How to make a DT 125 go fast and handle good

[GMC]

A mate is about to start an incredible, possibly unrealistic adventure.
He wants to hot up his DT 125 to make it go fast and handle good, for $43.65 as the letters to the editor would usually say.

It got me thinking, Yamaha had the DT2MX which was I believe Yamaha’s first production 250 MXer.
So did Yamaha have a 125 version ?
It seems they had the RT1MX & RT2MX which were the 360 versions

Can anyone offer up any hot up tips for a DT 125?

[TM BILL]

All shit aside Geoff fit one of your YZ 125A pipes and a bigger carb , lighten the magneto and Roberts your mums sisters brother 

[Lozza]

Ask Nathan were to send the box of bits.............................

[Nathan S]

Twin shock or mono?

The twin shock DT125 is basically an MX125A with a wide ratio gearbox and more crap hanging off it. It's not hard to make one into a respectable Pre-75 125 racer, especially if you use YZ or MX parts.
That said, the single best mod is a set of 250 forks and triples.

With an Uncle Lozza barrel, stock DT pipe, 32mm carby and YZ-E ignition, my DT is way faster than my stock YZ125A...

Monoshock DT125 is closely related to the IT125 and 1980-ish YZ100.

[NSR]

Here ya go Geoff,  A site with all he needs to know.  Translate and go for it.  A bit more than $43.65.  

http://dtmx-passion.forumactif.com/f5-preparation


    

[TM BILL]

Thats cool 

[bazza]

hey Bill put a couple of sinkers and some trace off it and DJ could ride it

[Canam370]

Let me guess....Dave H, right? I can't think of anyone else who'd derive joy from such a task.   You ought to know better than to get involved in these schemes! Now back to the shed and finish my job.

[Doggy Digger]

I'll bet it's Hammond too.  Of course.

This guy loves little Yammies!

(1972 AT2-MX matches DT2-MX era.   But it was just an AT1 with a GYT kit; while the 250 and 360 were new bikes.  Sexy beasts!)

[Nathan S]

AT1/1974.0 DT125A (444 prefix?) is an interesting choice...

Help us out here, Mr GMC!

[crossedup2]

If it's pre-mono shock going back to the AT1 then there are heaps of bits out there.

AT1 through to AT3 and DT125/175's ran a wide ratio box. The AT2MX (and the MX125 A & B) ran the same full close ratio box and can be had from http://www.nwvintagecycleparts.com/

1    265-17121-01-00    Gear, 4th Pinion, (16 Tooth)(Now #315-17121-0    $10.00    
2    265-17141-00-00    Gear, 4th Pinion, (22 Tooth), (AT1MX,LT2M,LTM        $10.00    
3    265-17151-00-00    Gear, 5th Pinion (23 Tooth), (AT1MX,LT2M,LTMX    $10.00    
4    265-17211-00-00    Gear, 1st Wheel, (34 Tooth), (AT1MX,LT2M,LTMX    $25.35    
5    265-17241-00-00    Gear, 4th Wheel, (24 Tooth), (AT1MX,LT2M,LTMX    $10.00    
6    265-17251-00-00    Gear, 5th Wheel, (22 Tooth), (AT1MX,LT2M,LTMX    $10.00    
7    265-17411-01-00    Axle, Main, (12 Tooth Drive Gear), (AT1MX,AT1       $15.00    

Third gear is common through the lot.

Ignition is simple, as Nathan said, run a YZ 125 X,D,E plate and rotor. Will need some work to mount on the backing plate but the rest is simple. While your at it, take out the 5 old thick plates and replace with YZ125 C,X  6 metal and fibre plates. Works a treat.

Carb and porting are open but 32mm carb is the ticket. A better reed block with a bigger opening is the go to but can't remember which bike it's from. Bring the head clearance down to. It really makes then crack. Pipe as well.

Get a fork brace and the 250/360 forks. (and find a 21" front wheel with the small hub to save weight).
13" rear shocks to find the front end again. (Not the 12") If your super keen add an 1" or 1.5" to the swing arm.

All this stuff is done to my AT1 which I race at Heaven.

Finally, and this is the most important part, don't start this project until the rider has lost weight and is no heavier than 65kg's.

