My MX250A project.

[Nathan S]

Interesting... This means that my old DT250A has a DT2/3 swingarm - and had it waaaay back in the past (it was an unloved paddock bike when I got it in 1990).
And it means that my freshly painted swingarm is a DT2/3 item.

Edit: My DT's swing arm is same length as the DT2 swing arm, but has a grease nipple in the same spot as MX250 swingarm.
The freshly painted swing arm for the MX is obviously a DT2 arm, as the grease nipple is in the bolt rather than the arm. And has the early style dust caps over the swingarm bushes. Duh! Its funny the evidence your brain can ignore when your brain has already decided that it knows...

Edit2: MX250/DT250 swing arm is ~445mm from pivot to middle position of axle slot. DT2 is ~470mm.
Interesting bit is that there seems to be about 5mm of variation in lengths between examples of the same type of swing arm!

Edit3: Sometimes I really don't understand Yamaha...
The DT2 (with its longer swingarm) handles better than a DT1.
The YZ125X (with its longer swingarm) handles better than a YZ125C.
The YZ125E (with its longer swingarm) handles better than a YZ125D.
Why were they so obessessed with fitting short swing arms, when they obviously knew that the longer arm improves handling?
My inner cynic suggests that it makes it really easy to make the updated model significantly better...

[mainline]

Posted by: Nathan S 
Insert Quote
Slow progress... 

You're kidding aren't you you should have the bloody thing finished this weekend at the rate you're going.

As far as tank schemes go, were the any other private hop-up companies from the era (eg. Noguchi) that you could use for inspiration?

[BJJ]

photos from Jan 22nd look very nice,,, Well done on your progress!!

[Nathan S]

Slow progress... Been stupid busy at work, and gotten involved in rally politics again*, which have combined with the heat and kinda sucked my motivation for going out into the shed...

Anyhow, the bike is to be metallic charcol. Figured that the grey would keep the pesky colour business to a minimum, while the metallic would stop it being entirely drab and business like.


I screwed up with the read guard, somehow. Cleaned up a really awful looking old guard (scraping, sanding, more sanding, etc) and painted it.
Then realised it was a 125 guard... 

Went to put the bearings into the cases tonight, then realsied it was 10pm, and decided against hitting stuff with a hammer...

While its got the usual chain damage in front of the sprocket, this case is the only MX/DT case I own that has the rear sprocket mount intact.

And another detail pic because it looked good in the shed - but looks shit in the photo!    Honeslty, there's no red dust anywhere on it - just something the camera has invented to make me look bad! Note the low-cost lower bearing seal, made from air-ilter foam - it supposed to be a rubber seal, but I've somehow managed to lose all of mine that aren't fitted to bikes...



*But the planets seem to align, for I'm on the right council, at the right time to actually make a positive difference.

[Marc.com]

Ah nothing like a bike of metallic in the fram paint, always works for me. Bright yellow tank would top it off nicely.

[matcho mick]

shouldn't joke about hammers/crankcases/mainbearings late at night,some might take it seriously

[matcho mick]

geezas,you wern't joking!,the bloody seals are in place , Nathan when you took the  "baked" cases out of the stove (to post cure the black case paint??),then would have been the time to take all the bearings out of the freezer and neatly drop them into their respective holes, ,

[Husk72]

Hey Nathan hows the re-build going.
This has totally grabbed my interest,and am looking forward to the next installment of pics,and words.
Looks like it going to be one sweet bike.



                                               Cheers,Mark.

[Nathan S]

Hey Nathan hows the re-build going.

Slowly! Spent a fair bit of the evening 'tinkering' in the shed, rather than getting stuck into it - doing stuff like sorting out bolts and tidying up.

Been trying to sort out gearbox stuff. In the pile from Jim, there were two complete bottom ends, a set of empty cases, and another two gear-sets. I'm using the empty cases and spare bits to build the motor, rather than pulling down either of the MX bottom ends (they've both got sub-standard big end bearings, so cannot be used as-in).
The first gearset turned out to be DT250 ratios, and the second gearset has an exploded 2nd gear.
Managed to confuse the hell out of myself, trying sort it all out... I've got a DT250 bottom end in pieces, and it appears that all three gearsets have different ratios to each other. But I really need to sit down and have a proper look when I'm less sleep-deprived.



