Mikuni jetting.

[Richard Toghill]

Help required before I go mad.Can any of you guys tell me what settings you are using for a 36mm mikuni on a 74 250 hare scrambler? I have tried numerous combinations but this thing runs hopelessly rich off the bottom, has a midrange hit like a 125 on steroids then goes flat on the top end.Also, is there any way to convert to a five speed box on the early black case motors?

[Larry Perkins]

Only Mikuni jet sizes I have are in a 250 HS with a reed valve.  I believe if you do a search here that there is a lot of info on Mikuni on 250 HS though.

Larry P

[Mick Milakovic]

What jets are in it?

Mick

[Larry Perkins]

Mick, do you mean in his Mikuni?

Larry P

[Richard Toghill]

27.5 pilot, 310 main, 6DH4 needle, Q6 needle jet, 3.0 slide. Running a K and N filter with a brand new repro pipe from pollino diron. Cylinder has original porting with a new 72mm wiseco piston abd the correct twinplug head. Ignition timing is spot on, rewound motoplat, non resistor llug cap, new crank seals!

[Mick Milakovic]

yes, the Mikuni.  I'll try to get to mine soon and report on my jets.

Mick

[skiracer]

In my 250 MC 5 with a 36 Mikuni, I ran a 400 main with a 35 pilot.

1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer

[JP Morgen]

A 36mm without a reed valve is on the big side, so jetting is going to hard to dial in. I suggest you try other needles if nobody gives a solid reference. What most don't realize is the needle is the only 'jet' that changes as the throttle position is changed. If you have the wrong needle you will chase your tail forever. My only experience is with a reed valve motor.

[skiracer]

So, you say a 36 is too big for a 250?  Then what size would you say is better, a 34mm?  Interesting.....

quote:Originally posted by JP Morgen

A 36mm without a reed valve is on the big side, so jetting is going to hard to dial in. I suggest you try other needles if nobody gives a solid reference. What most don't realize is the needle is the only 'jet' that changes as the throttle position is changed. If you have the wrong needle you will chase your tail forever. My only experience is with a reed valve motor.

1976 MC 5 Original Owner
1976 Penton 175 XC
1985 20' Hi Point trailer

[Larry Perkins]

They came with a 36mm Bing though JP.

Larry P

[Daniel P. McEntee]

In reading this forum for several years, I have read where some have used 34mm Mikunis primarily because they fit near the pipe better, a little more room to get the carb body set correctly. I think some one said that you can bore a 34mm Mikuni out to 36mm in order for it to flow better and have the best of both worlds. I would like to see or read how that is done. But some one in recent history has posted basic Mikuni settings for the 250, all you have to do is search and find it. What do plug readings tell you? Idle jet is for idle RPM up to when the throttle is cracked. The needle and it's jet commands up to 3/4 throttle and the main is for flat out top end. At least that is what I'm remembering from the old two stroke tuners books. I would think a reed doesn't make much difference in what an engine needs, it just make delivering that fuel supply more precise and efficient. Just my 2 pennies worth.
   Type at you later,
   Dan McEntee

[Richard Toghill]

I have done a search on this site and the only settings I can find are 30 pilot, 290 main, 6dh4 needle, q2 needle jet, 2.5 slide. I have a q2 needle jet but as yet havent had the chance to try it. Maybe I  have a Friday Afternoon Mikuni because this thing is truly awful, especially off the bottom where it will gas up at every opportunity. I was expecting a fairly useable power delivery  
or are they all basically unrideable? Mine is the only 250 HS being "raced" in the south of England and its proving to be a bit of an embarrassment when it should be an unfair advantage in the pre 74 class Looks lovely sat on the box though.

[Larry Perkins]

My reed valve 250 motor uses a 34mm Mikuni bored to 35.5mm so the airboot and body would fit.

Larry P

[Daniel P. McEntee]

That is what interests me, how that was done. Don't plan on doing it any time soon, just real curious as to how the carb slide still seals after boring?? Or did you order it that way from Sudco or some one else?
  Type at you later,
   Dan McEntee

[Larry Perkins]

They do not bore the part the slide is in but down the intake throat.  That is where the size of a carb is measured.  Bored bigger it flows more gas.  You have to have it done at a machine shop.

Larry P