Fine tuning the Mikuni VM20 carbs on the RD125

 After the basic tune of the Mikuni VM20 carbs (read make it running) I moved house and put the bike in the lounge for a couple of years. I was too busy with other personal and work related stuff to give it a perfect carb tune.

Yamaha RD125A in lounge

Now with the covid you're kind of doomed to stay home during your holidays but that's a good thing for fixing things around your house and I decided to take the old RD125A out for a proper carb tune.

Yamaha RD125 A Mikuni VM20 carb tuning

Yamaha RD125 tuning mikuni carburetors

 The fuel tap was leaking again so I decided to buy a new one at HVCCycle. Exellent copy of the original one and fast shipping.

Yamaha RD125A fuel tap

Yamaha RD125 A fuel tap

Most people say it's too risky to use the oil pump but mine is working fine and as the bike demands different levels of fuel/oil mix depending on speed/rpm it's better for the bike cause the pump will deliver and premix won't.

So what was in the two new Mikuni VM20-273 carbies when I put the bike away..

Pilot jet 22.5 (M28/1001)
Needle 4J13
Needle clip in 3rd (middle) position
Needle Jet O-6/N-6 (238/332 Series)
Main Jet 100 (4/042)
Air Jet 0.5
Slide Cutaway 2.0 (VM38/24)
Needle valve 1.5

 

Yamaha RD125 A Mikuni 22.5 pilot jet

Yamaha RD125 A Mikuni VM20-273 inside

The bike idled ok but I noticed the spark plugs were getting a bit wet after a while. So time for a leaner pilot jet. I tried the 20 and ended up with 17.5 pilot jets. When the engine is cold you need the choke to get it running otherwise it's too rich.

See youtube vid: https://youtu.be/UzTlYwpjJ-s

The carbs were only roughly synchronized 5 years ago so after fitting the new pilot jets I started re- synchronizing the 2 Mikuni carburetors.

First look at the carb slides. They should move simultaniously up and down and be at the same height all the time. If not you can adjust it with the cable adjuster screw on top of the carbie. Another good way to check this is with the icepop stick trick, stick two wooden ice lolly sticks underneath the carb slides (engine not running) and move the throttle. If everything is ok they will move simultaniously. A really small difference will be magnified by this trick so it's really accurate. 

Yamaha RD125 A with 2 Mikuni VM20 carbies

After this you can set the air/mixture screws and idle screws. First put the air screw in the postion mentioned in the manual. eg. 1 1/2 turn out. Now get the bike on normal operating temperature and disconnect one spark plug so the bike is running on only one cylinder. 

Turn the air screw 1/2 turn out and wait for the change in rpm for 10 seconds. If the rpm is increasing give it another 1/2 turn etc till the rpm starts to drop. Then turn air screw back to max rpm and give it 1/4 turn in  after you've reached max rpm again (richer). Now set the idle speed screw on ca 1250 rpm. Do the same with the other carbie.

When both carbies are done let the engine run on both cylinders, the rpm is now too high so turn back both idle speed screws the same amount till it's ok around 1250 rpm.

When tuning carbies it's a basic rule to start with the pilot jet, followed by the main jet and finish with the needle. But that said you need to get the bike running first so the needle should be close to the right needle and close to the best needle clip position. Also the main shouldn't be too far off. The Yamaha AS3 runs on Mikuni carbies (different ones of course) and uses 75 main jets.
I can use that as a ballpark figure. It's probably gonna be a bit bigger as I run the carbies with two pod filters. On average a main jet needs to be 20% bigger  when you use pod filters instead of the original air filter.
I also got rid of the fibre packing around the internal silencers cause it suffocated the engine too much. Now you need an even bigger main jet. So my guess was to start with a 95 or 100 or maybe even a 105 or 110. But the carbie can work different and pull maybe even more fuel from a smaller jet than the old mikuni on the Yamaha AS3. Time for some testdriving... well..  more then a couple :)

In order to tune the pilot, needle and main you need to divide your throttle in parts as the carbie has different circuits delivering fuel depending on the throttle position: 

0 - 1/4 open is Pilot jet
1/4 - 3/4 open is Needle and needle clip position
3/4 - FT open is Main jet (FT=Full Throttle also called WOT= Wide OpenThrottle)

This way you can testride your bike in the throttle position area you're tuning.
Remember it is not the speed or rpm that dictates what's the main fuel supplier  at that moment, only the throttle position counts.

The Mikuni VM20-273 carbies come with the needle clip in the middle position. There are 5 clip positions on this needle. The top position/groove is no.1 and the bottom one is called no.5. The leanest position is the top groove (no.1) and the richest no.5.

Yamaha RD125A Mikuni VM20 needle and slide

The bike was running but probably on the lean side in the midrange...it was kind of hard to get to the 3/4 FT position of the throttle without the bike hesitating, I had to go real slow with the trottle to get there.

So I decided to do a bit of basic needle clip tuning before looking into the best main jet and put the clips in the fourth groove which made the midrange a bit richer.. Way better pick up now.

Now I could easily get up to 3/4 throttle and tried a couple of different main jets from 85 till 100. 100 is still the best performing and maybe 105 is gonna be even better. But with all the  main jet changes the rubber seal between the main jet screw and carbie started to wear out so I've to order a couple of new ones before I can finish the fine tuning of the main jet carbies.

Next thing I did was putting the needle clip in the n0.5 groove. There's a very light flat spot somewhere between going from 1/4 to 1/2. Probably a bit on the rich side but the pick up from a bit over 1/2 to just over 3/4 is perfect which made me think to put the needle clip back into the fourth groove and swap the 100 main jets for 105 main jets.
The reason behind this is that there is always an overlap between the pilot jet and needle and the needle and main jet.

I'll post an update when it's all done :)
Now waiting for the seals.

Yamaha RD125A 1974

Yamaha RD125A 1974

Yamaha RD125A 1974 10.000 km

While tuning the Mikuni carbies I ticked just over 10.000 km. That's around 212 km a year ;)

New Mikuni VM20 carbs - Yamaha RD125 A

Although the bike was running after installing the original Teikei carbs, one of the carbs was on the rich side. It wasn't possible to tune it leaner and sometimes the throttle valve woudn't sink in all the way.
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.

Teikei carb with cracked tube

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.

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.

Mikuni VM20 carbies and Yamaha CT1 manifolds

Yamaha CT1 manifolds

Yamaha CT1 manifolds

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. 
 

New bolts for the VM20 carbs and CT1 manifolds

You also need to make your own manifold/reed gaskets or use gasket sealant as the original RD125 ones probably won't stay in place (No bolts to hold them in place...).

original teikei gasket

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.

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.

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

Original Mikuni Cable holder and the bigger Teikei cable holder

Yamaha CT1 manifolds on RD125 engine

Mikuni carburettors mounted on Manifolds

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

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