Also spent some time working out which bolts go where and stuff. Doesn't make for a particularly interesting photo, but...

The aluminium tube at the bottom will be the body for the muffler. Can't really do anything more on it, until the engine is in and the pipe is fitted up properly.
Gear lever is an IT folding tip welded onto a mangled DT lever - pretty pleased that I turned two bits of rubbish into a useful gearlever (the IT lever had stripped splines).

Discovered that the barrel is currently 1.7mm over size. I'm pretty sure that Wiseco make a +2.00mm piston, but I'm officially on the hunt for a less-rooted MX250A barrell...

[Husk72]

Coming along nicely,the gear lever looks factory.
Keep it up.
Post some more progress when you can.


                                           Cheers,Mark.

[Freakshow]

Wiseco makes it up to 2mm, it will be a 234-P8 (72mm), im sure but i would have a few here so can double check if needed.
Im not sure if that takes you over the 263cc though.....

[Nathan S]

Long overdue update!


(Crap photo because our camera died, so this was taken with my bat-phone).

Lost motivation for a while there - mostly because work has kept being stupidly busy and I arrive home exhausted most evenings. Seeing one of the two other guys at work who do the same thing as me put a router into his thumb, and put himself off for a fortnight, its been super busy.

Anyhow, there's been some trials and tribulations since the last post:

1. The MX gearset I've got doesn't fit into the MX cases I've got - when the cases are bolted together, they lock the gearbox solid. This happens with either or both shaft in place.
Lacking time to sort it out, I slapped a well-used DT250A motor into it. Its not rad or sexy, but it will work (I hope!). I also discovered that the top rear mount is too wide on the DT motor, due to the rubber mounting - of course, you only discover this after you've painted the bloody thing. Some careful hack-saw work had it under control.
Has been fitted with the MX ignition, 34mm carb and MX pipe, so it should still go OK.

2. The forks needed more work than I wanted to think about - or have time for... so I've 'borrowed' the forks & front wheel out of my DT125.

3. The tank was put into the too-hard basket, and I simply fitted a nice DT tank that came in the pile of bits from Jim - well, after welding in an MX filler neck (eagle-eyes might notice the MX cap and DT tap in the photo). After I did it, I realised that there's a simpler, quicker, neater way to fit thescrew-in cap to the DT tank.... Oh well, there's always next time, right?

4. Rear guard is awful... but should do the job!

5. Poverty forced me to fit a TT500 rear wheel that was ear-marked for my DT250, as I don't have the cash spare to buy a new sprocket to suit the MX wheel (TT wheel is same as YZ250A etc, so is pre-75 legal, BTW).

6. The flywheel removal (from the donor motor) was a nightmare. I owe Marty from Wright's Motorcycles a big thank you for his efforts, easpecially as I dumped the jobon him this morning, expecting it to be 5 minutes work... 

There's been a few 1am finishes and other such idiocy, but it might even make it to Broadford this weekend - and if it doesn't make it to Broadford the following weekend, then I'll be very unhappy.

The good news is that it fired on the first kick - but did not start. Getting it to run is a job for tomorrow evening.
I'm suspicious of the carby (seems like the float level is too high and/or the needle & seat is leaky), and there's some wiring to tidy up.

Beyond that, there's just a couple of little things to do: Fit the LHS hand grip (after the bar is plugged), mount the CDI box, and a few other trivial things.

[Nathan S]

And a HUGE thanks to vmx157 for the LHS side-cover at a very generous price.
  x 1000000

(And my apologies for not thanking you earlier - I've "Been being a self-obsessed wanker" as my wife so generously put it...).

[Lozza]

Dairy Farmer's won't be so kind when the Flying Squad catch up with you.....

[vandy010]

another quality whackittogetheratthelastminute bike.
 i mean that as a compliment by the way.
 i reckon i've seen those shocks on more than one of your bikes too Nathan. must be the best value for money shocks going.
a stock DT motor with the MX carb,pipe,ignition etc should still be sweet enough for most riders.
my MX is stock with a DT piston and external flywheel and i love it just the way it is.
smooth as...
cool bike