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....
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| 18 mm Teikei carburettor - Yamaha RD125A |
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| 18 mm Teikei carbies Type Y18P-1C |
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| Dirty fuel bowl Teikei carbie |
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| 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.
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| Removing old paint with paint stripper |
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| Ready for some new satin black paint |
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| Masked and ready for some new satin black paint |
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| Masked and ready for some new satin black paint |
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| Fuel bowl with 3 coats of VHT caliper satin black paint |
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| Teikei carb with 3 coats of VHT caliper satin black paint |
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| 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.
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| RD125 A, RD125 B and Yamaha AS3 carb specs and settings |
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| Keyster KY-0160 carb repair kit Yamaha RD125 |
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| Carbie parts 18mm Teikei Yamaha RD125 A |
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| #94 Main jet - 18mm Teikei Yamaha RD125 |
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| #44 Pilot jet - 18mm Teikei Yamaha RD125 A |
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| 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.
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| 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 !
