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My "new-to-me" 250 arrived with a Mikuni on it. I can find no numbers and it looks like a VM series.

The bike came with an air filter on it and presumably (according to the previous owner) it was all dialed in. I took the air filter off during some maintenance and the bike ran fine.

I have since put the air filter back on and the plug sooted up pretty darn fast although I had only started it and driven it around the block to test other things. Now I should mention that I did change the filter element so it may be that the filter I chose is more restrictive, but that's just speculation.

My wealth of knowledge favors the Dellorto SSI and I know nothing of Mikunis.

I have not looked to see what size of jets it has, or to throttle chop it and the like.

Advice on where to buy jets and a starting point would be appreciated. If you can tell me where to look to determine the actual model that would be helpful too.

Jim

These guys will have anything you need...

http://www.sudco.com/mikunijets.html

You could also try readjusting the idle mixture and dropping the needle a slot.


Bill

Thanks Bill,

I assume I would turn the idle mixture screw counter-clockwise which I expect would back it out and let more air in thus leaning out the mixture...

Is that how you remember it?

Jim

I assume I would turn the idle mixture screw counter-clockwise which I expect would back it out and let more air in thus leaning out the mixture...

Not on Mikuni's (and most other typical carbs), to lean it go clockwise which lets in less air, the air going through the passage venturi is what pulls the fuel up from the float bowl and into the idle jet. The idle circuit is very much like a very small version of the main intake of the carb, though the amount of air let in is much smaller in proportion the the amount of fuel it moves.
General rule of thumb is to turn the screw in/out until you get the highest idle, then back it out (enrich) so the idle goes down just a bit. Then set to the desired idle speed with the slide stop screw/adjust.


Bill

Thanks Bill.

Can someone explain this chart to me?



Is the fuel flow in litres per hour telling me that if I lower the needle I will go richer? (More l/h of fuel being expended?)

Jim

Lowering the needle blocks fuel coming from the main jet so is leaner. The chart shows the relative affect of the three needle positions (with position 3 as a baseline) to fuel flow volume at various throttle openings. Position 4 (raises the needle) flows 5-8 percent more fuel at that point than middle pos 3, while pos 2 flows 5-8 percent less than pos 3 (in a slightly different area due the the needle being moved relative to the slide).
Hope that makes sense...

Bill

Did about four miles on the bike today.

It cuts out at about 6000 RPM in any gear.

I dropped the bowl and pulled out what looks to like a "large hex" (as depicted on the Sudco site) jet.

The jet has the number "061" stamped on it.

I'm a little confused because the increments on the Sudco site are "5." Was it common to have jets available in finer resolution back in the day? Or could a 061 have been supplied in a stock carb on a specific bike by a manufacturer?

Jim

no smart remarks intended Jim but could it be 190 ?

Eldert

That's hilarious! It is 190!

What a numbnuts I am!

Thanks for helping me look at things in a different way.

Hello Jim,
I put a new VM 26 Mikuni on my (wifes) scrambler. 190 main, 22.5 pilot, 00 needle, 5E75 jet needle, 1.50 cutaway on the slide. We just got back from a 40 mile ride and with a foam sock filter. The plug center is brown and the outside of the plug is maybe on the rich side. Starts first kick hot or cold. This would be a good starting point for yours or I could sell you a really cool looking SS1 that I finally gave up on...I mean put away to save because it is such a wonderful carburetor
Tom
and SUDCO has any part you need at decent prices