Mikuni carb conversion questions for 73 125

[Dennis Jones]

Tommy, Do yourself a favor and keep the bing on your 175. Put a new needle and jets in and it will work great. Very easy to tune. Like the above posts state, most of these bikes have had a long hard life. Your bike seems to be a low hours and shouldn't be worn out.

Dennis Jones

[gooddirt]

quote:Originally posted by Dave Withrow

Do you have to do anything to adapt the Mikuni to the intake?

[sdm]

I was going to look for a piece of "Green Stripe" hose or something compatible with gas that is the right size and hose clamp it on to the engine. Unless someone knows the right combination to use...
I'll let you know what I come up with when everything gets here and I start working on it.
Sam

[john durrill]

Sam,
 Phil Schloss did this about 3 years ago for his sons 125 B engine Six-Day. here is what he had to do to make it work. THe bike ran well .
John D.

I purchased a 28 mm carburetor from my local dealer. He ordered it from Marshall, got it for about $85.00. Carburetor number is VM28SC-E0865.

The only changes I made were to change the pilot jet to 40, main jet to 160 and the needle to a 5F3. Needle is set at the first notch, or the richest position, (all the way UP). I'm sure there are a better needle and needle jet set ups, but this works great. The needle jet is a 169 N-8, which is standard on this carburetor. The air screw is out 1¼ turns.

To mount it on a SixDay I did the following:

Cut the air boot on the air box side one ring making sure the diameter is the same. The boot is now shorter.

The diameter of the boot side of the carburetor is similar to that on a JackPiner with a Bing. Use the same rubber sleeve, clamp it together and the air box side is done.

On the engine side of the carburetor I shortened the spigot by sanding it on my stationary belt sander. Remove enough metal that the spigot on the carburetor fits close to the spigot on the intake of the cylinder. They are of similar diameter on most cylinders so a hose and clamps will take care of the engine side of the carburetor.

Now you will notice the carburetor has to lean about 15 degrees to clear the pipe. I ran mine like this for a year with no problems. Recently I did some "fine tuning" on the pipe to make room for the carburetor to stand straight up. This took no more than a torch and a hammer.

That's about it. Hope it works for you as well as it has for me.

SkipperClyde

[sdm]

Thanks John
I knew I had seen that post here somewhere but when I did a search I must have done something wrong.
 Anyhow I would like to avoid cutting the spigot on the cylinder if at all possible but I will see how it all fits up when it gets here.
The jetting sounds pretty close to what I was figuring to start with so that will give me a good start.
  Thanks for the info and the link...
Sam

[tlanders]

Tom,

I blew my Six Day up in the last race in the fall of 2002. I had just finished a race on my Mint 400 and when I got on the Six Day, I held the throttle open at max revs for too long instead of risking a shift and the con rod bearings decided to give up. Due to all the other bikes having priority since then, I still haven't put it back together again but I have bought a 32mm Mikuni for it. Would you tell me the jetting info for your bike? With the non saddle time I am now faced with, I might finally get a chance to put it back together. I wonder if Doug would like to visit Missouri?

Thanks,

Teddy

[bentrims]

Hi Teddy,

I was quite fortunate in that when Donny Smith had his 6 Days on ebay about a month ago, I noticed he stated that it had a 30mm Mikuni. In an email he was kind enough to send me his specs.

I went slightly richer to compensate for a 32mm flowing more cfm's.
No Doubt tuning will be needed for each bike...riding area...electrical system.
Main Jet.......170
Pilot Jet.......55
Needle Jet..159-N8
Jet Needle..5LF14..(I raced Arizona with a 6DP17)5LF14 was on order.

Good Luck with those ribs. Makes me wonder how your CFMs are flowing.

[Don Hanson]

I just installed a 32 mm mikuni on my '73 125.  I ordered the bing fiber bushing from Al Buehner then machined the inner diameter to accept the bushing then cut four slots with a hacksaw so I could clamp it on stock cylinder spigot with stock bing clamp.  The air intake boot stretched enough to fit over the mikuni just fine.  The stock cable worked just fine.  As far as jetting goes, it came with a 200 main and a 35 pilot and at sea level seems to work amazingly well.

