So I decided to take the carbs off for a closer inspection.
Damn...The tube inwhich you screw in the needle valve had a crack in it... and when I tried to take out the main jet with needle valve it almost snapped completely off.
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| Teikei carb with cracked tube |
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| Teikei carb with cracked tube |
I tried to fix it with JB Weld but I'm afraid it won't hold long when submerged in petrol.
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| Trying to fix the crack with JB Weld |
So I started the hunt for a new Teikei carb, but I only could find used ones which were pretty expensive too.
On the aircooledrd forum I got a tip that you could use Mikuni carbs on the Yamaha RD125 A. but only if you mount extra manifolds on top of the original reed valves... The Teikei manifold is integrated in the carb but the Mikuni VM carb is just flat.
So what's the best size Mikuni VM carb for the Yamaha RD125? In a tuners manual I found a formula to calculate the Venturi / throat size of the carb:
For racing engines, the constant, K, is given as 0.8 to 0.9, so let's take 0.8 for the Yamaha RD125.
C=125/2/1000=0.063
N=9500 rpm according to the bike specs
Using the formula, The right venturi size (D) is around 20 mm
So the Mikuni VM20-273 is gonna be the best choice. I'm using the same size carburettor on my Yamaha LS3 100 with good results.
It is possible to mount the manifolds of a Yamaha CT1 175 (1972-1973) on top of the original Yamaha RD125 Reed valves. You can still find them on Ebay for around 15 dollar each. I bought the manifolds in the USA and the VM20 carbies here in Australia.
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| Mikuni VM20 carbies and Yamaha CT1 manifolds |
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| Yamaha CT1 manifolds |
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| Yamaha CT1 manifolds |
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| Mikuni VM20 carburettors with Yamaha CT1 manifolds and gasket |
You'll also need longer manifold/reed mounting bolts (M6 1.00 30mm long) and shorter carbie mounting bolts (M8 1.25 25mm long). I didn't use the spacers that came with the manifold because I wanted the distance from carb to cylinder as short as possible.
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| New bolts for the VM20 carbs and CT1 manifolds |
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| original teikei gasket |
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| cutting new carburettor gaskets |
Between the VM20 carbies and manifold I also used gaskets as the O-ring in the carbs didn't meet the manifolds surface all the way round (see pic below) The gaskets which are normally between the distance holder and manifold are perfect for the job.
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| Yamaha CT1 manifold wih Mikuni VM20 carburettor |
When you have gathered all the parts is pretty straight forward to fit the new carbies. I removed the original air filter box and used two pod air filters instead. With those "open" pod filters you need bigger main jets.
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| Pod air filters Yamaha RD125A |
You also have to exchange the cable holders between the original Teikei carbs and the new Mikuni carbs as the original cables are too "big" for the new carbies. The tread is the same so that's an easy swap
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| Original Mikuni Cable holder and the bigger Teikei cable holder |
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| Yamaha CT1 manifolds on RD125 engine |
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| Mikuni carburettors mounted on Manifolds |
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| Pod airfilters fitted (43 mm) |
This Mikuni is supplied with the following factory jetting installed:
Pilot jet 22.5 (M28/1001)
Needle 4J13
Needle Jet O-6/N-6 (238/332 Series)
Main Jet 180 (4/042)
Air Jet 0.5
Slide Cutaway 2.0 (VM38/24)
Needle valve 1.5
Probably not the right jetting for the Yamaha RD125 A. A search on internet about jetting similar engine sizes and carbs pointed in the direction of 90-140 Mikuni main jets. I'll start with the original jetting in place and work my way down to leaner jets. One of the nice things about the VM20 carb is that the main jet can be changed from the outside of the carb.
The bike started after 3 kicks and ran pretty smooth, but after a couple of minutes one of the cylinders cut out. When I checked the plugs they where completely wet... Time to start the most time consuming part; finding the right jetting for the bike. A correctly jetted carb makes a big difference in the torque, midrange pull, topend pull, and overall-revving of the engine.
It's just a matter of trail and error and knowing what to look for:
Throttle Ranges:
Pilot Jet/air screwtrottle slide valve: 0-1/4
Needle Jet:1/4-3/4
Main Jet: 3/4-Full open
I swapped the #120 for #115 main jets. A bit better but still too big... Anyway after some trial and error it seems like #100 and #105 are the best main size jets. Probably #100 in summer and #105 in winter.
Maybe it needs a pilot jet swap from #22.5 to #25, But first I've to buy and try 'm...
Here's a link to the bike's first start with the new Mikuni carbies on youtube: https://www.youtube.com/watch?v=85CCc4rf8Rg
