Mikuni manifolds and intake tract length

[JimF]

I talked to Chad @ Sudco (310-637-8330.) Good guy, knowledgeable...

He says the notion that a bleed versus primary needle jet is a 2-stroke versus 4-stroke application thing is an old wives tale.

The primary type does run many times richer but it has nothing to do with 2-stroke engines and the primary jet needle should not be pigeon-holed as only for 2-stroke applications.

It turns out I have a 26mm VM.

As I described my components Chad said that was reflective of a bone stock VM26 right off the shelf from Japan, save for mine is missing the air jet.

Way too rich for the Ducati 250 (as evidenced by my real world trial.)

Here's Sudco's recommendation (based on 40 years experience) for a Ducati 250 running a VM26:

140 main
30 pilot
182-02 needle jet
5DP7 needle
2.5 slide
0.5 air jet

My parts are on order. Stay tuned for an update after my carb is re-fitted as per Sudco's advice, probably late next week.

[DewCatTea-Bob]

" I talked to Chad @ Sudco "

____ I wonder how you came to find/pick this source (of all there must be to find) ?


" He says the notion that a bleed versus primary needle jet is a 2-stroke versus 4-stroke application thing is an old wives tale.
The primary type does run many times richer but it has nothing to do with 2-stroke engines and the primary jet needle should not be pigeon-holed as only for 2-stroke applications. "

____ I-myself have no problem accepting those notions as being actually true but, it makes fair sense to me that 2-strokers would be more apt to employ the 'primary' type (as your Mikuni has), and 4-strokers more apt to employ the 'bleed' type.


" It turns out I have a 26mm VM. "

____ This surprises me somewhat (as being rather small), and I'm still thinking that it may be a 28mm sized throat. _ So I'd sure like to see for myself that a 28mm washer could not fit through it.


" Chad said that was reflective of a bone stock VM26 right off the shelf from Japan, "

____ This is also a bit of a surprise, as I assume that that means to imply that it was originally assembled at it's factory to be intended as a generic carb for somewhat general-usage (very likely inclined for 2-stroke engines). _ And it doesn't make reasonable sense to purchase any-O factory-new carb that hasn't been (at least somewhat) set-up for generic engines not greatly different from ya'r own intended application (without realizing that much jetting-work would be needed).
So I'm still thinking that your Mikuni was more likely taken from some-O 2-stroke Jap.bike (with a 125 - 175cc engine) and then ignorantly (without any internal changes) bolted-on to your Duke, (just as has been done many numerous times by dorky Duke-owners in the past).
So still hoping to learn from the PO where he actually got that Mik.carb from.


" Here's Sudco's recommendation for a Ducati 250 running a VM26:

____ A 26mm-carb would be a good size-match for a stock 250-Monza cyl.head, but for a stock MI/Mk3 cyl.head, a 30mm-carb would be a better match.


" My parts are on order. "

____ If they're expensive, I'd first research for confirmation of those particular part-sizes.


Hopeful-Cheers,
-Bob

[JimF]

The PO says he bought the Mikuni from a fellow that told him to the effect it was either setup for or came off of a Ducati single. Clearly this was not truthful.

As for Chad's credentials, he does work for Sudco which has been noted as being a good US source of Mikuni carbs and parts.

Chad was mentioned as being a source of good information and as helpful in another thread on another board:

http://www.flattrack.com/index.php?opti ... ize=larger

That Chad was helpful I can corroborate. He quickly came up with these components off what I would imagine was some kind of a chart as I did not detect and lapses which could have led me to suspect he was making this stuff up as he went along.

Chad's knowledge of the circuits that each jet controls was evident in conversation.

He also stated that my fittings matched the factory default product set-up so here again he had some kind chart to work off of.

[DewCatTea-Bob]

" The PO says he bought the Mikuni from a fellow that told him to the effect it was either setup for or came off of a Ducati single. "

____ I-myself have little doubt that it very well could've indeed been taken-off from another Duke (that some-other dork had fitted it to).


" As for Chad's credentials, "

____ Any notion of that source being questionable, was not meant to be suspect !


" He also stated that my fittings matched the factory default product set-up "

____ THEN I'd suspect that your Mik.carb was probably once ordered/purchased as a new generic carb for some 2-stroke engine and then-later sold at a swap-meet (or something) to the first dork who next fitted it to some-other Duke in the more distant past,, (as a most-likely scenario).


Mickuni-Kooks,
-Bob

[JimF]

I have fitted the Sudco recommended needle, slide and jets, and the bike revs past 10K on the driveway and I've had it over 8K under load albeit briefly. The weather here had turned cold again but it is warming up again....

[vintagetour]

Thanks Jim. When I receive my K&N air filter I will know which direction to go when fine tuning mine.
Tom