Mikuni VM 26G setup on a narrow case 250

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JimF
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Mikuni VM 26G setup on a narrow case 250

Postby JimF » Thu Aug 03, 2017 4:19 am

Mikuni VM 26G Twin Float type carburetor setup

Refer to: http://www.mikuni.com/pdf/vmmanual.pdf

As used on a Ducati narrow case 250 engine #88103 believed to be circa 1964 production with no AAU (centrifugal advance.)

The engine is fitted with a period correct Del'Orto metal canister air filter (Ducati Scrambler type) containing within it a NAPA Gold 2719 Crankcase Ventilation pleated paper air filter (a restricted air intake is the point I am trying to make) which by the way is an excellent fit for the canister.

The exhaust system is a lightly baffled aftermarket megaphone (very little restriction.)

The subject 250 fitted as described is a 2-kick cold engine starter; a first kick is given with the choke on, a second kick with the choke off and the engine starts.

With regards to the Mikuni, this is what worked for me. Your results may vary:


Jet Needle 5DP7

Jet Needle "E-ring" Clip position: 2nd notch from the top.

Needle Jet: "PRIMARY type" 182 (NOT "Bleed type")

Slide cutaway: 2.5

120 Main Jet

Pilot Jet 40

Air Jet 0.5

Idle adjusting screw: 7.5mm from carb body to bottom of screw adjust (total distance the spring spans)

Air Screw: Set at 1.0mm from bottoming out.




The long story is the bike was sold to me with the Mikuni fitted. At 3000 rpm the bike would stop making power. Removal of the spark plug revealed the plug was wet and fouled, the mixture was far too rich. I told the previous owner (who had assured me the bike ran) what was happening and he said rather matter-of-factly that the engine ran just the same way for him, as if it was somehow normal for a Ducati 250.

The previous owner had sourced the Mikuni from a fellow that told him something to the effect that the Mikuni was jetted so as to be virtually a plug and play fit for a Ducati 250. Clearly this was not the case.

I found a company called Sudco (http://www.sudco.com/carburetors.html) and described the fitted jets to a fellow there. He said that the jets fitted were simply what the factory puts in that model by default in Japan for shipment everywhere in the world. No consideration for engine displacement, make or model, just some jets and such so the carb would be delivered with some guts in it. He also said he had a list of jets and needles that Sudco recommended as a good starting point for a Ducati 250.

I purchased the 'Ducati 250' kit and and there was some improvement, but it was far from an acceptable solution.

Working from the throttle closed position I incrementally tuned each fuel circuit from the pilot to the air jet. At least twice I found I had not ordered enough range in whatever jet I was working on (choosing several jets above and/or below the 'Ducati 250 kit' I was first sold), and I would place a subsequent order to Sudco after which I would have to wait for the new jets to arrive so I could proceed further.

Honestly though, I have a SSI29 on a Mach 1 250 and it's temperamental and difficult. It's difficult to start (the short throw of the Mach 1's curved kick-starter adds to the problem) and I've never been able to get the engine to idle with any kind of air filter, even a highly breathable foam sock filter. It's either an open velocity stack or the engine simply refuses to run. Maybe the valve sizes have something to do with it too, I can't say. The big SSI just seems a bit more crude in terms of the amount of circuits between closed throttle and wide-open throttle which presents very limited tuning options.

It's easy to see that the added complexity of the Mikuni makes for a more manageable and even-tempered throttle response, from starting and idle to full throttle. The only bad aspect of the Mikuni is that it's a Japanese carburetor on an Italian bike. Sacrilege! Forgive me Dr. T for I have sinned.

In my defense I am able to ride this bike anywhere and any time, so much so that this over-50 year-old motorcycle is my daily driver. The prominent vintage Ducati scrambler air cleaner hanging off the right-hand side boasts the name "Del'Orto' on it, and to date no one has ever knelt down and peered at the pot metal lump between the air cleaner and the engine so much as to ask "Isn't that a Mikuni?"


Jim

Nick
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Joined: Sat Jun 01, 2013 4:57 pm
Location: Paradise

Re: Mikuni VM 26G setup on a narrow case 250

Postby Nick » Thu Aug 03, 2017 5:06 am

Excellent post, thanks. Maybe send it to Sudco so they can improve their jet package.

And this, dear readers, is how one turns an old NC Ducati single (250 or 350) into a reliable, easy starting, steady idling daily rider - Put a Mikuni on it!
Put a Mikuni on it!

Jordan
Posts: 1380
Joined: Fri Dec 24, 2010 11:29 am

Re: Mikuni VM 26G setup on a narrow case 250

Postby Jordan » Thu Aug 03, 2017 5:20 am

Nothing at all wrong with a Mikuni VM, possibly the best Amal carb ever mass produced.

