Hi all,

I'm new here so first of all hi, and thank you in advance for any help provided and I look forward to reading more of your posts.
I’m based in the uk and always been a triumph man and whilst competent in building and maintain good British bikes I’m completely new to these lovely stink wheels and become quite smitten with them.

I’ve obtained something of a bitsa AT2 and I’m slowly turning it something more akin to a stripped back ATMX ready for some fun both on and off road.

The bike came set up for premix with the autolube system removed and I’m happy with that as I would like the whole bike to be as light as possible (I broke my spine a little while back so lightweights are the way forwards now).

So ….this leads to my questions and advance as I fear I may spend money in the wrong place or worse ultimately end up with something that’s not as enjoyable to ride as I know these bikes can be. 

The bike is fitted with a DT175 top end from an early twin shock model, original AT2 22mm (I think) carb and a badly fitting cracked original exhaust. I’m happy once I’ve done the chassis, made new mudguards etc to have an exhaust professionally made, however I want to ensure I have the rest of the set up as good as it can be to get the biggest benefits of doing so so advice here would be greatly appreciated.
I am making my own webco style airbox along with reproducing the guards in Ali so again i need to ensure the carb I’m using is the final one before undertaking this work.

Again, thank you so much for any advice, if anyone has any recommendations for a two stroke guru/exhaust fabricator in the uk that would also be great.

couple of photos of where i am with it at the moment…, just waiting on rims and spokes to finish the wheel build.

Mark


   

Martyn Newlyn at MGN Fabrications is well regarded amongst the RD crowd for bespoke exhausts, so it might be worth contacting him.

What numbers are stamped on your carb body, 316? I think that you need one with 248 stamped on it, with jetting as per the relevant service manual (find these in the Tech Library, see top toolbar).

You'll likely need to change out the 'O' ring in the jet setter, and also both sets of brake shoes, unless you know for sure that they aren't too old.

Which 3rd party are you using for photos?

Hi bud,
Thanks for the info I will check him out for sure.

I just loaded my photos as attachments directly from my iPad and all worked well.

Alan- I noticed some people have been able to post photos again.  

MarkDrury-   first step is to make a positive ID of what you have.  You mentioned it's a "AT2" and if that's the prefix to the VIN number, it ID's it as 73-75 UK/Euro model.  Full VIN number can ID the exact model as outside the USA the 125 came as "electric start" with a 12v dynamo or "kick start only" with a 6v magneto.

It looks like the cylinder has a reed valve.  Top edge of carb flange should have numbers stamped in it if original.  The 125 and 175 twin shocks both had the same basic 24mm carbs with jetting differences on the reed valve models.  (Earlier piston port 125 and 175 carbs were identical)

Hi Mark,

thanks for the reply, I will check the vin this weekend and see what’s what. 
the carb I have I’m pretty sure is a fake, I was wondering about going for a mikuni 28mm flat slide as if I’ve understood it that would work well with my 175 top end - would love your opinion. 

I would need a different manifold of course and I don’t really understand the whole reed set up yet however. 

thanks again
mark


  


 

"It looks like the cylinder has a reed valve. "

Good spot MarkT, disappointed that I didn't notice that myself! 

What I have just noticed is that the fork legs are each on the wrong side - speedo drive and brake operation are on the left side, as you sit on the bike.

As far as I have been able to make out, here in the UK we got very few pre 1973 Yamaha enduros/trail bikes. Funnily enough, I was looking through some old photos from the 1980s the other day and realised that a mate actually had an old AT3, although we all called it a DT125 at the time.

Ha, yes I know the fork legs are on the wrong way round bud ~ it was purely to see how my polishing and Cerekote looked together and the bike was pushed up against the garage door, that said it is weird to me having the brake plate on the left with doing British bikes for so long it’s alien

Yep. First bike I ever had, age 12 was a 1972 DT125, it was not called the AT2 as in the US.

UK bought originally from padgetts of batley according to the dealer sticker on the ( dented ) tank, so when I had it as a field bike, it had not moved more than 3 miles from the supplying dealer.
 It had a turquoise-ish blue metallic paint job on the tank with stripe just like the OP bike. Oil tank on left with " 125 torque induction" badge on it, signifying it had a reed valve system.

It had simple flywheel mag ignition and no electric foot, although I believe the electric start version was more common in the UK.

The exhaust pipe ended at the expansion chamber, about level with the rear of the seat,with the baffle practically fitted in the expansion chamber itself. It did not have an extension pipe and muffler, nor even just an extension pipe as I have seen on some pics. Oil used to run down the exhaust and drip onto the passenger footrest area. Bet the 1970s girlfriends were pissed when they found 2 stroke spooge dripping all over their best flared pants from the shorty exhaust after riding pillion.
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Cannot comment on original fenders because mine, being a stripped down field bike had white plastic, preston petty "mudder" fenders fitted. I believe the originals should have been steel?

I can tell with a glance that the cylinder on the OP bike is not original.  Mine had regular slots cut into the cooling fins.

VM24 carbs work great.  A 28 is too big unless maybe you're doing a lot of modifications, I don't think a flatside 28 will physically fit without some kind of offset manifold to raise it up.

Stock 24 carbs work really good, even on modified engines. A CT3 land speed bike some members here built went nearly 100 mph with a stock carb size.  A larger carb slowed it down.