Truenotch

Truenotch's Tiny Tracker - Yamaha MX100

Recommended Posts

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! 

  • Like 6

Share this post


Link to post
Share on other sites

Forgot to add: 

Current sprockets are 14/48 (3.429:1)

Desired sprockets are: 14/39 (2.786:1) 

18.75% increase in gearing and means I might actually use the lower gears on the open road. 

Share this post


Link to post
Share on other sites

Fuel can, panini storage, tank bag and hypercolour handlebar bag fitted. The rack is from the FS1:

B4C2C47D-5483-4F90-81C1-3A9DF868BE99.thumb.jpeg.1362d0d9f379ea8d83b5f25be709cc5d.jpeg

BECD60D5-753C-40FE-ACF1-49F157053B75.thumb.jpeg.45278bc78ec289e6889f45980786d075.jpeg

I’ll need a backpack for undies and Brian Lowns too. 

  • Like 9

Share this post


Link to post
Share on other sites
5 minutes ago, TimShadboltfan27 said:

Total Andy /ling but gizz 17.5 pilot to have a play with my RX125... will swap you a 30 if you need spare bits 

I’ve got a 17.5 from my spare carb if you want to have a play? 

  • Like 1

Share this post


Link to post
Share on other sites

Yea foreshore and seabed! Keen for campsite carb rebuilds in Waikaremoana tbh. Keep it in your back pocket Jim.
Mine runs a 30 standard and is suuuuuper rich at idle and 1/4 throttle I’m interested in anything that will lean it out down low. 

  • Like 3

Share this post


Link to post
Share on other sites

Hey @Truenotch how does the oil system work? I see you have changed to premix but is the oiling adjustable, or some way you can see if it was working properly? I put a drill to pump oil on both my scoots to prime them/check them. My Suzuki has a non adjustable oil setup but the aprilia is adjustable

Share this post


Link to post
Share on other sites

@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).

  • Like 2

Share this post


Link to post
Share on other sites

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 :)

  • Like 1

Share this post


Link to post
Share on other sites

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. 

46915D59-9F24-42E4-8FAC-22C574962CC1.jpeg.7337729d9dcb17d92c3559687bd149f3.jpeg

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!

E71F555B-B08A-48BF-82C2-B12E3FB9C3C7.thumb.jpeg.2565fbf44ebe31df59cb77d687c6295f.jpeg

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. 

  • Like 6

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.