A Dell'Orto VHB square slide carburettor gives excellent results too, in Italian.

ranton_rambler
Posts: 439
Joined: Sun Jun 04, 2017 11:33 am
Location: Stafford UK

Re: Mikuni VM 26G setup on a narrow case 250

Postby ranton_rambler » Thu Aug 03, 2017 1:13 pm

I'm on my second day trying to dismantle the MB22 carb off my 175. The float is out now but the throttle slide is not budging.
I was thinking I might put a small VHB on anyway, as new carbs and parts seem fairly easy to get, and I have VHB's on both of my Guzzis.
Anyone have a starting point for jets and needle for a 175? I'm a long way away from the bike running but can collect information as I go along.

narrow_monza
Posts: 131
Joined: Wed Sep 04, 2013 11:51 pm

Re: Mikuni VM 26G setup on a narrow case 250

Postby narrow_monza » Thu Aug 03, 2017 3:35 pm

Thanks for the great info Jim!
Can you share your intake manifold and air filter adaptor setup?

JimF
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Joined: Fri Oct 23, 2009 2:49 am

Re: Mikuni VM 26G setup on a narrow case 250

Postby JimF » Fri Aug 04, 2017 12:52 am

I should mention I was fortunate enough to have a Mach 1/Mark 3 tank with the indentation under the tank. Looking at the fit of the carb I think the implementation would have been far more difficult without the indent as the top of the carb would likely be in conflict with the tank otherwise.

I suspect the intake manifold is a generic Ducati single part, perhaps a 27mm intake manifold as I suspect the 250 wore a SSI 27 in it's previous life.
At the local automotive supply store I bought a 1-foot length of an extremely thick rubber hose: 24020 Vulco 1-1/4" I.D.

For reference, not an endorsement of this seller:
https://picclick.com/Gates-24020-Straig ... id=1&pid=1

I cut a short couple-inch section and used a pair of stainless steel hose clamps to attach the carb to the intake manifold, and another short section with another couple inches of the hose (and two more clamps) to connect the carb inlet to the air cleaner.

When I got the 250 it came to me with a regular (non-indented) tank and the Mikuni, so there is way to fit it. I just can't remember how it was fitted or how much physical contention was present.

IMG_20170804_171244.jpg


IMG_20170804_171223.jpg


Jim
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Nick
Posts: 243
Joined: Sat Jun 01, 2013 4:57 pm
Location: Paradise

Re: Mikuni VM 26G setup on a narrow case 250

Postby Nick » Sun Aug 06, 2017 2:51 am

Mikuni makes a curved, 90° elbow for the top of the carb that eases this installation. You have to re-solder the cable to install it, but it eliminates potential cable kinking. (Have yet to do this on my Mikuni-equipped Sebring after many, many years though.....)

I use a lightweight foam air cleaner to reduce the amount of weight hanging onto the back of the carb.

Can't see how your air cleaner is mounted, but having an additional support somewhere would be ideal. Even a tie-wrap or spring attached to the frame for support would be nice.
Look closely at the Mikuni set-ups on the dirt trackers back in the day. Most of them were fitted with large K&N air cleaners and almost all had some sort of support for the air cleaners that reduced their leverage on the carbs. Much rougher riding of course.

The OD of my Sebring's original Ducati manifold was so much smaller than the OD of the Mikuni spigot that I machined a piece of PVC pipe to the same OD of the Mikuni, but with an ID that slipped over the stock Due manifold and also matched the ID of the Mikuni. Also, I cut notches in the manifold hose to allow it to clear the mounting studs and slide up flush with the flange.
Put a Mikuni on it!

JimF
Site Admin
Posts: 1124
Joined: Fri Oct 23, 2009 2:49 am

Re: Mikuni VM 26G setup on a narrow case 250

Postby JimF » Sun Aug 06, 2017 3:35 pm

As for the mating to the intake manifold, the intake tract length is actually tuned to be that specific distance between the carb and the cylinder head....

NOT! :D

Thanks for pointing that out. I had to take the carb off to capture the information about the jetting, and re-mounting the carb somewhere after midnight with work looming five hours in the future gave me little time to wrestle the hose over the manifold.

As I look at it, there seems to be some witness marks on the manifold that I probably had the hose further overlapping the manifold before I took the carb off for the jet inventory.

You are quite right about the air filter weight leveraging against the carb. I do have an elastic strap supporting the air cleaner by its elbow to a point on the frame's backbone just behind the tank.

No one noticed, but the float bowl is missing a screw. Not that I would have noticed it either, I didn't realize it was gone until I started disassembling the carb.


Jim


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