1975 RD 125 Carb upgrade

[2strokesforever]

Anybody ever retrofit some 28mm carbs to a 125 twin?   

The TA 125s ran 20mm carbs and I think the stock tiekees are 16 or 18 mm>

The barrels being used are ported- nothing crazy and the pipes will be a variant of the TA 125 pipes.

Going to run the bike as a little in-town ripper

[Dvsrd]

Anybody ever retrofit some 28mm carbs to a 125 twin?   

The TA 125s ran 20mm carbs and I think the stock tiekees are 16 or 18 mm>

The barrels being used are ported- nothing crazy and the pipes will be a variant of the TA 125 pipes.

Going to run the bike as a little in-town ripper

28 mm on 62.5 cc cylinders with moderate porting sounds like too much. 85cc MXers use carbs of that size, but they are in a pretty hot state of tune. For street use, I would try something between 18 and 22. I guess you could look at hop up kits for aircooled 50cc mopeds to get an idea of suitable carb sizes.

[Dvsrd]

Here is a link to a Norwegian vendor. 70 cc tuning cylinder for a Honda 50 cc. Reed valve AC single cyl engine, so pretty similar to half a  RD125 I'd say.

https://speedoptions.no/athena-70cc-honda-mt5

They recommend 19-21 mm carbs for it, output  up to a claimed 12 hp. Surely depending on exhaust, with the 21 mm carb.
Based on that, I would definitely say 22 mm is absolutely max for your RD. I guess the reed valves will help it allow slightly larger carbs than  a piston ported TA,
Also, will TA pipes work well with a mildly ported RD? Maybe some added length in header and belly section is a good idea for street use?

[SoCal250]

The RD125 stock carb is a 18mm TeiKei Y18P (the RD200 uses 20mm TK Y20P). They're junk!
RD125 & RD200 specs
The TA125 carb from the factory is a Mikuni VM26SC. A VM28SC was also available with the optional GYT kit.
TA125 specs
The TA uses large carbs because it was designed to run flat-out all the time, and hence has a narrow powerband up near it's 13K redline. Carbs this big would probably be overkill on a street 125cc. 

The changeover to Mikuni carbs has been done and it's something I've been planning. The TeiKei carbs are nearly impossible to work with due to the near zero availability of parts and difficulty in tuning. The last genuine jets I saw were going for $15-20 EACH and are very hard to find!

Because the TKs have the carb manifold integrated into the body you need to install angled carb joints that will allow other flange-mount carb brands to bolt up. You can use Yamaha CT2 / AT2 / DT125A/B carb joints for this. Yamaha part #314-13565-00-00. I picked up some for about $10ea a while back off Ebay for my future project. (see the attached photos). I am planning to use Mikuni VM20s

I found this write up that details the parts and process.
https://yamaha-rd125.blogspot.com/2016/12/new-mikuni-vm20-carbs-yamaha-rd125-a.html

[2strokesforever]

As always, Great info Socal250!

Ive actually found that the outer 4 bolt pattern is the same as the RD250/350 intake manifolds. I sliced off the reed stuffers and they bolted directly on to the 125 cylinders. Ive got a pretty decent supply of spare parts here so Im using stuff I have before buying more.

Im going to give the 28s a shot. Ive got them here. If they are too much carb, I can always find some banshee 26s and go at it again.

Ive had a Honda Elite scooter that I put a 70cc kit on and that kit had an intake and a HUGE 30mm flatslide. It jetted in pretty easily and was actually really responsive.

I'm going to clean up the reed cages as I do with all my bikes by getting rid of the casting lines and smoothing them out some.


Im going to be playing with the pipe dimensions before I settle on a final design. Im not too worried about the bike being peaky, ive built a lot of the yamaha spec race chambers and they always surprise me how decent they are on the street, even with stock porting

[m in sc]

fyi, ds7 carbs are 26 as well.

[SoCal250]

Many years ago when I was compaing parts in the garage in an attempt to find a solution for the carb issue on the small RDs I did a comparison of intake joints and found that the RD350/RD400/Banshee bolt holes lined up with the RD125/RD200. Attached is a tracing I did probably 10-12 years ago.

Also attached is pipe specs for the TA125. I'll look around in my files, I may have some other pipe designs for the 125. I contacted a lot of pipe builders (a couple guys in the UK, a guy in the Netherlands, Jon at Jemco, etc.) to inquire about making pipes for a hot-rodded RD125 or RD200.  Jon said he could make them, but I'm not sure if he's still building pipes. The guy in the Netherlands said he had a kit but it would need to be welded.

[SoCal250]

I found this in my old files from 2010. Just for FYI
RD125B reed cage dimensions

[Dvsrd]

Looks like a good candidate for removing (or narrowing) the bridge and fitting a single petal.
Also, in stock form, it would seem to be the limiting factor when increasing carb size.

[SoCal250]

I found some more old photos

[SoCal250]

And a few more for reference