Mikuni Carb Settings

[Duccout]

After the last ride with the needle at the highest setting the mixture was obviously too rich, so I went back to the centre needle notch, and also to the 25 pilot jet because I had the air adjustment screw three turns out.

Today I went out for a run, and the bike did not want to run without the cold-start lever on. Eventually I turned the air screw all the way in to richen the pilot, and it ran! I turned the screw one turn out and left it like that for the run out, but when I got home and adjusted the screw it does not make any difference. I may have to ring MotoCarb and see what they say as obviously, the mixture screw makes a huge difference to how the engine runs, which means that the pilot jet size is critical. Apparently, the adjuster should be between 1-2 turns out.

[Dan C]

Hi Colin,

Weird that adjusting the mixture screw makes no difference. Regardless of pilot jet size (unless its way out) it should make some difference. I'm wondering if that indicates an air leak somewhere?

Also Phil Hitchcock (Road and Race) advised me to consider removing the air jet. I haven't found that necessary on the 450 but it has improved the 350.

Cheers Dan

[Duccout]

Hi Dan,

I don't know; I'm sure that I read somewhere that if the adjuster does not affect the running then it shows that the pilot jet is way out, although my experience yesterday when starting shows that the adjuster is doing something!

Colin

[Duccout]

A little bit more experimentation: this morning I tried some start ups and the pilot air adjustment is critical when the engine is cold. I first started the engine a couple of times and it would not run with the throttle open at all, but when I turned the pilot air adjuster in a quarter of a turn, to richer the mixture, it would. This is puzzling as the pilot jet only affects the running of the engine up to one eighth of throttle opening. I'm guessing that my pilot jet, at 25, is spot-on, because the adjuster is 1 1/2 turns out from the stop.

[linker48x]

Hi, I posted way up above, to the effect that the quickest way to get Mikuni carburetor jetted in the ballpark is to ask Sudco for jetting specs for a comparable motorcycle (single cylinder, 4 stroke, 25-28 hp, same size carburetor ) for the VM Mikuni. In this case, that would be a 250 four stroke Street/ trail or Enduro type bike with the same sized carburetor, or one cylinder of a Triumph 500 Daytona twin carb. As I said above, the Mikuni VM round slide is/was head and shoulders above any other brand of pre-1970 carburetor but the increments of change in jet sizes are so close it is very difficult to make headway and it is really easy to get lost. I think folks posting here are experiencing this. These carbs are relatively responsive to changes in pilot, somewhat responsive to main jet changes, and far less so to needle jet and jet needle and needle changes and you can spend a lot of time wearing out the screws and spend a lot of money on brass if you don’t start relatively close. I learned all this from the hard way. The first motorcycle I put one of these on was more than 50 years ago, my 1969 CZ 250 motocrosser. I was told to start with a 250 Yamaha DT-1 GYT Kit carburetor on it (same size engine, close to same horsepower, also 2 stroke) and I tried to save a few dollars and demonstrate my smarts by buying a Generic Mikuni off the shelf version and it took me many many many tries to get it even close. I have put a lot of Mikunis on a lot of bikes since then and the experience is always the same—you can get them perfect but it’s best to start with a very comparable engine or cylinder. A couple other thoughts—these carbs work best on the middle needle clip or one up or down, on a pilot jet that’s 1-1/2 turns out, using a 20-35 pilot, 2.5 or 3 slide cutaway, with a 2.5 or so air jet ( this riches the mix at high rpm so it lets you jet the main leaner at lower rpm). Jet to those settings first— trying to correct wrong jets by going up or down on needles or slide cutaway or air screw suggests you aren’t there yet. I’ve jumped whole series of needle jets—like P to Q to R, just trying to get in the ball park, the increments are very close. The starting jet is huge, because this started as a 2 stroke carb, don’t expect it to run and warm up with it on, unless you drill and tap it to fit an air jet out of a more modern and comparable flat slide Mikuni for a 4 stroke. On my race Triumph 750 and Yamaha 2 stroke 430 cc bikes I would only depress the starter on one side then turn it off immediately before it fouled a plug and warm it up manually. Having to use the starter system after the bike is warm is a signal you are way too lean on the pilot or needle jet or main. And be careful introducing steps in the diameter of the intake either before or after the carb with plumbing parts or cheap Chinese K&N , this can cause real problems and VMs put their air correction and pilot orifices in the carb intake at the outer part of its bell mouth—for instance Chinese knockoff K&N style carbs can partially block or obscure these. I still use the original Dellorto on my Mark 3 because I like them and raced a Parilla Wildcat with one in the 60’s or I might have some jetting specs so all I can say is, good luck!

[Duccout]

Thanks for your thoughts, very helpful. I was surprised at how much difference the pilot air screw made to the running, but now I have found that out I think that I'm getting close.


Colin

[Duccout]

Tried dropping the needle all the way down....... Engine started easily and ran perfectly - until I tried to ride it! Pulling away and running at 1/4 throttle it was obviously way too weak, jerking and spitting back, but higher up at 1/2 - 3/4 throttle it ran cleanly and revved properly.

Called Motocarb for a chat, with the result that I am going back to the original needle jet (169-02) and am going up again on the pilot jet to a 30. So, we will see. I was going to try a different needle, but Motocarb advised against it.

Colin

[Blaine.Hale]

This sounds exactly like what I'm experiencing on my 250 Monza and sudco jetted vm 26. I was thinking I need a richer pilot as off idle, it just isn't getting enough fuel but after that, it's great. But it hangs when coming down from rev and won't go back to idle until I fiddle with the idle speed adjuster. Works just fine when the choke is on.

[Duccout]

Blaine, it sounds like you need a richer pilot jet, if the engine runs with the choke on, then the pilot is too weak. What I have found is that the pilot air adjustment makes a massive difference to how the engine runs, so screwing in the adjuster really helps with weakness at low throttle openings. I am waiting to try a no 30 pilot jet, but if that does not do the job then I may have to ditch the Mikuni and buy an Amal. I suspect that I need a needle with a smaller diameter at the top, to allow more fuel through at small openings, but the UK agent does not advise changing the needle, and there are so many variations that I don't fancy blowing a fortune experimenting.


Colin

[Blaine.Hale]

Y'all are correct. I bumped from a 30 to 35 pilot and it runs fantastic. Got to put 20 miles on it for its very first run since the rebuild and had no issues. That mikuni is really great! I'm going to 3d model and print a hose to attach it to the stock Monza airbox to keep things looking good and stock. I'm not a big fan of pod filters sticking out and it would be pretty hard to fit one here.
I could really do with a heavier throttle return spring but Sudco say one does not exist. Would putting a second, identical, spring in there work?