Fixing the Teikei carbies

One of the things I still had to clean were the 2 Teikei carburettors. They're not as common as the Mikuni and Keihin carbs so there's not a lot of info about them and parts are hard to find.

Incredible how the bike still ran with those dirty float chambers....

18 mm Teikei carburettor - Yamaha RD125A

18 mm Teikei carbies Type Y18P-1C

Dirty fuel bowl Teikei carbie

Dirty Teikei carbie

The satin black paint was flaking, time for a fresh coat. VHT caliper paint is very close to the original and maybe even more important it's chemical resistant after 1 hour baking at max 93 degrees Celcius.
After stripping the paint I masked the carbies and applied 3 coats of the caliper paint.

Removing old paint with paint stripper

Ready for some new satin black paint

Masked and ready for some new satin black paint

Masked and ready for some new satin black paint

Fuel bowl with 3 coats of VHT caliper satin black paint

Teikei carb with 3 coats of VHT caliper satin black paint

VHT caliper satin black paint

Next thing to look at were the jets. According to the original RD125A parts list the 18 mm Teikei carburettors have #96 main jets. The later RD125B uses the same carbs with #94 main jets.

In my RD125A carbs I found the following parts:

#94 main jet

#44 pilot jet
N80 needle jet

4D50-2 Jet needle - clip position 

2.5 Cut away 

16 Needle valve and seat assembly 

The Keyster kit I bought (advertised as suitable for all RD125 Yamaha's...):

#66 main jet

#44 pilot jet
Y54 Jet needle 

16 Needle valve and seat assembly 

So the new main jets are way to small and the needle is 0.5 mm longer. 

RD125 A, RD125 B and Yamaha AS3 carb specs and settings

Keyster KY-0160 carb repair kit Yamaha RD125

Carbie parts 18mm Teikei Yamaha RD125 A

#94 Main jet - 18mm Teikei Yamaha RD125

#44 Pilot jet - 18mm Teikei Yamaha RD125 A

Old and new needle 18 mm Teikei carb - Yamaha RD125 A

I still needed other main jets or keep on using the old ones. Original Teikei parts are really hard to find but I've read that some Keihin jets will fit as well. Look for the 99101-116 series, they have almost the same dimensions to replace the old main jets and more important the same thread M4x0.75.

Keihin 99101-116 series main jets

So, what's the best size to use? Some say go for a smaller jet.. the story behind going for a smaller jet when the capacity increases is a bigger bore creates a higher vacuum, pulling more fuel then before. Hmm.. I think this only make sense when you go for a really big overbore.

In my case it's only first oversize pistons and we're talking about an increase of ≈ 1.5 cc

The formula to calculate the capacity of an engine:

Capacity (in cc)  =  (π/4 x bore² x stroke x # cylinders)/1000   (note: π ≈ 3.14)

Original engine: (3.14/4 x 43² x 43 x 2)/1000  ≈  124.8 cc

Rebuild engine with 43.25 mm pistons:

 (3.14/4 x 43.25² x 43 x 2)/1000  ≈  126.3 cc

An increase of 1.5 cc !

For now I'll start with the original #94 main jets and then try the #100 Keihin jets, if it's too rich the #98's will be the next jets to try out.

Checking the float height (ca 20 mm) on both Teikei carburettors

Last but not least make sure the air supply is ok. Pack your mufflers, use airfilters in the airfilter box and make sure the rubber boots are in good shape. Like with most older bikes the original rubber boots on the RD125 were hard (especially on the carb site) which made it almost impossible to seal 'm properly. 

Old and new carb/airfilter boots Yamaha RD125 A

Cutting the new carb boots - originals are for the AS3

Re packing the exhaust muffler Yamaha RD125

The original Yamaha RD125 airfilters are hard to find, so when I found some old Honda pre-oiled air filters on special for 2 dollar each it was an easy choice, just a matter reshaping them a bit.

Pre oiled air filter

Time to start the engine....

https://www.youtube.com/watch?v=6t2mZJeN0AM

Yamaha RD125 A 1974

Yamaha RD125 A 1974 two stroke