Don

[bentrims]

Don,
Glad it works...your bike is really nice. I have a baby tank like yours coming for mine and hope to get it before Gaineville. The larger one is tough on the cubes during moto combat. Please e-mail me direct Don. Thanks,
Tom Benolkin
[email protected]

[brokenb23]

Sam,
 I used radiator hose with a slight bend to adapt my mikuni. Check your local Yamaha shop for a used 28mm. I beleive either the Blasters or Warriors came with a 28mm. Also a Keihn will work very will. Bottom seam of exhaust pipe comes very close to top of carb. I have been using a 28 on my 72 125/6a. Easy start, smooth thru all ranges, even lugs very well without loading up(when racing with only 3 gears). Good Luck
Bob Brizzee

[bentrims]

Right on to what Bob says,:)
Last year I took a boot into a Checker Auto store and asked if I could go behind the counter and see all the options they had hanging on the wall. You end up buying a hose that may be 20" long to cut out the 3 " you need. Ain't that the wasteful American way...GetRdun[]

[sdm]

Well my carb showed up today and it was pretty painless on the
installation part. I went with a new 26 because the cycle salvage
wanted as much as a new carb for a used. So instead of taking the chance on buying some elses problem I went that way. I had an old piece of fuel delivery hose laying around that I cut for an intake manifold hose and on the airbox side I needed to shim the diameter of the intake bell to meet the diameter of the air boot. I found a piece of radiator hose I had trimmed that fit perfectly and the intake boot between the carb and airbox is sealed in the same manner as with the Bing.
  As delivered the Mikuni is very rich for this application but it runs and is a good starting place.
It came with a 190 mj 60 pj 182nj
I'll be off to the shop tomorrow to get a hand full of jets to start fiddling with it.
I'll let you know what I settle on when it is done..
Thanks for all the help and advise
As stated many times before the folks here are the best !
Sam

[Ernie Phillips]

Installed a new 28mm Miknui on Christopher's '72 1256B this weekend.  Marvolus, Wonderful, ... The bike takes 3-6 kicks cold and then only one tap when hot.  Fall over, pick up and starts one tap.  GREAT!  Pulls clean top to bottom.  Need to fine tune, a little rich on top and a little lean just off bottom.  Will race this weekend at Evansville and report back how things turnout.  Installation - 1-3/8" radiator hose, 2 hose clamps, stock throttle cable.  So easy even I could do it.  No interference with pipe, air booth sorta contorted but sealed - need to sort out better.

Jets: 180 main, 35 pilot, 5F21 needle, ?? needle jet.  Plan on trying 160-170 main, 40-45 pilot.

Bing?  I haven't given up and still run a 30mm on my race 5A.  But, I'll be carefully evaluating how Christopher's rig runs.  



Ernie P.
Chattanooga, TN

[Ron]

Ernie,

I have a 30mm Mikuni that works flaw less. I have a PVL so that helps it start, but less than 1/2 a kick when its warm.
My jetting is patterned after Donnie Smiths recomendations.
My only deviation has been a smaller pilot and up a little on the main jet (I haven't had a chance to really hold it open.)

Needle:  6d917 (middle clip position)
Needle jet: N8
Pilot jet: 50
Main jet: 180

RonW

[Ernie Phillips]

RonW,

Tell me about the fit-up of the 30mm.  How does it mount to the cylinder & air boot?  Did you have to bend the pipe?  The reason I went with 28mm is the smaller carb body ... but the 28s are getting harder to find.  I do have a 30mm and 32mm Mik that I'd like to try if it isn't going to cause me to have to use a sledge hammer to get it to fit .  1/2 kick on the warm start is great!  I'm sure the PVL helps but on Christopher's bike, all I did was change carb and presto-change-o, it starts like magic.  Thanks for the jetting specs.

Ernie P.
Chattanooga, TN