I know from experience that 100kg's is a big handicap.

They are great fun. They are cheap and there are bucket loads of bits every where.

If it's and early mono shock, YZ125D or E gear set goes straight in (same cases), bigger carb, porting, head, YZ ignition goes straight in. Finding the bits will be a little harder as gear boxes are almost rocking horse stuff! Ignitions are too......

This model hot up is possibly the quickest and easiest.

Either way, an hour at a wrecker or two hours at Nathans and you'll have more than enough stuff to start a fun new life throwing around a little old DT!

Peter B

[Tony T]

Watching Mr. Hammond wringing the neck of that little YZ up the hill on the Harrow mx track was a blast. 

If this is another weirdo small bore project, I'd be interested to see what comes of it!

Not many of us old fat bastards can still do a 100 or 125 justice these days........... 

[Tex]

Here ya go Geoff

That's a nice lookin' DT!

I've got a '77 twin shock DT125 and I found that the best way for me to get it to "go fast and handle good" was to get someone else to ride it! 

I'll be interested to see what comes out of this project too.

Tex

[GMC]

AT1/1974.0 DT125A (444 prefix?) is an interesting choice...

Help us out here, Mr GMC!

The 74 DT appears to have the 444 prefix
What makes it interesting?


AT1B   -   1970   -   AT1 Enduro
AT1C   -   1971   -   AT1 Enduro
AT1E   -   1969   -   AT1 Enduro
AT1M  -   1969   -   AT1M Enduro
AT1MB -  1970   -   AT1 Enduro
AT1MX -  1971   -   AT1MX Enduro
AT2     -  1972   -   AT2 Enduro
AT2M   -  1972   -   AT2M Enduro     close ratio
AT3     -  1973   -   AT3 Enduro
ATMX   -  1973   -   ATMX Enduro     close ratio

DT125A  -  1974  
DT125B  -  1975
DT125C  -  1976
DT125E  -  1978
DT125F  -  1979
DT125G  -  1980
DT125H  -  1981

[Nathan S]

Because they're basically an AT1 - a 'quirky' Pre-70 VMX bike, but fundamentally pretty ordinary, particularly among Pre-75 (and newer) bikes.
As the old saying goes "if I wanted to go there, I wouldn't start from here"...
To put it in context, ask whether he/you would want to hot up and AG100, 'cause they're the same basic bike.

If the sow's ear challenge appeals (sorry for repeating a lot of what Peter B has already posted - I forgot he'd posted that up):

Close Ratio gearbox is very useful for MX, probably a significant liability for Vinduro.
Motor shares bore, stroke, crank and basic port lay-out with the YZ/MX so there's lots of potential there.
Too much power will see the gearbox input shaft spin in second gear.
34mm Yamaha forks and triples, and matching 21" front wheel should be considered essential for staying alive above 40kph. If you stick with the stock drinking straw forks, a YZ100 or IT125 front wheel goes straight in and has an alloy rim.
Swing arm is too short, and the rubber bushes make it feel flexy.
Frame is flexy. They should benefit from the old DT1 frame mods, too.
Internal gearing is very short - something like 17/44 sprockets on my old one was still too short for MX. The idea of riding across those paddocks at Harrow on it, causes me to shudder...
The reed block and intake boot are tiny - don't think you'd get more than a 30mm carby in there.
Airbox is also restrictive. AT1M airbox is an improvement, but I reckon it will still limit power.
Bassani pipe seems better than GYT.
CDI ignition from an AG100 goes straight on. I assume the AG's advance curve is rubbish, but you'd be able to swap CDI boxes until you found one that suited.
Footpeg mounts can bend under hard use (my paddock bike bent them all the time, the racer never did!?).
Cush drive rear hub can be swapped with a modified monoshock DT125/175 hub. This makes it pretty much a YZ125A hub, and is legal for Pre-75.

Oh, and because I think I managed to not answer the original question properly:
The short lived 1974.0 model DT125A with the green tank (and maybe other colours) is a mildly rehashed AT2. The few I've seen have all been electric start models.

The 1974.5 DT125A with the brown/burgundy tank is a variation on the 1974.0 DT175/YZ125A/MX125A platform, and is a much better bike.