and you're sorted.  

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  • 5 months later...

I've decided that knobbly life is not for me, so the holeshot has been retired and there's now an SR241 GOLDENBOY gracing the rear of the MX. @Mitch.W and I went for a test hoon yesterday and the bike felt fine. The rear feels WAY better on the road and at least as good on the hard-ish gravel we were hooning yesterday. 
20201123_170404.jpg.5d0bb7cea8d7dc917b06d4c017c7c34b.jpg
 

Aliexpress fuel can arrived today too. Good timing! 

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Revised paneer for the other side: 

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The rear wheel bearings were pretty loose in the wheel, so I grabbed a new set and @BLIZZO kindly punched my hib and goo'd the bearings into place with retaining compound. Should feel a bit more solid after that treatment! 

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