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
Dear sir:
ReplyDeleteI congratulate you for your yamaha and your work! I also have this model and i am currentely in need of new carbs. I am able to find the Mikunis VM20-273 but I am having trouble finding the CT1 manifolds for a correct instalation. You wouldnt happen to know where i can find them?
Thank you and good luck!
Corto,
DeleteCheck Ebay, that's where I got mine.
Yamaha part # 314-13565-00-00
Used on the following models:
1972 AT2 - Intake
1972 CT2 - Intake
1973 AT3 - Intake
1973 CT3 - Intake
1974 DT125A - Intake
1975 DT125B - Intake
Hope that helps. Good luck.
-Russ (aka SoCal250)
I cannot find this manifold anywhere. What happens with no manifold? Is the angle of carb.s bad?
DeleteHi, nice restoration project,fyi there is mikuni vm20 variant with round end,in indonesia, mikuni made it for Suzuki satria 120r, i'm planning to buy the original vm20 but relized it has no oil intake? So i wonder since vm 20 has no oil intake? do you drill some oil intake to the manifold?
ReplyDeleteBtw currently im using the imitation of mikuni vm22 for yamaha f1zr since it has oil intake.
I think you can also fit the vm20 carburetors from Suzuki GT185 bikes. Plus the manifold from late Yamaha RD125 DX. I do this Conversion on my 1E7.The Reeds should be modified and enlarged a little bit
ReplyDeleteWhat is the benefit of having the carbs closer to the reeds by eliminating the manifold spacer?
ReplyDeleteCould you also eliminate the spacer between the reeds and engine?
I am putting the VM20 carb.s on my RD125 also. Did you do it without the spacers? I cannot find spacers. Did it work? What about the angle of the carb.s?
DeleteHi , I am currently rebuilding my rd125 twin dx , the 125 engine is completely destroyed anyway I have a rd200 dx engine but no Carburetors for either engine,
ReplyDeleteAnd old racing guy said suzuki gt185 Carburetors work perfectly with the rubber adapters on connected to reed blocks,
It starts 1st time and runs smoothly, then the engine locked up, now the suzuki gt185 Carburetors work fine you just need to take 2mm off Carburetors where you push inside the rubber adapters,
Then they push in and oring lines up spot on
I discovered why rd200 dx engine locked up, the original owner said rd200 dx engine was a good running engine and he had done a top engine rebuild.
Nothing was rebuilt, only new barrel and head gaskets,
The barrel bores the left was had serval deep scores. ,
After cleaning pistons and rings left and right barrels
They were now running smoothly,
It appears at some stage while the bike was left in bits for around 30 years someone had removed sparkplugs and condensation was allowed into the crankshaft,
The strange thing is before I lubed up barrels and crankshaft it turned over smoothly and quietly, but as two oil oiled the crankshaft up it uncovered bad pitting on crankshaft and didn't lock engine up but basically made it so tight that you hear that familiar noise
I put loads of WD40 into crankshaft and its all turning over smoothly but I fear because previous owner wasn't truthful,
It will probably need either a reconditioned crankshaft
Rd125 dx and rd200 dx are basically the same bikes and becoming super expensive and rare
Its getting to the point here in the UK 🇬🇧, that vintage mopeds and vintage motorbikes are now reaching far too much money for the average person to afford ,
So hence why I have to buy bikes that were taken apart when original owners were young men and then got forgotten about for 30 to 40 years plus family lol.
I've been fortunate to.buy this bike in pieces but overall 85% COMPLETE,
Cost was £400
And I allowed total budget including buying bike of £1000,
And take a look at what you can get in the UK for £1000, all you can find is bikes that were left outside for 30 to 40 years where most parts are rotten and so rusty your washing your time,
Next is around £2000 to £2500 for complete bike but still need new tyres and probably another £500 more needed,
Then you have the really nice bikes that can be ridden on the road straight away but they are £3000.to £4000.
,
So we do have a choice but only if you have the money to spend,,
So I feel I was quite fortunate as most of the parts are dirty but in very good condition and hence if I can make a complete rd125 dx for £1000 and time is not an issue as I'm retired, the bike Will be a good buy fingers crossed.